Archive for the ‘Male Genetics’ Category

Fans greeted the news that Sam Neill, Laura Dern and Jeff Goldblum will join the cast of Jurassic World 3 with a roar of approval. Is this a shot in the arm the franchise needs after the relative disappointment of Fallen Kingdom, or is this Universal shamelessly pandering to nostalgia?

Maybe its both. The return of the beloved principals of the first Jurassic Park is certainly welcome, but how effective that return is depends on how well they are used.

The first Jurassic World in 2015 was a huge success, making $652 million here and $1.7 billion worldwide, making it the franchise leader before you adjust for inflation. Although some critics knocked it for being sexist and too derivative of Jurassic Park, it was generally well reviewed, with 72% on Rotten Tomatoes.

Fallen Kingdom came out three yearslater, and something was missing enthusiasm. The sequel made $417 million here, a disappointment somewhat salvaged by making $1.3 billion worldwide, but the reviews hit a franchise low of 48%. Fans and critics alike noted the dour tone, with a structure that seemed too close to The Lost World. Both the 1997 and 2018 contrived reasons to return to dinosaur islands failed to capture the wonder of their predecessors. Both movies also unleashed the dinosaurs on the mainland.

One aspect of the sequel that burned some fans was that the trailers highlighted an appearance by Goldblums Ian Malcolm, but he turned out only to have a cameo, with almost his entire performance contained in the trailer. One hopes Universal isnt making the same mistake again with Dern and Neill, although it sounds like all three will be more central to the story this time.

Jurassic World 3, due out in June 2021, will deal with the aftermath of the dinosaurs reaching the mainland. The Lost Worlds third act dealt with this too, but it cleaned up the mess relatively quickly. That movie had one dinosaur stomping around, while this one will have a whole bunch.

Colin Trevorrow, returning to the directors chair after only co-writing Fallen Kingdom, told EW the storywill be focused storytelling with dinosaurs all over the world. We really wanted this technology, this genetic power, to go open-source at the end of the film. What were suggesting is not just that these specific animals that we care about that were in captivity were freed, but also that the ability to create these animals has gone a little bit wider than our friend Dr. Wu. The open-sourcing of any technology, like nuclear power, thats the scary side for me.

In other words, it sounds like we get not only your traditional T-Rexs and velociraptors, but your mutant hybrid dinosaurs that caused so much trouble in the previous movies. Those greedy humans just never learn, do they? Chris Pratt and Bryce Dallas Howard return as well.

Many fans were delighted to see Neill, Dern and Goldblum back in the mix, with one person on Instagram saying, BEST DAY EVER! YOU GUYS KNOW HOW TO MAKE HAPPY THE FANDOM! A more cynical commenter on EW said, The original title of this story was: Universal resorts to Disney tactics by playing on Gen X nostalgia. (and Millennials disdain for watching ancient 90s movies.)

It almost goes without saying that Jurassic World 3 will make a ton of money. One hopes, however, that Universal isnt just handing out the rose-colored glasses of nostalgia. Sam Neill and Laura Dern came back for Jurassic Park III, and that movie in some corners is like the forgotten stepchild, so theres no guarantee their return will get the franchise back on course.

To be fair, dinosaurs roaming the mainland isnt something the franchise has played with for an entire movie, and if it fulfills the promise of the short film that came out, then Jurassic World 3 will make a good capper for the franchise provided they stop there.

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'Jurassic World 3': The Real Reason the Movie Needs Original Characters to Return - Showbiz Cheat Sheet

Cat owners and enthusiasts have heard a number of fascinating myths about male calico cats. While they are relatively rare, with an estimated one male in 3,000 calico cat births, there is no extraordinary demand for them. They do not make good breeding studs because almost all male calicos are sterile. In fact, only about one in 10,000 male calicos is fertile.

Some people have the misconception that calico kittens and cats comprise a specific cat breed. However, calico is the description of a cat's coloration. Cats of many breeds can be a calico, or true tricolor, as a result of their genetic heritage.

Unlike cats with tortoiseshell coloring, the coats of calico cats are of three distinct colors - red, black and white, or a variation of those colors.

Male calicos are a genetic anomaly. Cats, like humans, have two sex chromosomes. Chromosomes carry genes and determine an animal's traits. The required red color for a calico cat is passed only on a female (X) chromosome. How then can a male cat inherit the red colored required for a calico cat?

To put it simply, two chromosomes determine gender. Each parent contributes one chromosome to the offspring. The mother, who has only X chromosomes, always contributes an X chromosome. The father who has both X and Y chromosomes, can contribute either an X or a Y chromosome to his offspring. Thus, it is the father who determines the sexes of his kittens. The red color gene cannot be passed to a male offspring due to unusual characteristics of the gene in question. Under certain conditions, when the red genes are passed to a female offspring, she displays not the expected red or orange coat, but the tricolor coat of a true calico cat.

How then can a male be a true calico? Sometimes there is an incomplete division of the chromosome pair when the chromosomes are separating at the time of fertilization. When that happens, the incomplete chromosome ends up attached to another of the two required chromosomes, giving the offspring one of the following combinations:

In both cases, the result is a male cat who can inherit the trait for a true calico coat. Among humans, this genetic arrangement is called Klinefelter syndrome. A male calico usually cannot sire offspring because the genetics described above almost always guarantees that he will be sterile.

One might suppose that male calicos would bring a high price among breeders because of their rarity. You may even see some websites claiming a purebred male calico cat can fetch a price as high as $1,000 to $2,000. The truth is, while they are an interesting phenomenon, they are of little interest to breeders because they are sterile. It's possible a pet owner might want to pay that amount of money to own a cat that's a rarity, but chances are if you're looking to buy a male calico cat don't expect to pay much more than you would for any regular unpedigreed house cat.

Although most male calicos are sterile, it is a good idea to neuter them to deter spraying and other unwelcome male behaviors. Despite their limitations, they are still boys at heart!

As mentioned earlier, male calico cats have distinctive tricolor coats, but they are not a separate breed. In fact, as many as 16 different cat breeds can have calico coloration, and male calicos can occur among any of those breeds. Some common breeds that may have calico coloration are:

Male calico cats are the offspring with a genetic anomaly of parents representing many possible cat breeds. While female calico cats are quite common, true male calicos are rare and of particular interest for the combination of their unique coloration and sex. While it's a myth that they can command a high price among cat fanciers, if you happen to own a male calico, you can treasure him for his rare condition and other wonderful feline attributes!

Excerpt from:
Myths and Facts About Male Calico Cats | LoveToKnow

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Selecting Male Genetics for Cannabis Plants - School of ...

Androgen insensitivity syndrome (AIS) is an intersex condition that results in the partial or complete inability of the cell to respond to androgens.[1][2][3] The unresponsiveness of the cell to the presence of androgenic hormones can impair or prevent the masculinization of male genitalia in the developing fetus, as well as the development of male secondary sexual characteristics at puberty, but does not significantly impair female genital or sexual development.[3][4] As such, the insensitivity to androgens is clinically significant only when it occurs in genetic males (i.e. individuals with a Y-chromosome, or more specifically, an SRY gene).[1] Clinical phenotypes in these individuals range from a normal male habitus with mild spermatogenic defect or reduced secondary terminal hair, to a full female habitus, despite the presence of a Y-chromosome.[1][5][6][7][8][9]

AIS is divided into three categories that are differentiated by the degree of genital masculinization: complete androgen insensitivity syndrome (CAIS) is indicated when the external genitalia are that of a normal female; mild androgen insensitivity syndrome (MAIS) is indicated when the external genitalia are that of a normal male, and partial androgen insensitivity syndrome (PAIS) is indicated when the external genitalia are partially, but not fully, masculinized.[1][2][5][6][7][10][11][12][13] Androgen insensitivity syndrome is the largest single entity that leads to 46,XY undermasculinized genitalia.[14]

Management of AIS is currently limited to symptomatic management; no method is currently available to correct the malfunctioning androgen receptor proteins produced by AR gene mutations. Areas of management include sex assignment, genitoplasty, gonadectomy in relation to tumor risk, hormone replacement therapy, genetic counseling, and psychological counseling.

The human androgen receptor (AR) is a protein encoded by a gene located on the proximal long arm of the X chromosome (locus Xq11-Xq12).[15] The protein coding region consists of approximately 2,757 nucleotides (919 codons) spanning eight exons, designated 1-8 or A-H.[1][3] Introns vary in size between 0.7 and 26 kb.[3] Like other nuclear receptors, the AR protein consists of several functional domains: the transactivation domain (also called the transcription-regulation domain or the amino / NH2-terminal domain), the DNA-binding domain, the hinge region, and the steroid-binding domain (also called the carboxyl-terminal ligand-binding domain).[1][2][3][13] The transactivation domain is encoded by exon 1, and makes up more than half of the AR protein.[3] Exons 2 and 3 encode the DNA-binding domain, while the 5' portion of exon 4 encodes the hinge region.[3] The remainder of exons 4 through 8 encodes the ligand binding domain.[3]

The AR gene contains two polymorphic trinucleotide microsatellites in exon 1.[2] The first microsatellite (nearest the 5' end) contains 8 [16] to 60 [17][18] repetitions of the glutamine codon "CAG" and is thus known as the polyglutamine tract.[3] The second microsatellite contains 4 [19] to 31 [20] repetitions of the glycine codon "GGC" and is known as the polyglycine tract.[21] The average number of repetitions varies by ethnicity, with Caucasians exhibiting an average of 21 CAG repeats, and Blacks 18.[22] In men, disease states are associated with extremes in polyglutamine tract length; prostate cancer,[23] hepatocellular carcinoma,[24] and intellectual disability [16] are associated with too few repetitions, while spinal and bulbar muscular atrophy (SBMA) is associated with a CAG repetition length of 40 or more.[25] Some studies indicate that the length of the polyglutamine tract is inversely correlated with transcriptional activity in the AR protein, and that longer polyglutamine tracts may be associated with male infertility [26][27][28] and undermasculinized genitalia in men.[29] However, other studies have indicated no such correlation exists.[30][31][32][33][34][35] A comprehensive meta-analysis of the subject published in 2007 supports the existence of the correlation, and concluded these discrepancies could be resolved when sample size and study design are taken into account.[11] Some studies suggest longer polyglycine tract lengths are also associated with genital masculinization defects in men.[36][37] Other studies find no such association.[38]

As of 2010, over 400 AR mutations have been reported in the AR mutation database, and the number continues to grow.[2] Inheritance is typically maternal and follows an X-linked recessive pattern;[1][39] individuals with a 46,XY karyotype always express the mutant gene since they have only one X chromosome, whereas 46,XX carriers are minimally affected. About 30% of the time, the AR mutation is a spontaneous result, and is not inherited.[10] Such de novo mutations are the result of a germ cell mutation or germ cell mosaicism in the gonads of one of the parents, or a mutation in the fertilized egg itself.[40] In one study,[41] three of eight de novo mutations occurred in the postzygotic stage, leading to the estimate that up to one-third of de novo mutations result in somatic mosaicism.[1] Not every mutation of the AR gene results in androgen insensitivity; one particular mutation occurs in 8 to 14% of genetic males,[42][43][44][45] and is thought to adversely affect only a small number of individuals when other genetic factors are present.[46]

Some individuals with CAIS or PAIS do not have any AR mutations despite clinical, hormonal, and histological features sufficient to warrant an AIS diagnosis; up to 5% of women with CAIS do not have an AR mutation,[2] as well as between 27[6][47] and 72%[48] of individuals with PAIS.

In one patient, the underlying cause for presumptive PAIS was a mutant steroidogenic factor-1 (SF-1) protein.[49] In another patient, CAIS was the result of a deficit in the transmission of a transactivating signal from the N-terminal region of the normal androgen receptor to the basal transcription machinery of the cell.[50] A coactivator protein interacting with the activation function 1 (AF-1) transactivation domain of the androgen receptor may have been deficient in this patient.[50] The signal disruption could not be corrected by supplementation with any coactivators known at the time, nor was the absent coactivator protein characterized, which left some in the field unconvinced that a mutant coactivator would explain the mechanism of androgen resistance in CAIS or PAIS patients with a normal AR gene.[1]

Depending on the mutation, a person with a 46,XY karyotype and AIS can have either a male (MAIS) or female (CAIS) phenotype,[51] or may have genitalia that are only partially masculinized (PAIS).[52] The gonads are testes regardless of phenotype due to the influence of the Y chromosome.[53][54] A 46,XY female, thus, does not have ovaries or a uterus,[55] and can neither contribute an egg towards conception nor gestate a child.

Several case studies of fertile 46,XY males with AIS have been published,[4][56][57][58][59] although this group is thought to be a minority.[13] Additionally, some infertile males with MAIS have been able to conceive children after increasing their sperm count through the use of supplementary testosterone.[1][60] A genetic male conceived by a man with AIS would not receive his father's X chromosome, thus would neither inherit nor carry the gene for the syndrome. A genetic female conceived in such a way would receive her father's X chromosome, thus would become a carrier.

Genetic females (46,XX karyotype) have two X chromosomes, thus have two AR genes. A mutation in one (but not both) results in a minimally affected, fertile, female carrier. Some carriers have been noted to have slightly reduced body hair, delayed puberty, and/or tall stature, presumably due to skewed X-inactivation.[3][4] A female carrier will pass the affected AR gene to her children 50% of the time. If the affected child is a genetic female, she, too, will be a carrier. An affected 46,XY child will have AIS.

A genetic female with mutations in both AR genes could theoretically result from the union of a fertile man with AIS and a female carrier of the gene, or from de novo mutation. However, given the scarcity of fertile AIS men and low incidence of AR mutation, the chances of this occurrence are small. The phenotype of such an individual is a matter of speculation; as of 2010, no such documented case has been published.

Individuals with partial AIS, unlike those with the complete or mild forms, present at birth with ambiguous genitalia, and the decision to raise the child as male or female is often not obvious.[1][40][61] Unfortunately, little information regarding phenotype can be gleaned from precise knowledge of the AR mutation itself; the same AR mutation may cause significant variation in the degree of masculinization in different individuals, even among members of the same family.[1][39][52][62][63][64][65][66][67][68] Exactly what causes this variation is not entirely understood, although factors contributing to it could include the lengths of the polyglutamine and polyglycine tracts,[69] sensitivity to and variations in the intrauterine endocrine milieu,[52] the effect of coregulatory proteins active in Sertoli cells,[21][70] somatic mosaicism,[1] expression of the 5RD2 gene in genital skin fibroblasts,[62] reduced AR transcription and translation from factors other than mutations in the AR coding region,[71] an unidentified coactivator protein,[50] enzyme deficiencies such as 21-hydroxylase deficiency,[4] or other genetic variations such as a mutant steroidogenic factor-1 protein.[49] The degree of variation, however, does not appear to be constant across all AR mutations, and is much more extreme in some.[1][4][46][52] Missense mutations that result in a single amino acid substitution are known to produce the most phenotypic diversity.[2]

The effects that androgens have on the human body (virilization, masculinization, anabolism, etc.) are not brought about by androgens themselves, but rather are the result of androgens bound to androgen receptors; the androgen receptor mediates the effects of androgens in the human body.[73] Likewise, under normal circumstances, the androgen receptor itself is inactive in the cell until androgen binding occurs.[3]

The following series of steps illustrates how androgens and the androgen receptor work together to produce androgenic effects:[1][2][3][13][18][74][75]

In this way, androgens bound to androgen receptors regulate the expression of target genes, thus produce androgenic effects.

Theoretically, certain mutant androgen receptors can function without androgens; in vitro studies have demonstrated that a mutant androgen receptor protein can induce transcription in the absence of androgen if its steroid binding domain is deleted.[76][77] Conversely, the steroid-binding domain may act to repress the AR transactivation domain, perhaps due to the AR's unliganded conformation.[3]

Human embryos develop similarly for the first six weeks, regardless of genetic sex (46,XX or 46,XY karyotype); the only way to tell the difference between 46,XX or 46,XY embryos during this time period is to look for Barr bodies or a Y chromosome.[79] The gonads begin as bulges of tissue called the genital ridges at the back of the abdominal cavity, near the midline. By the fifth week, the genital ridges differentiate into an outer cortex and an inner medulla, and are called indifferent gonads.[79] By the sixth week, the indifferent gonads begin to differentiate according to genetic sex. If the karyotype is 46,XY, testes develop due to the influence of the Y chromosomes SRY gene.[53][54] This process does not require the presence of androgen, nor a functional androgen receptor.[53][54]

Until around the seventh week of development, the embryo has indifferent sex accessory ducts, which consist of two pairs of ducts: the Mllerian ducts and the Wolffian ducts.[79] Sertoli cells within the testes secrete anti-Mllerian hormone around this time to suppress the development of the Mllerian ducts, and cause their degeneration.[79] Without this anti-Mllerian hormone, the Mllerian ducts develop into the female internal genitalia (uterus, cervix, fallopian tubes, and upper vaginal barrel).[79] Unlike the Mllerian ducts, the Wolffian ducts will not continue to develop by default.[80] In the presence of testosterone and functional androgen receptors, the Wolffian ducts develop into the epididymides, vasa deferentia, and seminal vesicles.[79] If the testes fail to secrete testosterone, or the androgen receptors do not function properly, the Wolffian ducts degenerate.[81]

Masculinization of the male external genitalia (the penis, penile urethra, and scrotum), as well as the prostate, are dependent on the androgen dihydrotestosterone.[82][83][84][85] Testosterone is converted into dihydrotestosterone by the 5-alpha reductase enzyme.[86] If this enzyme is absent or deficient, then dihydrotestosterone is not created, and the external male genitalia do not develop properly.[82][83][84][85][86] As is the case with the internal male genitalia, a functional androgen receptor is needed for dihydrotestosterone to regulate the transcription of target genes involved in development.[73]

Mutations in the androgen receptor gene can cause problems with any of the steps involved in androgenization, from the synthesis of the androgen receptor protein itself, through the transcriptional ability of the dimerized, androgen-AR complex.[3] AIS can result if even one of these steps is significantly disrupted, as each step is required for androgens to activate the AR successfully and regulate gene expression.[3] Exactly which steps a particular mutation will impair can be predicted, to some extent, by identifying the area of the AR in which the mutation resides. This predictive ability is primarily retrospective in origin; the different functional domains of the AR gene have been elucidated by analyzing the effects of specific mutations in different regions of the AR.[3] For example, mutations in the steroid binding domain have been known to affect androgen binding affinity or retention, mutations in the hinge region have been known to affect nuclear translocation, mutations in the DNA-binding domain have been known to affect dimerization and binding to target DNA, and mutations in the transactivation domain have been known to affect target gene transcription regulation.[3][80] Unfortunately, even when the affected functional domain is known, predicting the phenotypical consequences of a particular mutation (see Correlation of genotype and phenotype) is difficult.

Some mutations can adversely impact more than one functional domain. For example, a mutation in one functional domain can have deleterious effects on another by altering the way in which the domains interact.[80] A single mutation can affect all downstream functional domains if a premature stop codon or framing error results; such a mutation can result in a completely unusable (or unsynthesizable) androgen receptor protein.[3] The steroid binding domain is particularly vulnerable to the effects of a premature stop codon or framing error, since it occurs at the end of the gene, and its information is thus more likely to be truncated or misinterpreted than other functional domains.[3]

Other, more complex relationships have been observed as a consequence of mutated AR; some mutations associated with male phenotypes have been linked to male breast cancer, prostate cancer, or in the case of spinal and bulbar muscular atrophy, disease of the central nervous system.[9][23][87][88][89] The form of breast cancer seen in some men with PAIS is caused by a mutation in the AR's DNA-binding domain.[87][89] This mutation is thought to cause a disturbance of the AR's target gene interaction that allows it to act at certain additional targets, possibly in conjunction with the estrogen receptor protein, to cause cancerous growth.[3] The pathogenesis of spinal and bulbar muscular atrophy (SBMA) demonstrates that even the mutant AR protein itself can result in pathology. The trinucleotide repeat expansion of the polyglutamine tract of the AR gene that is associated with SBMA results in the synthesis of a misfolded AR protein that the cell fails to proteolyze and disperse properly.[90] These misfolded AR proteins form aggregates in the cell cytoplasm and nucleus.[90] Over the course of 30 to 50 years, these aggregates accumulate and have a cytotoxic effect, eventually resulting in the neurodegenerative symptoms associated with SBMA.[90]

The phenotypes that result from the insensitivity to androgens are not unique to AIS, thus the diagnosis of AIS requires thorough exclusion of other causes.[14][64] Clinical findings indicative of AIS include the presence of a short vagina [91] or undermasculinized genitalia,[1][63][82] partial or complete regression of Mllerian structures,[92] bilateral nondysplastic testes,[93] and impaired spermatogenesis and/or virilization.[1][5][6][9] Laboratory findings include a 46,XY karyotype[2] and normal or elevated postpubertal testosterone, luteinizing hormone, and estradiol levels.[2][14] The androgen binding activity of genital skin fibroblasts is typically diminished,[3][94] although exceptions have been reported.[95] Conversion of testosterone to dihydrotestosterone may be impaired.[3] The diagnosis of AIS is confirmed if androgen receptor gene sequencing reveals a mutation, although not all individuals with AIS (particularly PAIS) will have an AR mutation (see Other Causes).[2][6][47][48]

Each of the three types of AIS (complete, partial, and mild) has a different list of differential diagnoses to consider.[1] Depending on the form of AIS suspected, the list of differentials can include:[53][54][96][97][98]

AIS is broken down into three classes based on phenotype: complete androgen insensitivity syndrome (CAIS), partial androgen insensitivity syndrome (PAIS), and mild androgen insensitivity syndrome (MAIS).[1][2][5][6][7][10][11][12][13] A supplemental system of phenotypic grading that uses seven classes instead of the traditional three was proposed by pediatric endocrinologist Charmian A. Quigley et al. in 1995.[3] The first six grades of the scale, grades 1 through 6, are differentiated by the degree of genital masculinization; grade 1 is indicated when the external genitalia is fully masculinized, grade 6 is indicated when the external genitalia is fully feminized, and grades 2 through 5 quantify four degrees of decreasingly masculinized genitalia that lie in the interim.[3] Grade 7 is indistinguishable from grade 6 until puberty, and is thereafter differentiated by the presence of secondary terminal hair; grade 6 is indicated when secondary terminal hair is present, whereas grade 7 is indicated when it is absent.[3] The Quigley scale can be used in conjunction with the traditional three classes of AIS to provide additional information regarding the degree of genital masculinization, and is particularly useful when the diagnosis is PAIS.[2][99]

Management of AIS is currently limited to symptomatic management; no method is currently available to correct the malfunctioning androgen receptor proteins produced by AR gene mutations. Areas of management include sex assignment, genitoplasty, gonadectomy in relation to tumor risk, hormone replacement therapy, genetic counseling, and psychological counseling.

Estimates for the incidence of androgen insensitivity syndrome are based on a relatively small population size, thus are known to be imprecise.[1] CAIS is estimated to occur in one of every 20,400 46,XY births.[100] A nationwide survey in the Netherlands based on patients with genetic confirmation of the diagnosis estimates that the minimal incidence of CAIS is one in 99,000.[62] The incidence of PAIS is estimated to be one in 130,000.[101] Due to its subtle presentation, MAIS is not typically investigated except in the case of male infertility,[82] thus its true prevalence is unknown.[2]

Preimplantation genetic diagnosis (PGD or PIGD) refers to genetic profiling of embryos prior to implantation (as a form of embryo profiling), and sometimes even of oocytes prior to fertilization. When used to screen for a specific genetic sequence, its main advantage is that it avoids selective pregnancy termination, as the method makes it highly likely that a selected embryo will be free of the condition under consideration.

In the UK, AIS appears on a list of serious genetic diseases that may be screened for via PGD.[102] Some ethicists, clinicians, and intersex advocates have argued that screening embryos to specifically exclude intersex traits are based on social and cultural norms as opposed to medical necessity.[103][104][105][106][107]

Recorded descriptions of the effects of AIS date back hundreds of years, although significant understanding of its underlying histopathology did not occur until the 1950s.[1] The taxonomy and nomenclature associated with androgen insensitivity went through a significant evolution that paralleled this understanding.

The first descriptions of the effects of AIS appeared in the medical literature as individual case reports or as part of a comprehensive description of intersex physicalities. In 1839, Scottish obstetrician Sir James Young Simpson published one such description[117] in an exhaustive study of intersexuality that has been credited with advancing the medical community's understanding of the subject.[118] Simpson's system of taxonomy, however, was far from the first; taxonomies or descriptions for the classification of intersexuality were developed by Italian physician and physicist Fortun Affaitati in 1549,[119][120] French surgeon Ambroise Par in 1573,[118][121] French physician and sexology pioneer Nicolas Venette in 1687 (under the pseudonym Vnitien Salocini),[122][123] and French zoologist Isidore Geoffroy Saint-Hilaire in 1832.[124] All five of these authors used the colloquial term "hermaphrodite" as the foundation of their taxonomies, although Simpson himself questioned the propriety of the word in his publication.[117] Use of the word "hermaphrodite" in the medical literature has persisted to this day,[125][126] although its propriety is still in question. An alternative system of nomenclature has been recently suggested,[127] but the subject of exactly which word or words should be used in its place still one of much debate.[97][128][129][130][131]

"Pseudohermaphroditism" has, until very recently,[127] been the term used in the medical literature to describe the condition of an individual whose gonads and karyotype do not match the external genitalia in the gender binary sense. For example, 46,XY individuals who have a female phenotype, but also have testes instead of ovaries a group that includes all individuals with CAIS, as well as some individuals with PAIS are classified as having "male pseudohermaphroditism", while individuals with both an ovary and a testis (or at least one ovotestis) are classified as having "true hermaphroditism".[126][127] Use of the word in the medical literature antedates the discovery of the chromosome, thus its definition has not always taken karyotype into account when determining an individual's sex. Previous definitions of "pseudohermaphroditism" relied on perceived inconsistencies between the internal and external organs; the "true" sex of an individual was determined by the internal organs, and the external organs determined the "perceived" sex of an individual.[117][124]

German-Swiss pathologist Edwin Klebs is sometimes noted for using the word "pseudohermaphroditism" in his taxonomy of intersexuality in 1876,[133] although the word is clearly not his invention as is sometimes reported; the history of the word "pseudohermaphrodite" and the corresponding desire to separate "true" hermaphrodites from "false", "spurious", or "pseudo" hermaphrodites, dates back to at least 1709, when Dutch anatomist Frederik Ruysch used it in a publication describing a subject with testes and a mostly female phenotype.[132] "Pseudohermaphrodite" also appeared in the Acta Eruditorum later that same year, in a review of Ruysch's work.[134] Also some evidence indicates the word was already being used by the German and French medical community long before Klebs used it; German physiologist Johannes Peter Mller equated "pseudohermaphroditism" with a subclass of hermaphroditism from Saint-Hilaire's taxonomy in a publication dated 1834,[135] and by the 1840s "pseudohermaphroditism" was appearing in several French and German publications, including dictionaries.[136][137][138][139]

In 1953, American gynecologist John Morris provided the first full description of what he called "testicular feminization syndrome" based on 82 cases compiled from the medical literature, including two of his own patients.[1][3][140] The term "testicular feminization" was coined to reflect Morris' observation that the testicles in these patients produced a hormone that had a feminizing effect on the body, a phenomenon now understood to be due to the inaction of androgens, and subsequent aromatization of testosterone into estrogen.[1] A few years before Morris published his landmark paper, Lawson Wilkins had shown through experiment that unresponsiveness of the target cell to the action of androgenic hormones was a cause of "male pseudohermaphroditism".[64][108] Wilkins' work, which clearly demonstrated the lack of a therapeutic effect when 46,XY women were treated with androgens, caused a gradual shift in nomenclature from "testicular feminization" to "androgen resistance".[82]

A distinct name has been given to many of the various presentations of AIS, such as Reifenstein syndrome (1947),[141] Goldberg-Maxwell syndrome (1948),[142] Morris' syndrome (1953),[140] Gilbert-Dreyfus syndrome (1957),[143] Lub's syndrome (1959),[144] "incomplete testicular feminization" (1963),[145] Rosewater syndrome (1965),[146] and Aiman's syndrome (1979).[147] Since it was not understood that these different presentations were all caused by the same set of mutations in the androgen receptor gene, a unique name was given to each new combination of symptoms, resulting in a complicated stratification of seemingly disparate disorders.[64][148]

Over the last 60 years, as reports of strikingly different phenotypes were reported to occur even among members of the same family, and as steady progress was made towards the understanding of the underlying molecular pathogenesis of AIS, these disorders were found to be different phenotypic expressions of one syndrome caused by molecular defects in the androgen receptor gene.[1][13][64][148]

AIS is now the accepted terminology for the syndromes resulting from unresponsiveness of the target cell to the action of androgenic hormones.[1] CAIS encompasses the phenotypes previously described by "testicular feminization", Morris' syndrome, and Goldberg-Maxwell syndrome;[1][149] PAIS includes Reifenstein syndrome, Gilbert-Dreyfus syndrome, Lub's syndrome, "incomplete testicular feminization", and Rosewater syndrome;[148][150][151] and MAIS includes Aiman's syndrome.[152]

The more virilized phenotypes of AIS have sometimes been described as "undervirilized male syndrome", "infertile male syndrome", "undervirilized fertile male syndrome", etc., before evidence was reported that these conditions were caused by mutations in the AR gene.[58] These diagnoses were used to describe a variety of mild defects in virilization; as a result, the phenotypes of some men who have been diagnosed as such are better described by PAIS (e.g. micropenis, hypospadias, and undescended testes), while others are better described by MAIS (e.g. isolated male infertility or gynecomastia).[1][58][59][151][153][154]

In the film Orchids, My Intersex Adventure, Phoebe Hart and her sister Bonnie Hart, both women with CAIS, documented their exploration of AIS and other intersex issues.[155]

Recording artist Dalea is a Hispanic-American Activist who is public about her CAIS. She has given interviews about her condition[156][157] and founded "Girl Comet, a non-profit diversity awareness and inspiration initiative.[158]

In 2017, fashion model Hanne Gaby Odiele disclosed that she was born with the intersex trait androgen insensitivity syndrome. As a child, she underwent medical procedures relating to her condition,[159] which she said took place without her or her parents' informed consent.[160] She was told about her intersex condition weeks before beginning her modelling career.[160]

In the 1991 Japanese horror novel Ring, by Koji Suzuki (later adapted into Japanese, Korean, and American films), the central antagonist Sadako has this syndrome.

In season 2, episode 13 ("Skin Deep") of the TV series House, the main patient's cancerous testicle is mistaken for an ovary due to the patient's undiscovered CAIS.

In season 2 of the MTV series Faking It, a character has CAIS. The character, Lauren Cooper, played by Bailey De Young, was the first intersex series regular on American television.[161][162]

In season 8, episode 11 ("Delko for the Defense") of the TV series CSI: Miami, the primary suspect has AIS which gets him off a rape charge.

In series 8, episode 5 of Call the Midwife, a woman discovers that she has AIS. She attends a cervical smear and brings up that she has never had a period, and is concerned about having children as she is about to be married. She is then diagnosed with "testicular feminisation syndrome", the old term for AIS. [163]

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Androgen insensitivity syndrome - Wikipedia

From bald eagles to Bruce Willis, bald spots are a common sight and part of the fabric of our society. Its often assumed that baldness has a genetic component to it, and thats absolutely true; it does. But its also commonly believed that baldness is inherited from your maternal grandfather. That part isnt entirely true. As with many concepts in genetics, theres a lot more to it than that!

Both men and women experience hair loss, but research has historically focused primarily on male subjects (and efforts to link the two have shown that female pattern hair loss is not predicted by the same genetic markers). Because of this, significantly less is known about female hair loss. We do know that approximately 30% of males experience some degree of hair loss (including simple hair thinning or a receding hairline) by the age of 30, 50% by the age of 50, and 80% by the age of 701.

Male pattern baldness (MPB) is a condition where hair loss occurs in multiple parts of the scalp, ultimately leading to a bald region surrounded by hair in a horseshoe-like pattern3.The process of going bald is more complex than simply hair falling out, though. For starters, individuals with MPB are known to have smaller hair follicles on their scalp. Hair follicles are made of multiple cell types, each one dedicated to a particular process in building hair, which is actually a long chain of proteins (mostly keratin, which you can read about here) outside those cells. These follicles are where hair gains its unique features like curliness and color. Individuals with MPB not only have smaller follicles, but those follicles produce less hair, which contributes to the hair thinning process. Eventually, these follicles die, which produces a bald spot1-4.

But why do some people go bald while others dont?

Large scale genetic studies have shown that DNA plays a big part in determining whether MPB will develop2-4. A common saying is that hair loss can be traced back to a persons grandfather on their mothers side. While this isnt entirely true, there is some genetic evidence behind it. One of the well-known genes related to hair loss is the AR gene which codes for the androgen receptor protein. Among other functions, this protein helps hair follicle cells detect androgen hormones (like testosterone) that circulate through the body. Testosterone and other androgens can affect when, where, and how much a persons hair grows1. The AR gene is located on the X chromosome, which means that, for males, it was inherited from their mother. While this seems to lend credence to the notion that baldness is inherited from a persons maternal grandfather, research indicates that the story is more complex than that. Recent studies report that MPB is a polygenic condition, meaning there are many genetic variants involved2. In fact, many of the genetic variants associated with MPB are not located on sex chromosomes. When considered together, these variants have been found to be more predictive of MPB development than variants that are located on sex chromosomes2.

MPB can be inherited from either side of a persons family

Although scientists have found DNA variants that seem to predict the likelihood of MPB development, its not entirely clear how these minor changes in the DNA lead to hair loss. Many of these variants are located in or near genes involved in the process of forming and maintaining hair follicle cells, indicating that these changes somehow affect the biology of hair follicles. Lots of proteins are involved in making and maintaining hair follicles, and we need to take all of them into account if we want to find the most complete answer1.

DNA cannot be used to predict everything about a persons future, but it can be used to make useful estimates of how likely it is that a person will have certain physical traits. MPB is a good example of this. Scientists can determine how many MPB associated DNA variants a person has, and use them to estimate their likelihood of experiencing hair loss. Individually, each gene may be associated with slightly higher odds of going bald; however, a persons chances increase with each additional variant they inherit. Some people inherit a specific combination of variants that increases their likelihood of developing MPB by 58%2. This kind of analysiswhere multiple genetic variants are taken into considerationis common in genetics and helps strengthen the predictive ability of some types of genetic tests.

So, whats the bald truth on baldness? MPB can be inherited from either side of a persons family, and there are ways you can learn more through a DNA test. In the Helix Store, HumanCodes BABYglimpse and DNAPassport can give you insights into your predisposition for male pattern baldness. And if the evidence comes back strong, who knows? You might just be the next Samuel L. Jackson.

2Hagenaars, Saskia P. et al. Genetic Prediction of Male Pattern Baldness. Ed. Markus M. Noethen. PLoS Genetics 13.2 (2017): e1006594. PMC. Web. 11 Dec. 2017.

3Heilmann-Heimbach, Stefanie et al. Meta-Analysis Identifies Novel Risk Loci and Yields Systematic Insights into the Biology of Male-Pattern Baldness. Nature Communications 8 (2017): 14694. PMC. Web. 11 Dec. 2017.

4Pirastu, Nicola et al. GWAS for Male-Pattern Baldness Identifies 71 Susceptibility Loci Explaining 38% of the Risk. Nature Communications 8 (2017): 1584. PMC. Web. 11 Dec. 2017.

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The genetics of baldness: More complex than you might ...

Why does it matter if infertility has a genetic cause?

Developed in the early 1990s, assisted reproduction in the form of IVF and ICSI (intracytoplasmic sperm injection) is a revolutionary laboratory technique in which a single sperm is placed directly inside an egg for fertilization. This technique has opened the door to fertility for men who formerly had few available treatment options, as it allows men who were previously considered severely infertile or sterile the possibility of fatherhood. However, with ICSI sperm are chosen by laboratory technicians and not by nature and because of this, it is not clear what barriers to natural selection are altered. Thus, along with this technology comes the possibility of passing on to a child certain genetic issues that may have caused the fathers infertility, or even more severe conditions. Another reason to know whether male infertility is genetic or not is because classic treatments such asvaricocelerepair or medications given to improve male infertility. In fact, Dr Turek was one of the first to publishonthis issue, showing thatvaricocelerepair was not effective in improving fertility in men with genetic infertility. Because he recognized these issues early on, Dr. Turek, while at UCSF in 1997, founded the first formal genetic counseling and testing program for infertility in the U.S. Called the Program in the Genetics of Infertility (PROGENI), Dr.Tureksprogram has helped over 2000 patients at risk for genetic infertility to navigate the decision-making waters that surround this condition.

Men with infertility should be seen by a urologist for a thorough medical history, physical examination, and appropriate medical testing. If genetic infertility is a possibility, then a genetic counselor can help couples understand the possible reasons, offer appropriate genetic testing, and discuss the complex emotional and medical implications of the test results. The approach taken early on by Dr. Turek is outlined in Figure 1. Just like the medical diagnosis from a urologist or fertility specialist, information about family history plays a critical role in genetic risk assessment. This approach to genetic evaluation, termed non-prescriptive, has been the cornerstone of Dr. Tureks critically acclaimed clinical program that now has over a dozen publications contributing to our current knowledge in the field. It is important to note that a lack of family history of infertility or other medical problems does not eliminate or reduce the risk of genetic infertility. In fact, a family history review will often be unremarkable. However, family history can provide crucial supporting information toward making a genetic diagnosis (such as a family history of recurrent miscarriages or babies born with problems). Dr. Turek has published thathaving a genetic counselor obtainfamily history information is much more accurate than simply giving patients a written questionnaire to fill out and bring to their visit. A genetic counselor can also discuss appropriate genetic testing options and review the test results in patients in a meaningful way.

When speaking to Dr.Tureksgenetic counselor about genetic testing, keep in mind that he or she will not tell you what to do. Genetic counselors are trained to provide information, address questions and concerns, and support you in the decision making process. A genetic counselor does not assume which decisions are most appropriate for you.

Among the various infertility diagnoses that men have, some are more commonly associated with genetic causes. Diagnoses that can have genetic causes include men nonobstructive azoospermia (no sperm count), oligospermia (low sperm count), and congenital absence of the vas deferens. A list of some of the best- described causes of genetic male infertility and their frequencies and associated conditions are listed in Table 1.

Nonobstructiveazoospermiais defined aszero sperm countin the ejaculate due to an underlying sperm production problem within the testicles. This is quite different from obstructive azoospermia in which sperm production within the testes is normal, but there is a blockage in the reproductive tract ducts that prevents the sperm from leaving the body. There can be changes in the levels of reproductive hormones, such as follicle stimulating hormone (FSH), observed withnonobstructiveazoospermia. Most commonly, the FSH is elevated in this condition, which is an appropriate and safe hormone responseofthe pituitary gland to states of low or no sperm production. This diagnosis is associated with a 15%chance forhaving chromosome abnormalities(Figure 2) and a 13% chance for having gene regions missing on the Y chromosome (termed Y chromosome microdeletions, Figure3). To detect these changes, blood tests are typically offered to men with nonobstructive azoospermia.

Oligospermiathat places men at risk for genetic infertility occurs when the ejaculate contains a sperm concentration of <5 million sperm/mLsemen. Similar to nonobstructive azoospermia, this is most commonly due to an underlying sperm production problem. With this diagnosis, there is a 2% risk for chromosome abnormalities and 6-8% risk of Y chromosomemicrodeletions.Ingeneral, the lower the sperm count, the higher the chance that a genetic cause is present. Again the appropriate testing includes akaryotypeand Y chromosome microdeletion analysis. Thus far, there are noestablished guidelines for applying these genetic tests in cases of low spermmotility(movement) or poor sperm morphology (shape).

Congenital absence of the vas deferens is characterized by the malformation or absence of the ducts that allow sperm to pass from the testicles into the ejaculate and out of the body during ejaculation. The duct that is affectedinthis condition is thevasdeferens. This is the sameductthat is treated during a vasectomy, a procedure for men who want birth control. Men with this condition are essentially born with a natural vasectomy. This congenital condition is associated with mutations and/or variations in the genes for cystic fibrosis (the CFTR gene) in 70-80% men if thevasdeferensis absent on both sides, but less than this if the duct is missing on only one side. For most men with this condition with a mutation in the cystic fibrosis gene, the missingvasdeferensis the only problem that results from this genetic change and they do not have the full spectrum of symptoms associated with cystic fibrosis, the most common genetic disease in the U.S.andgenerally lethal in early adulthood.

A less common reason for mento havea zero sperm count (azoospermia) than nonobstructive azoospermia is obstructive azoospermia. In essence, this is an unexplained zero sperm count due to a blockage of the reproductive tract ducts leading from the testicle to the ejaculate. Blockages are most commonly found in theepididymisbut can also be located in thevasdeferensor ejaculatory ducts. Most cases of obstructive azoospermia are amendable to surgical repair and naturally fertility is common. However, a high proportion of these men (47%) have mutations in the cystic fibrosis gene (CFTR) or harbor variations in the CFTR gene, termed 5T alleles. As such, genetic counseling and testing is also important in these patients.

These conditions represent only the most common genetic conditions encountered when evaluating men for genetic infertility. For this reason, consider readingDr.Turekspublished paperthat discusses most of the currently understood syndromes and conditions that are associated with infertility. It is also important to remember that if all genetic test results are normal, there is still a possibility that the infertility has a genetic cause. However, in many cases, medical science is currently unable to offer testing to detect it.

If a man has a chromosome abnormality identified as the cause of infertility, then depending on the chromosome abnormality detected, there may be a higher risk for children to be born with birth defects or mental impairment. This occurs as a result of a child inheriting from the father an imbalance in chromosome material. A genetic counselor can provide more detailed information about such potential risks, and offer other resources for individuals who have been diagnosed with a chromosome abnormality. There may be support organizations available to help men with genetic diagnoses and their partners cope with the impact of this information. Some couples find it helpful to talk to others in similar circumstances.

If a man is diagnosed with a Y chromosome deletion, then he will pass on that Y chromosome deletion to anysonhe conceives. To his daughters, he will pass on his X chromosome, instead of the Y chromosome. It is assumed that any son inheriting a Y chromosome deletion from his father will also have infertility. It is unclear whether the type and severity of the infertility will be different from the fathers. So far, there have only been a few reports of sons born to fathers with Y chromosome deletions after conception by assisted reproduction. As expected, there has not been an increase in the rate of birth defects or other problems for these boys, although this group is still small in number, and too young to have fertility evaluations.

Transmission of CFTR mutations in cases of infertility due to congenital absence of the vasdeferensis somewhat more complex than either Ymicrodeletionsor a chromosome abnormality. This is because there are over 1400 described mutations in the CFTR gene and the impact of mutations differs depending on which one is present. In general, the partner of an affected man should be tested as well, so that the residual risk of a child having either congenital absence of the vas deferens or full-blown cystic fibrosis can be estimated.

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Is Male Infertility Genetic? | Hereditary Fertility Issues ...

The human genome is the complete set of instructions for building a human.

It is made up of 23 pair of chromosomes which each have somewhere between 500 5,000 individual genes, each of which are responsible for a particular trait hair color, eye color, etc. 22 of the chromosome pairs are the same in men and women, but the last set is the sex chromosome. Women have 2 X chromosomes (XX), while Men have an X and a Y (XY).When a baby is conceived the babys DNA comes from both the father and the mother. The egg contains a single set of 23 chromosomes and the sperm contains the other 23. When the sperm fertilizes the egg, the two sets of chromosomes combine to form a full set of DNA for the new little person. Interestingly, the gender of the baby is determined by the man. Sperm can carry either an X or a Y chromosome which will determine whether the baby will be a boy or a girl.

Genetic disorders can be divided into a few different categories. Numerical chromosomal abnormalities occur when a person receives an extra chromosome, which can cause various disorders such as Downs or Turners syndrome. Another thing that can happen is pieces of a gene can get deleted during duplication. Most of the time, large deletions cause severe enough disorders that dont enable an embryo to develop. However, some very small deletions, known as micro-deletions can lead to a variety of congenital defects including infertility. Microdeletions in the Y chromosome have a profound impact on sperm production and is a fairly common cause of infertility in men. Finally, mutations or trans-locations are disorders where parts of a gene form abnormally or get mixed up with other genes causing it to malfunction. This can cause disorders such as cystic fibrosis or sickle cell anemia. Sometimes these mutations can impact fertility.

As mentioned above, the most well-known of the numeric abnormalities is trisomy 21 or Downs Syndrome. This occurs when someone receives an extra copy of the 21st chromosome and happens in about 1 in every 700 births. It is also fairly common to have extra copies of either the X or the Y chromosome. Some women may have a an extra X resulting in a condition known as triple X. Men can receive either an extra X or an extra Y chromosome, resulting in XXY or XYY. Rarely, babies are born that are genetically female with two X chromosomes but present as males.

The most common genetic cause of male infertility is a condition known as Klinefelter Syndrome. About 1 in every 500 boys are born with an extra X chromosome in their genetic makeup XXY. This condition is known as Klinefelter Syndrome, and it has been shown to drastically reduce the mans fertility.

Microdeletions make up another large portion of the genetic causes of male infertility. Most of the time deletions occur on the Y chromosome.

The Y chromosome is by far the smallest chromosome and is primarily responsible for the creation of sperm and the development of tissue in the testicle. Because it is only passed from man to man via sperm, and sperm are continually being made by the body, it has a higher risk of mutating from one generation to the next when compared to the other genes. When a man has Y microdeletions, it is kind of like having a few bad sectors on a hard drive, it really doesnt affect anything EXCEPT trying to make sperm. Its estimated that somewhere around 10% of azoospermic men have these micro-deletions but the number could be bigger as it has been traditionally difficult and expensive to diagnose.

Another genetic disorder caused by microdeletions is Prader-Willi Syndrome. This syndrome is incredibly rare occurring once in every 25,000 births. It is caused by microdeletions on the 15th chromosome and is commonly diagnosed via genetic testing at birth. Like autism, Prader-Willi is a spectral disorder with a range of symptoms from mild to severe. The most common symptoms include hypgonadism, infertility, small hands and feet, and obesity stemming from an uncontrollable appetite.

Scientists estimate that there are around 2,300 genes involved in male reproduction. Each gene has several possible mutations, making it nearly impossible to isolate all genetic causes of infertility. The rise in genetic sequencing and other genetic testing techniques has dramatically increased our understanding of how genes impact our health and fertility. Here are some of the more common genetic mutations that can impact fertility.

Cystic Fibrosis is one of the most well known genetic mutation disorders. It occurs when there is a mutation in the CFTR gene (located on the 7th chromosome). This gene is responsible for helping your body regulate use of salt and over 1,200 mutations of the gene have been identified by scientists. Full blown CF occurs when an individual has two copies of a mutated gene, which happens about once in every 2,500 births. Thanks to modern medicine, CF patients are living longer, healthier lives and many are able to start families of their own. However, a hidden side-effect of CF is that it can often cause a natural vasectomy by preventing the formation of the vas deferens. Skilled urologists specialized in fertility are able to help men with CF become fathers.A lesser known fact about CF is 1 in 25 people carry one copy of the mutated gene. Some mutations specifically impact the formation of the vas deferens, so even men who dont have full blown CF may be genetic carriers with unexplained obstructive azoospermia. A skilled urologist should be able to detect the absence of the vas deferens during a physical evaluation and recommend genetic testing if a CF mutation is suspected.

Mutations in key genes involved with fetal development can prevent the testicles from descending. Other mutations can cause abnormal development of the ducts that connect the testicles to the body. Since sperm production is regulated by hormones, genetic problems with the endocrine system may also create conditions that are unfavorable for or preclude sperm production. Some of the known syndromes stemming from genetic mutations that affect male fertility include: Noonan Syndrome, Androgen Insensitivity Syndrome, Kallman Syndrome, Myotonic Dystrophy, and Kartageners Syndrome.

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Genetic Causes of Male Infertility | Common male fertility ...

The development of in vitro fertilitzation (IVF) has allowed many couples to have the families they might otherwise have been unable to create independently. At the same time, this technology has allowed researchers to study the genetic make-up of the earliest stages of embryos. These advances are providing insights into the link between genetics and infertility and how defects (mutations) in specific genes may result in male or female infertility. It is possible that many cases of unexplained infertility will one day be found to have a clear genetic basis.

What has been learned in the last two decades of assisted reproduction is that some cases of severe male factor infertility are clearly related to gene deletions, mutations or chromosomal abnormalities.

Some men with very severe male factor infertility will be found, upon testing their blood chromosomes (known as a "karyotype") to have an extra X chromosome. That is, instead of having a 46 XY karyotype, they have a 47 XXY karyotype. This condition is known as "Klinefelter Syndrome" and can result in failure to achieve puberty or even when puberty is achieved, these men often have male infertility. Some men with Klinefelter Syndrome can father pregnancies through the use of in vitro fertilitzation (IVF) with Intra-Cytoplasmic Sperm injection (ICSI). So far, we are not seeing an increased risk of Klinefelter Syndrome or other chromosome abnormalities in the offspring achieved in these cases.

Also discovered in recent years is that some men with very severe low sperm counts will be found to have deletions in a certain part of their Y chromosome, known as the DAZ gene. Their karyotype is normal (46 XY) but close inspection of the Y chromosome shows there are sections of the chromosome that are missing. A portion of these men will have no recoverable sperm in the ejaculate or on testicular surgery and donor sperm is the only option. With other deletions in the DAZ gene, there is a small amount of sperm present and conception with IVF-ICSI is possible. In these cases, the male offspring which will always inherit their father's Y chromosome, will also have this deletion, and will themselves be infertile.

A single gene mutation in the gene for Cystic Fibrosis (CF) is associated with absence of the part of the tube (the "vas deferens") that leads from the testicle to the urethra in the penis. These men are usually carriers for the CF gene mutation and do not themselves have the disease of Cystic Fibrosis. Sperm can be recovered from the testicles in these men to be used for IVF with ICSI but it is imperative that their wife (or egg provider) be fully tested for CF mutations as well, otherwise there is significant risk of having a child with Cystic Fibrosis.

For men with sperm counts routinely in the less than 5 million total motile sperm range, testing for genetic conditions is warranted so that these men or couples can be made aware of the genetic issues and how these issues might affect their offspring.

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Genetics and Male Infertility - pacificfertilitycenter.com

The relationship between biology and sexual orientation is a subject of research. While scientists do not know the exact cause of sexual orientation, they theorize that a combination of genetic, hormonal, and social factors determine it.[1][2][3] Hypotheses for the impact of the post-natal social environment on sexual orientation, however, are weak, especially for males.[4]

Biological theories for explaining the causes of sexual orientation are favored by scientists[1] and involve a complex interplay of genetic factors, the early uterine environment and brain structure.[5] These factors, which may be related to the development of a heterosexual, homosexual, bisexual, or asexual orientation, include genes, prenatal hormones, and brain structure.

A number of twin studies have attempted to compare the relative importance of genetics and environment in the determination of sexual orientation. In a 1991 study, Bailey and Pillard conducted a study of male twins recruited from "homophile publications", and found that 52% of monozygotic (MZ) brothers (of whom 59 were questioned) and 22% of the dizygotic (DZ) twins were concordant for homosexuality.[6] 'MZ' indicates identical twins with the same sets of genes and 'DZ' indicates fraternal twins where genes are mixed to an extent similar to that of non-twin siblings. In a study of 61 pairs of twins, researchers found among their mostly male subjects a concordance rate for homosexuality of 66% among monozygotic twins and a 30% one among dizygotic twins.[7] In 2000 Bailey, Dunne and Martin studied a larger sample of 4,901 Australian twins but reported less than half the level of concordance.[8] They found 20% concordance in the male identical or MZ twins and 24% concordance for the female identical or MZ twins. Self reported zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt. Other researchers support biological causes for both men and women's sexual orientation.[9]

Bearman and Brckner (2002) criticized early studies concentrating on small, select samples[10] and non-representative selection of their subjects.[11] They studied 289 pairs of identical twins (monozygotic, or from one fertilized egg) and 495 pairs of fraternal twins (dizygotic, or from two fertilized eggs) and found concordance rates for same-sex attraction of only 7.7% for male identical twins and 5.3% for females, a pattern which they say "does not suggest genetic influence independent of social context".[10]

A 2010 study of all adult twins in Sweden (more than 7,600 twins)[12] found that same-sex behavior was explained by both heritable factors and individual-specific environmental sources (such as prenatal environment, experience with illness and trauma, as well as peer groups, and sexual experiences), while influences of shared-environment variables such as familial environment and social attitudes had a weaker, but significant effect. Women showed a statistically non-significant trend to weaker influence of hereditary effects, while men showed no effect of shared environmental effects. The use of all adult twins in Sweden was designed to address the criticism of volunteer studies, in which a potential bias towards participation by gay twins may influence the results;

Biometric modeling revealed that, in men, genetic effects explained .34.39 of the variance [of sexual orientation], the shared environment .00, and the individual-specific environment .61.66 of the variance. Corresponding estimates among women were .18.19 for genetic factors, .16.17 for shared environmental, and .64.66 for unique environmental factors. Although wide confidence intervals suggest cautious interpretation, the results are consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior.[12]

Twin studies have received a number of criticisms including self-selection bias where homosexuals with gay siblings are more likely to volunteer for studies. Nonetheless, it is possible to conclude that, given the difference in sexuality in so many sets of identical twins, sexual orientation cannot be attributed solely to genetic factors.[13]

Another issue is the finding that even monozygotic twins can be different and there is a mechanism which might account for monozygotic twins being discordant for homosexuality. Gringas and Chen (2001) describe a number of mechanisms which can lead to differences between monozygotic twins, the most relevant here being chorionicity and amniocity.[14] Dichorionic twins potentially have different hormonal environments because they receive maternal blood from separate placenta, and this could result in different levels of brain masculinisation. Monoamniotic twins share a hormonal environment, but can suffer from the 'twin to twin transfusion syndrome' in which one twin is "relatively stuffed with blood and the other exsanguinated".[15]

Chromosome linkage studies of sexual orientation have indicated the presence of multiple contributing genetic factors throughout the genome. In 1993 Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families.[16] Hamer et al. found that the gay men had more gay male uncles and cousins on the maternal side of the family than on the paternal side. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles. In another finding, thirty-three of the forty sibling pairs tested were found to have similar alleles in the distal region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. This was popularly dubbed the "gay gene" in the media, causing significant controversy. Sanders et al. in 1998 reported on their similar study, in which they found that 13% of uncles of gay brothers on the maternal side were homosexual, compared with 6% on the paternal side.[17]

A later analysis by Hu et al. replicated and refined the earlier findings. This study revealed that 67% of gay brothers in a new saturated sample shared a marker on the X chromosome at Xq28.[18] Two other studies (Bailey et al., 1999; McKnight and Malcolm, 2000) failed to find a preponderance of gay relatives in the maternal line of homosexual men.[17] One study by Rice et al. in 1999 failed to replicate the Xq28 linkage results.[19] Meta-analysis of all available linkage data indicates a significant link to Xq28, but also indicates that additional genes must be present to account for the full heritability of sexual orientation.[20]

Mustanski et al. (2005) performed a full-genome scan (instead of just an X chromosome scan) on individuals and families previously reported on in Hamer et al. (1993) and Hu et al. (1995), as well as additional new subjects. In the full sample they did not find linkage to Xq28.[21]

Results from the first large, comprehensive multi-center genetic linkage study of male sexual orientation were reported by an independent group of researchers at the American Society of Human Genetics in 2012.[22] The study population included 409 independent pairs of gay brothers, who were analyzed with over 300,000 single-nucleotide polymorphism markers. The data strongly replicated Hamer's Xq28 findings as determined by both two-point and multipoint (MERLIN) LOD score mapping. Significant linkage was also detected in the pericentromeric region of chromosome 8, overlapping with one of the regions detected in the Hamer lab's previous genomewide study. The authors concluded that "our findings, taken in context with previous work, suggest that genetic variation in each of these regions contributes to development of the important psychological trait of male sexual orientation". Female sexual orientation does not seem to be linked to Xq28,[18][23] though it does appear moderately heritable.[24]

In addition to sex chromosomal contribution, a potential autosomal genetic contribution to the development of homosexual orientation has also been suggested. In a study population composed of more than 7000 participants, Ellis et al. (2008) found a statistically significant difference in the frequency of blood type A between homosexuals and heterosexuals. They also found that "unusually high" proportions of homosexual males and homosexual females were Rh negative in comparison to heterosexuals. As both blood type and Rh factor are genetically inherited traits controlled by alleles located on chromosome 9 and chromosome 1 respectively, the study indicates a potential link between genes on autosomes and homosexuality.[25][26]

The biology of sexual orientation has been studied in detail in several animal model systems. In the common fruit fly Drosophila melanogaster, the complete pathway of sexual differentiation of the brain and the behaviors it controls is well established in both males and females, providing a concise model of biologically controlled courtship.[27] In mammals, a group of geneticists at the Korea Advanced Institute of Science and Technology bred a female mice specifically lacking a particular gene related to sexual behavior. Without the gene, the mice exhibited masculine sexual behavior and attraction toward urine of other female mice. Those mice who retained the gene fucose mutarotase (FucM) were attracted to male mice.[28]

In interviews to the press, researchers have pointed that the evidence of genetic influences should not be equated with genetic determinism. According to Dean Hamer and Michael Bailey, genetic aspects are only one of the multiple causes of homosexuality.[29][30]

In 2017, Nature published an article with a genome wide association study on male sexual orientation. The research consisted of 1,077 homosexual men and 1,231 heterosexual men. A gene named SLITRK6 on chromosome 13 was identified.[31] The research supports another study which had been done by Simon LeVay. LeVay's research suggested that the hypothalamus of gay men is different from straight men.[32] The SLITRK6 is active in the mid-brain where the hypothalamus is. The researchers found another gene, named "thyroid stimulating hormone receptor" (TSHR) on chromosome 14 which dna sequence is different also for gay men.[31] TSHR stimulates thyroid and grave disease interrupted the function of TSHR. The previous research also indicated that grave disease had been seen more in gay men than in straight men.[33] Research indicated that gay people have lower body weight than straight people. It had been presumed that the overactive TSHR hormone lowered body weight in gay people.[34][35]

In 2018, Ganna et al. performed another genome wide association study on sexual orientation of men and women with data from 26,890 people who had at least one same-sex partner and 450,939 controls. The data in the study was meta-analyzed and obtained from the UK Biobank study and 23andMe. The researchers identified four variants more common in people who reported at least one same-sex experience on chromosomes 7, 11, 12, and 15. The variants on chromosomes 11 and 15 were specific to men, with the variant on chromosome 11 located in an olfactory gene and the variant on chromosome 15 having previously been linked to male-pattern baldness. The four variants were also correlated with mood and mental health disorders; major depressive disorder and schizophrenia in men and women, and bipolar disorder in women. However, none of the four variants could reliably predict sexual orientation.[36]

A study suggests linkage between a mother's genetic make-up and homosexuality of her sons. Women have two X chromosomes, one of which is "switched off". The inactivation of the X chromosome occurs randomly throughout the embryo, resulting in cells that are mosaic with respect to which chromosome is active. In some cases though, it appears that this switching off can occur in a non-random fashion. Bocklandt et al. (2006) reported that, in mothers of homosexual men, the number of women with extreme skewing of X chromosome inactivation is significantly higher than in mothers without gay sons. 13% of mothers with one gay son, and 23% of mothers with two gay sons, showed extreme skewing, compared to 4% of mothers without gay sons.[37]

Blanchard and Klassen (1997) reported that each additional older brother increases the odds of a man being gay by 33%.[38][39] This is now "one of the most reliable epidemiological variables ever identified in the study of sexual orientation".[40] To explain this finding, it has been proposed that male fetuses provoke a maternal immune reaction that becomes stronger with each successive male fetus.This maternal immunization hypothesis (MIH) begins when cells from a male fetus enter the mother's circulation during pregnancy or while giving birth.[41]Male fetuses produce H-Y antigens which are "almost certainly involved in the sexual differentiation of vertebrates".These Y-linked proteins would not be recognized in the mother's immune system because she is female, causing her to develop antibodies which would travel through the placental barrier into the fetal compartment. From here, the anti-male bodies would then cross the blood/brain barrier (BBB) of the developing fetal brain, altering sex-dimorphic brain structures relative to sexual orientation, increasing the likelihood that the exposed son will be more attracted to men than women.[41]It is this antigen which maternal H-Y antibodies are proposed to both react to and 'remember'. Successive male fetuses are then attacked by H-Y antibodies which somehow decrease the ability of H-Y antigens to perform their usual function in brain masculinisation.[38]

However, the maternal immune hypothesis has been criticized because the prevalence of the type of immune attack proposed is rare compared with the prevalence of homosexuality.[42]

The "fraternal birth order effect" however, cannot account for between 71-85% of male homosexual preference.[43] Additionally, it does not explain instances where a firstborn child displays male homosexual preference (MHP).[44]

In 2017, researchers discovered a biological mechanism of gay people who tend to have older brothers. They think Neuroligin 4 Y-linked protein is responsible for a later son being gay. They found that women had significantly higher anti-NLGN4Y levels than men. The result also indicates that number of pregnancies, mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons.[45]

In 2004, Italian researchers conducted a study of about 4,600 people who were the relatives of 98 homosexual and 100 heterosexual men. Female relatives of the homosexual men tended to have more offspring than those of the heterosexual men. Female relatives of the homosexual men on their mother's side tended to have more offspring than those on the father's side. The researchers concluded that there was genetic material being passed down on the X chromosome which both promotes fertility in the mother and homosexuality in her male offspring. The connections discovered would explain about 20% of the cases studied, indicating that this is a highly significant but not the sole genetic factor determining sexual orientation.[46][47]

Research conducted in Sweden[48] has suggested that gay and straight men respond differently to two odors that are believed to be involved in sexual arousal. The research showed that when both heterosexual women and gay men are exposed to a testosterone derivative found in men's sweat, a region in the hypothalamus is activated. Heterosexual men, on the other hand, have a similar response to an estrogen-like compound found in women's urine.[49] The conclusion is that sexual attraction, whether same-sex or opposite-sex oriented, operates similarly on a biological level. Researchers have suggested that this possibility could be further explored by studying young subjects to see if similar responses in the hypothalamus are found and then correlating these data with adult sexual orientation.[citation needed]

A number of sections of the brain have been reported to be sexually dimorphic; that is, they vary between men and women. There have also been reports of variations in brain structure corresponding to sexual orientation. In 1990, Dick Swaab and Michel A. Hofman reported a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men.[50] In 1992, Allen and Gorski reported a difference related to sexual orientation in the size of the anterior commissure,[51] but this research was refuted by numerous studies, one of which found that the entirety of the variation was caused by a single outlier.[52][53][54]

Research on the physiologic differences between male and female brains are based on the idea that people have male or a female brain, and this mirrors the behavioral differences between the two sexes. Some researchers state that solid scientific support for this is lacking. Although consistent differences have been identified, including the size of the brain and of specific brain regions, male and female brains are very similar.[55][56]

Simon LeVay, too, conducted some of these early researches. He studied four groups of neurons in the hypothalamus called INAH1, INAH2, INAH3 and INAH4. This was a relevant area of the brain to study, because of evidence that it played a role in the regulation of sexual behaviour in animals, and because INAH2 and INAH3 had previously been reported to differ in size between men and women.[57]

He obtained brains from 41 deceased hospital patients. The subjects were classified into three groups. The first group comprised 19 gay men who had died of AIDS-related illnesses. The second group comprised 16 men whose sexual orientation was unknown, but whom the researchers presumed to be heterosexual. Six of these men had died of AIDS-related illnesses. The third group was of six women whom the researchers presumed to be heterosexual. One of the women had died of an AIDS-related illness.[57]

The HIV-positive people in the presumably heterosexual patient groups were all identified from medical records as either intravenous drug abusers or recipients of blood transfusions. Two of the men who identified as heterosexual specifically denied ever engaging in a homosexual sex act. The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been primarily or exclusively heterosexual "on the basis of the numerical preponderance of heterosexual men in the population".[57]

LeVay found no evidence for a difference between the groups in the size of INAH1, INAH2 or INAH4. However, the INAH3 group appeared to be twice as big in the heterosexual male group as in the gay male group; the difference was highly significant, and remained significant when only the six AIDS patients were included in the heterosexual group. The size of INAH3 in the homosexual men's brains was comparable to the size of INAH3 in the heterosexual women's brains.

However, other studies have shown that the sexually dimorphic nucleus of the preoptic area, which include the INAH3, are of similar size in homosexual males who died of AIDS to heterosexual males, and therefore larger than female. This clearly contradicts the hypothesis that homosexual males have a female hypothalamus. Furthermore, the SCN of homosexual males is extremely large (both the volume and the number of neurons are twice as many as in heterosexual males). These areas of the hypothalamus have not yet been explored in homosexual females nor bisexual males nor females. Although the functional implications of such findings still haven't been examined in detail, they cast serious doubt over the widely accepted Drner hypothesis that homosexual males have a "female hypothalamus" and that the key mechanism of differentiating the "male brain from originally female brain" is the epigenetic influence of testosterone during prenatal development.[58][59]

William Byne and colleagues attempted to identify the size differences reported in INAH 14 by replicating the experiment using brain sample from other subjects: 14 HIV-positive homosexual males, 34 presumed heterosexual males (10 HIV-positive), and 34 presumed heterosexual females (9 HIV-positive). The researchers found a significant difference in INAH3 size between heterosexual men and heterosexual women. The INAH3 size of the homosexual men was apparently smaller than that of the heterosexual men, and larger than that of the heterosexual women, though neither difference quite reached statistical significance.[60]

Byne and colleagues also weighed and counted numbers of neurons in INAH3 tests not carried out by LeVay. The results for INAH3 weight were similar to those for INAH3 size; that is, the INAH3 weight for the heterosexual male brains was significantly larger than for the heterosexual female brains, while the results for the gay male group were between those of the other two groups but not quite significantly different from either. The neuron count also found a male-female difference in INAH3, but found no trend related to sexual orientation.[60]

A 2010 study, Garcia-Falgueras and Swaab asserted that "the fetal brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed or organized into our brain structures when we are still in the womb. There is no indication that social environment after birth has an effect on gender identity or sexual orientation."[61]

The domestic ram is used as an experimental model to study early programming of the neural mechanisms which underlie homosexuality, developing from the observation that approximately 8% of domestic rams are sexually attracted to other rams (male-oriented) when compared to the majority of rams which are female-oriented. In many species, a prominent feature of sexual differentiation is the presence of a sexually dimorphic nucleus (SDN) in the preoptic hypothalamus, which is larger in males than in females.

Roselli et al. discovered an ovine SDN (oSDN) in the preoptic hypothalamus that is smaller in male-oriented rams than in female-oriented rams, but similar in size to the oSDN of females. Neurons of the oSDN show aromatase expression which is also smaller in male-oriented rams versus female-oriented rams, suggesting that sexual orientation is neurologically hard-wired and may be influenced by hormones. However, results failed to associate the role of neural aromatase in the sexual differentiation of brain and behavior in the sheep, due to the lack of defeminization of adult sexual partner preference or oSDN volume as a result of aromatase activity in the brain of the fetuses during the critical period. Having said this, it is more likely that oSDN morphology and homosexuality may be programmed through an androgen receptor that does not involve aromatisation. Most of the data suggests that homosexual rams, like female-oriented rams, are masculinized and defeminized with respect to mounting, receptivity, and gonadotrophin secretion, but are not defeminized for sexual partner preferences, also suggesting that such behaviors may be programmed differently. Although the exact function of the oSDN is not fully known, its volume, length, and cell number seem to correlate with sexual orientation, and a dimorphism in its volume and of cells could bias the processing cues involved in partner selection. More research is needed in order to understand the requirements and timing of the development of the oSDN and how prenatal programming effects the expression of mate choice in adulthood.[62]

The early fixation hypothesis includes research into prenatal development and the environmental factors that control masculinization of the brain. Some studies have seen pre-natal hormone exposures as the primary factor involved in determining sexual orientation.[63][64][65] This hypothesis is supported by both the observed differences in brain structure and cognitive processing between homosexual and heterosexual men. One explanation for these differences is the idea that differential exposure to hormone levels in the womb during fetal development may change the masculinization of the brain in homosexual men. The concentrations of these chemicals is thought to be influenced by fetal and maternal immune systems, maternal consumption of certain drugs, maternal stress, and direct injection. This hypothesis is connected to the well-measured effect of fraternal birth order on sexual orientation.

Daryl Bem, a social psychologist at Cornell University, has theorized that the influence of biological factors on sexual orientation may be mediated by experiences in childhood. A child's temperament predisposes the child to prefer certain activities over others. Because of their temperament, which is influenced by biological variables such as genetic factors, some children will be attracted to activities that are commonly enjoyed by other children of the same gender. Others will prefer activities that are typical of another gender. This will make a gender-conforming child feel different from opposite-gender children, while gender-nonconforming children will feel different from children of their own gender. According to Bem, this feeling of difference will evoke psychological arousal when the child is near members of the gender which it considers as being 'different'. Bem theorizes that this psychological arousal will later be transformed into sexual arousal: children will become sexually attracted to the gender which they see as different ("exotic"). This proposal is known as the "exotic becomes erotic" theory.[66]

Bem sought support from published literature but did not present new data testing his theory.[67] Research cited by him as evidence of the "exotic becomes erotic" theory includes the study Sexual Preference by Bell et al. (1981)[67] and studies showing the frequent finding that a majority of gay men and lesbians report being gender-nonconforming during their childhood years. A meta-analysis of 48 studies showed childhood gender nonconformity to be the strongest predictor of a homosexual orientation for both men and women.[68] In six "prospective" studiesthat is, longitudinal studies that began with gender-nonconforming boys at about age 7 and followed them up into adolescence and adulthood 63% of the gender nonconforming boys had a homosexual or bisexual orientation as adults.[69]

Sexual practices that significantly reduce the frequency of heterosexual intercourse also significantly decrease the chances of successful reproduction, and for this reason, they would appear to be maladaptive in an evolutionary context following a simple Darwinian model (competition amongst individuals) of natural selectionon the assumption that homosexuality would reduce this frequency. Several theories have been advanced to explain this contradiction, and new experimental evidence has demonstrated their feasibility.[70]

Some scholars[70] have suggested that homosexuality is indirectly adaptive, by conferring a reproductive advantage in a non-obvious way on heterosexual siblings or their children. By way of analogy, the allele (a particular version of a gene) which causes sickle-cell anemia when two copies are present, also confers resistance to malaria with a lesser form of anemia when one copy is present (this is called heterozygous advantage).[71]

Scholars have also pointed out that Darwin himself described kin selection in The Origin of Species, so under a Darwinian model of evolution, not only individuals, but family groups (bloodlines) can compete for selection.

Brendan Zietsch of the Queensland Institute of Medical Research proposes the alternative theory that men exhibiting female traits become more attractive to females and are thus more likely to mate, provided the genes involved do not drive them to complete rejection of heterosexuality.[72]

In a 2008 study, its authors stated that "There is considerable evidence that human sexual orientation is genetically influenced, so it is not known how homosexuality, which tends to lower reproductive success, is maintained in the population at a relatively high frequency." They hypothesized that "while genes predisposing to homosexuality reduce homosexuals' reproductive success, they may confer some advantage in heterosexuals who carry them". Their results suggested that "genes predisposing to homosexuality may confer a mating advantage in heterosexuals, which could help explain the evolution and maintenance of homosexuality in the population".[73]

However, in the same study, the authors noted that "nongenetic alternative explanations cannot be ruled out" as a reason for the heterosexual in the homosexual-heterosexual twin pair having more partners, specifically citing "social pressure on the other twin to act in a more heterosexual way" (and thus seek out a greater number of sexual partners) as an example of one alternative explanation. Also, the authors of the study acknowledge that a large number of sexual partners may not lead to greater reproductive success, specifically noting there is an "absence of evidence relating the number of sexual partners and actual reproductive success, either in the present or in our evolutionary past".

The heterosexual advantage hypothesis was given strong support by the 2004 Italian study demonstrating increased fecundity in the female matrilineal relatives of gay men.[46][47] As originally pointed out by Hamer,[74] even a modest increase in reproductive capacity in females carrying a "gay gene" could easily account for its maintenance at high levels in the population.[47]

The "gay uncle hypothesis" posits that people who themselves do not have children may nonetheless increase the prevalence of their family's genes in future generations by providing resources (e.g., food, supervision, defense, shelter) to the offspring of their closest relatives.

This hypothesis is an extension of the theory of kin selection, which was originally developed to explain apparent altruistic acts which seemed to be maladaptive. The initial concept was suggested by J. B. S. Haldane in 1932 and later elaborated by many others including John Maynard Smith, W. D. Hamilton and Mary Jane West-Eberhard.[75] This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive.

Vasey and VanderLaan (2010) tested the theory on the Pacific island of Samoa, where they studied women, straight men, and the fa'afafine, men who prefer other men as sexual partners and are accepted within the culture as a distinct third gender category. Vasey and VanderLaan found that the fa'afafine said they were significantly more willing to help kin, yet much less interested in helping children who aren't family, providing the first evidence to support the kin selection hypothesis.[76][77]

The hypothesis is consistent with other studies on homosexuality, which show that it is more prevalent amongst both siblings and twins.[76][77][78][bettersourceneeded] Since both twins and non-twin siblings share genes and therefore have a higher factor of genetic redundancy, there is less genetic familial risk if the strategy is expressed. It is speculated that environmental and hormonal stress factors (linked to resource feedbacks) may act as triggers.

Since the hypothesis solves the problem of why homosexuality has not been selected out over thousands of years, despite it being antithetical to reproduction, many scientists consider it the best explanatory model for non-heterosexual behaviour such as homosexuality and bisexuality. The natural bell curve variation that occurs in biology and sociology everywhere, explains the variable spectrum of expression.

Vasal and VanderLaan (2011) provides evidence that if an adaptively designed avuncular male androphilic phenotype exists and its development is contingent on a particular social environment, then a collectivistic cultural context is insufficient, in and of itself, for the expression of such a phenotype.[79]

Some studies have found correlations between physiology of people and their sexuality; these studies provide evidence which suggests that:

Whether genetic or other physiological determinants form the basis of sexual orientation is a highly politicized issue. The Advocate, a U.S. gay and lesbian newsmagazine, reported in 1996 that 61% of its readers believed that "it would mostly help gay and lesbian rights if homosexuality were found to be biologically determined".[106] A cross-national study in the United States, the Philippines, and Sweden found that those who believed that "homosexuals are born that way" held significantly more positive attitudes toward homosexuality than those who believed that "homosexuals choose to be that way" or "learn to be that way".[107][108]

Equal protection analysis in U.S. law determines when government requirements create a suspect classification" of groups and therefore eligible for heightened scrutiny based on several factors, one of which is immutability.

Evidence that sexual orientation is biologically determined (and therefore perhaps immutable in the legal sense) would strengthen the legal case for heightened scrutiny of laws discriminating on that basis.[109][110][111]

The perceived causes of sexual orientation have a significant bearing on the status of sexual minorities in the eyes of social conservatives. The Family Research Council, a conservative Christian think tank in Washington, D.C., argues in the book Getting It Straight that finding people are born gay "would advance the idea that sexual orientation is an innate characteristic, like race; that homosexuals, like African-Americans, should be legally protected against 'discrimination;' and that disapproval of homosexuality should be as socially stigmatized as racism. However, it is not true." On the other hand, some social conservatives such as Reverend Robert Schenck have argued that people can accept any scientific evidence while still morally opposing homosexuality.[112] National Organization for Marriage board member and fiction writer Orson Scott Card has supported biological research on homosexuality, writing that "our scientific efforts in regard to homosexuality should be to identify genetic and uterine causes... so that the incidence of this dysfunction can be minimized.... [However, this should not be seen] as an attack on homosexuals, a desire to 'commit genocide' against the homosexual community.... There is no 'cure' for homosexuality because it is not a disease. There are, however, different ways of living with homosexual desires."[113]

Some advocates for the rights of sexual minorities resist linking that cause with the concept that sexuality is biologically determined or fixed at birth. They argue that sexual orientation can shift over the course of a person's life.[114] At the same time, others resist any attempts to pathologise or medicalise 'deviant' sexuality, and choose to fight for acceptance in a moral or social realm.[112] Chandler Burr has stated that "[s]ome, recalling earlier psychiatric "treatments" for homosexuality, discern in the biological quest the seeds of genocide. They conjure up the specter of the surgical or chemical "rewiring" of gay people, or of abortions of fetal homosexuals who have been hunted down in the womb."[115] LeVay has said in response to letters from gays and lesbians making such criticisms that the research "has contributed to the status of gay people in society".[112]

See the article here:
Biology and sexual orientation - Wikipedia

Please don't bother my friend!

I reckon that you are quiet young, so the thought of going bald scares you.

I had that too when I was in my early 20s, because baldness is a common thing too in my family.

I'm almost 40 now and YES I'm almost bald too LOL! But in reality going bald is a very slow process. Nobody (some rare cases excepted) is completely bald in their early 20s.

You see this is what I'm trying to say: -When you are 20 you don't wanna look like a bald old man (and that's not gonna happen I promise)

BUT: -When you are 40+ you don't wanna look like a 20 year old! (Although the media wants to make us believe that "young" is the way to be)

So when you reach the age of 40 you won't bother about a little or more baldness because all of of your male generation members have the same "problem" (which isn't a problem)

Since in prehistory man was hairy like an ape and now we are allmost hairless I think the ability of loosing hair is a step ahead in evolution! And I feel that being a little or more bald is very masculine!

View post:
male pattern baldness and genetics? | Yahoo Answers

February 6, 2019

When researchers say they have sequenced the human genome, there is a caveat to this statement: a lot of the human genome is sequenced and assembled, but there are regions that are full of repetitive elements, making them difficult to map. One piece that is notoriously difficult to sequence is the Y chromosome.

Now, researchers from the University of Rochester have found a way to sequence a large portion of the Y chromosome in the fruit fly Drosophila melanogasterthe most that the Y chromosome has been assembled in fruit flies. The research, published in the journal GENETICS, provides new insights into the processes that shape the Y chromosome, and adds to the evidence that, far from a genetic wasteland, Y chromosomes are highly dynamic and have mechanisms to acquire and maintain genes, says Amanda Larracuente, an assistant professor of biology at Rochester.

Y chromosomes are sex chromosomes in males that are transmitted from father to son; they can be important for male fertility and sex determination in many species. Even though fruit fly and mammalian Y chromosomes have different evolutionary origins, they have parallel genome structures, says Larracuente, who co-authored the paper with her PhD student Ching-Ho Chang. Drosophila melanogaster is a premier model organism for genetics and genomics, and has perhaps the best genome assembly of any animal. Despite these resources, we know very little about the organization of the Drosophila Y chromosome because most of it is missing from the genome assembly.

Thats in part because most Y chromosomes do not undergo standard recombination. Typically, genes from the mother and father are shuffledor, cross overto produce a genetic combination unique to each offspring. But the Y chromosome does not undergo crossing over, and, as a result, its genes tend to degenerate, while repetitive DNA sequences accumulate.

Each chromosome is made up of DNA. When mapping a genome, traditional sequencing methods chop up strands of DNA and reador sequencethem, then try to infer the order of those sequences and assemble them back together.

But, there is a difference between sequencing a genome and assembling a genome, Larracuente says. There are so many repetitive strands on the Y chromosome that the pieces tend to look the same. It is difficult, therefore, to figure out where they come from and how to reassemble the strandslike trying to put together a puzzle when all of the pieces are exactly the same color. When we try to take those bits of DNA and assemble them to see what the chromosome looks like, we cant fill in some of those gaps. We might have the sequence, but we dont know where it goes.

Using sequence data generated by new technology that reads long strands of individual DNA molecules, Chang and Larracuente developed a strategy to assemble a large part of the Y chromosome and other repeat-dense regions. By assembling a large portion of the Y chromosome, they discovered that the Y chromosome has a lot of duplicated sequences, where genes are present in multiple copies. They also discovered that although the Y chromosome does not experience crossing over, it undergoes a different type of recombination called gene conversion. While crossing over involves the shuffle and exchange of genes between two different chromosomes, gene conversion is not reciprocal, Larracuente says. You dont have two chromosomes that exchange material, you have one chromosome that donates its sequence to the other part of the chromosome and the sequences become identical.

The Y chromosome has therefore found a way to maintain its genes via a process different from crossing over, Larracuente says. We usually think of the Y chromosome as a really harsh environment for a gene to survive in, yet these genes manage to get expressed and carry out their functions that are important for male fertility. This rampant gene conversion that were seeing is one way that we think genes might be able to survive on Y chromosomes.

Tags: Amanda Larracuente, Arts and Sciences, Department of Biology, genetics, research finding

Category: Science & Technology

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Male Y chromosomes not 'genetic wastelands' : NewsCenter

Male pattern baldness, also called androgenic alopecia, is the most common type of hair loss in men. According to the U.S. National Library of Medicine (NLM), more than 50 percent of all men over the age of 50 will be affected by male pattern baldness to some extent.

One cause of male pattern baldness is genetics, or having a family history of baldness. Research has found that male pattern baldness is associated with male sex hormones called androgens. The androgens have many functions, including regulating hair growth.

Each hair on your head has a growth cycle. With male pattern baldness, this growth cycle begins to weaken and the hair follicle shrinks, producing shorter and finer strands of hair. Eventually, the growth cycle for each hair ends and no new hair grows in its place.

Inherited male pattern baldness usually has no side effects. However, sometimes baldness has more serious causes, such as certain cancers, medications, thyroid conditions, and anabolic steroids. See your doctor if hair loss occurs after taking new medications or when its accompanied by other health complaints.

Doctors use the pattern of hair loss to diagnose male pattern baldness. They may perform a medical history and exam to rule out certain health conditions as the cause, such as fungal conditions of the scalp or nutritional disorders.

Health conditions may be a cause of baldness when a rash, redness, pain, peeling of the scalp, hair breakage, patchy hair loss, or an unusual pattern of hair loss accompanies the hair loss. A skin biopsy and blood tests also may be necessary to diagnose disorders responsible for the hair loss.

Male pattern baldness can begin in your teenage years, but it more commonly occurs in adult men, with the likelihood increasing with age. Genetics plays a big role. Men who have close relatives with male pattern baldness are at a higher risk. This is particularly true when their relatives are on the maternal side of the family.

If your hair loss begins at the temples or the crown of the head, you may have male pattern baldness. Some men will get a single bald spot. Others experience their hairlines receding to form an M shape. In some men, the hairline will continue to recede until all or most of the hair is gone.

Medical treatment isnt necessary if other health conditions arent a cause. However, treatments are available for men who are unhappy with the way they look and would like the appearance of a fuller head of hair.

Men with limited hair loss can sometimes hide hair loss with the right haircut or hairstyle. Ask your hairstylist for a creative cut that will make thinning hair look fuller.

Wigs can cover thinning hair, receding hairlines, and complete baldness. They come in a variety of styles, colors, and textures. For a natural look, choose wig colors, styles, and textures that look similar to your original hair. Professional wig stylists can help style and fit wigs for an even more natural look.

Hair weaves are wigs that are sewn into your natural hair. You must have enough hair to sew the weave into. The advantage to weaves is they always stay on, even during activities such as swimming, showering, and sleeping. The disadvantages are they must be sewn again whenever new hair growth occurs, and the sewing process can damage your natural hair.

Minoxidil (Rogaine) is a topical medication applied to the scalp. Minoxidil slows hair loss for some men and stimulates the hair follicles to grow new hair. Minoxidil takes four months to one year to produce visible results. Hair loss often happens again when you stop taking the medication.

Possible side effects associated with minoxidil include dryness, irritation, burning, and scaling of the scalp. You should visit the doctor immediately if you have any of these serious side effects:

Finasteride (Propecia, Proscar) is an oral medication that slows hair loss in some men. It works by blocking the production of the male hormone responsible for hair loss. Finasteride has a higher success rate than minoxidil. When you stop taking finasteride, your hair loss returns.

You must take finasteride for three months to one year before you see results. If no hair growth occurs after one year, your doctor will likely recommend that you stop taking the medication. The side effects of finasteride include:

Although its rare, finasteride can cause breast cancer. You should have any breast pain or lumps evaluated by a doctor immediately.

Finasteride may affect prostate-specific antigen (PSA) tests used to screen for prostate cancer. The medication lowers PSA levels, which causes lower-than-normal readings. Any rise in PSA levels when taking finasteride should be evaluated for prostate cancer.

A hair transplant is the most invasive and expensive treatment for hair loss. Hair transplants work by removing hair from areas of the scalp that have active hair growth and transplanting them to thinning or balding areas of your scalp.

Multiple treatments are often necessary, and the procedure carries the risk of scarring and infection. The advantages of a hair transplant are that it looks more natural and its permanent.

Going bald can be a big change. You may have trouble accepting your appearance. You should seek counseling if you experience anxiety, low self-esteem, depression, or other emotional problems because of male pattern baldness.

Theres no known way to prevent male pattern baldness. A theory is that stress may cause hair loss by increasing the production levels of sex hormones in the body. You can reduce stress by participating in relaxing activities, such as walking, listening to calming music, and enjoying more quiet time.

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Male Pattern Baldness: Causes, Identification, and Prevention

Fertility Center and Applied Genetics of Florida is a Fertility Center providing comprehensive fertility services (IVF, IUI, PGD, PGS, Family Balancing/Sex Selection, Reproductive Surgeries, egg donation, surrogacy) for Tampa Bay, Sarasota, Bradenton, Orlando, Ft. Myers, Naples, all Florida, U.S., and International patients. Dr. Pabon is a fertility doctor (Reproductive Endocrinologist and Infertility Specialist) specializing in IVF, Tubal Reversals, Preimplantation genetic diagnosis, egg donation, surrogacy, and general infertility with offices in Sarasota and Bonita Springs, Florida, U.S.A. We enjoy helping traditional couples, single men or women, and the LGBT community.

Dr. Pabon is a nationally recognized Reproductive Endocrinologist and Infertility Specialist that has received Top Doctor designation by U.S. News and World Report and by the Castle Connolly agency for 2013, 2014, 2015, 2016, 2017.

Medical Tourism IVF and Tubal Reversals at Fertility Center and Applied Genetics of Florida:

Tampa, Tampa Bay, Orlando, Sarasota, Bradenton, Ft. Myers, Naples, Florida, and many international patients from the United Kingdom, Spain, and Canada have discovered that we are a destination where the highest technology and current science come together in a uniquely personal and compassionate setting. Patients enjoy the care at one of the best IVF clinics while relaxing in Floridas West Coast. Miles and miles of white sandy beaches, fishing, golf, tennis, and sun sports are quite a draw for Medical treatment tourists.

Dr. Pabon is a nationally and internationally recognized physician, reproductive surgeon, author, lecturer, and a leader in the implementation of new technologies in his area of expertise. He is a graduate with honors of The Univ. of Texas at Austin, Baylor College of Medicine, The University of Louisville, and is a clinical assistant professor for Florida State University College of Medicine. He is the past president, current board member and secretary of the Florida Society of Reproductive Endocrinology and Infertility (FSREI).

Since our first IVF procedures in Sarasota in 1997, we have implemented new technologies such as office based IVF, ICSI, laser assisted hatching, egg donor IVF, surrogacy, Day 3 pre-implantation genetics, trophectoderm blastocyst day 5 & 6 biopsies for pre-implantation genetics, fluorescent in situ hybridization, complete genomic hybridization for 24 chromosome PGS/PGD, laser embryology, fast freeze vitrification, antagonist protocols, agonist triggers, all freeze IVF protocols, family balancing and vitrification. Dr. Pabon is also one of the most experienced tubal reversal surgeons in the world. He has perfected his technique for the outpatient procedure since 1992.

Our Mission Goals:

We are better because we genuinely care about each patient. We do not screen out challenging patients in order to pad our results. Patients are given realistic information about the limits of current technology. While we aim to please, patients must understand that not all clinics are a perfect fit for all patients and that there are some patients that dont succeed despite our best efforts. It is our privilege and honor to have our patients confidence to help build healthy families.

AWorld Class Center for excellence in Reproductive Technologies and Surgery Dr. Pabon is among the most experienced reproductive surgeons in the world. Moreover, our center is recognized as one of the most successful clinics in the United States with pregnancy rates consistently above the National average. Triplets and higher order pregnancies occur in less than 1% of our cases. Our first successful pregnancy after pre-implantation genetic diagnosis was achieved 1999-2000. We are proud to announce the first pregnancy in Florida (Oct 2009) using new PGD/PGS technology through the new microarray technology called gene security parental support and most recently in 2012 the progression of our pre-implantation genetic diagnosis program from the previous multicellular embryo biopsies (1999-2012) to laser assisted trophectoderm blastocyst biopsies and next generation sequencing of the genome of each embryo.

Dr. Pabon is one of the most experienced reproductive surgeons. He specializes in Outpatient Tubal Reversals with microsurgical techniques. Dr. Pabon is one of few surgeons who uses a microscope to perform these delicate surgeries in an outpatient setting. The surgical microscope technique gives the highest magnification possible for the highest accuracy in performing this surgery. He has been performing these surgeries with high success since 1992. He enjoys quite a following at TubalReversalSurgeon/Facebook.

Read What Patients Say About Their Experience:

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Fertility Center & Applied Genetics of Florida and the offices of Julio E. Pabon, M.D., P.A. (formerly Fertility Center of Sarasota) is an extremely uniqueprivate practice where patients receive personal care from one Reproductive Endocrinologist and Infertility Specialist. Our Medical and Laboratory Director, J. E. Pabon, M.D., F.A.C.O.G. takes the time to know his patients, their history and their specific needs. Dr. Pabon and the staff of FC & AG of FL are happy to serve Lee County and Collier County through our south office formerly in Naples (since 2004) and now through the new Bonita Springs office (since 2010)

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Fertility Center & Applied Genetics of Florida

After conducting the test, as expected, Mr. Brown verifies that all three have exactly the same Y-DNA STR marker profile. After speaking with his grand-uncle, he was able to trace distant relatives in Europe who share his surname. After contacting various members of his European line, he obtained 9 participants and the results of the test show the following:

Mr. Brown and his cousin share the same Y-DNA STR marker profile. He also shares the same Y-DNA STR marker profile as group 2 and group 5 of his European line. There is a single mutation in group 3 and group 4, indicating that although they are related, it is more distant, and that groups 3 and 4 are closely related to each other. Group 7, however is not related to this particular Brown family line.

After finding out this exciting information, his newfound European family lines were able to bring more extended family into the surname project, and within a few months, Mr. Brown was able to connect and piece together a large puzzle of his ancestry.

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Home - DNA Ancestry Project

Genetics, study of heredity in general and of genes in particular. Genetics forms one of the central pillars of biology and overlaps with many other areas, such as agriculture, medicine, and biotechnology.

Since the dawn of civilization, humankind has recognized the influence of heredity and applied its principles to the improvement of cultivated crops and domestic animals. A Babylonian tablet more than 6,000 years old, for example, shows pedigrees of horses and indicates possible inherited characteristics. Other old carvings show cross-pollination of date palm trees. Most of the mechanisms of heredity, however, remained a mystery until the 19th century, when genetics as a systematic science began.

Genetics arose out of the identification of genes, the fundamental units responsible for heredity. Genetics may be defined as the study of genes at all levels, including the ways in which they act in the cell and the ways in which they are transmitted from parents to offspring. Modern genetics focuses on the chemical substance that genes are made of, called deoxyribonucleic acid, or DNA, and the ways in which it affects the chemical reactions that constitute the living processes within the cell. Gene action depends on interaction with the environment. Green plants, for example, have genes containing the information necessary to synthesize the photosynthetic pigment chlorophyll that gives them their green colour. Chlorophyll is synthesized in an environment containing light because the gene for chlorophyll is expressed only when it interacts with light. If a plant is placed in a dark environment, chlorophyll synthesis stops because the gene is no longer expressed.

Genetics as a scientific discipline stemmed from the work of Gregor Mendel in the middle of the 19th century. Mendel suspected that traits were inherited as discrete units, and, although he knew nothing of the physical or chemical nature of genes at the time, his units became the basis for the development of the present understanding of heredity. All present research in genetics can be traced back to Mendels discovery of the laws governing the inheritance of traits. The word genetics was introduced in 1905 by English biologist William Bateson, who was one of the discoverers of Mendels work and who became a champion of Mendels principles of inheritance.

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heredity

clear in the study of genetics. Both aspects of heredity can be explained by genes, the functional units of heritable material that are found within all living cells. Every member of a species has a set of genes specific to that species. It is this set of genes that provides

Although scientific evidence for patterns of genetic inheritance did not appear until Mendels work, history shows that humankind must have been interested in heredity long before the dawn of civilization. Curiosity must first have been based on human family resemblances, such as similarity in body structure, voice, gait, and gestures. Such notions were instrumental in the establishment of family and royal dynasties. Early nomadic tribes were interested in the qualities of the animals that they herded and domesticated and, undoubtedly, bred selectively. The first human settlements that practiced farming appear to have selected crop plants with favourable qualities. Ancient tomb paintings show racehorse breeding pedigrees containing clear depictions of the inheritance of several distinct physical traits in the horses. Despite this interest, the first recorded speculations on heredity did not exist until the time of the ancient Greeks; some aspects of their ideas are still considered relevant today.

Hippocrates (c. 460c. 375 bce), known as the father of medicine, believed in the inheritance of acquired characteristics, and, to account for this, he devised the hypothesis known as pangenesis. He postulated that all organs of the body of a parent gave off invisible seeds, which were like miniaturized building components and were transmitted during sexual intercourse, reassembling themselves in the mothers womb to form a baby.

Aristotle (384322 bce) emphasized the importance of blood in heredity. He thought that the blood supplied generative material for building all parts of the adult body, and he reasoned that blood was the basis for passing on this generative power to the next generation. In fact, he believed that the males semen was purified blood and that a womans menstrual blood was her equivalent of semen. These male and female contributions united in the womb to produce a baby. The blood contained some type of hereditary essences, but he believed that the baby would develop under the influence of these essences, rather than being built from the essences themselves.

Aristotles ideas about the role of blood in procreation were probably the origin of the still prevalent notion that somehow the blood is involved in heredity. Today people still speak of certain traits as being in the blood and of blood lines and blood ties. The Greek model of inheritance, in which a teeming multitude of substances was invoked, differed from that of the Mendelian model. Mendels idea was that distinct differences between individuals are determined by differences in single yet powerful hereditary factors. These single hereditary factors were identified as genes. Copies of genes are transmitted through sperm and egg and guide the development of the offspring. Genes are also responsible for reproducing the distinct features of both parents that are visible in their children.

In the two millennia between the lives of Aristotle and Mendel, few new ideas were recorded on the nature of heredity. In the 17th and 18th centuries the idea of preformation was introduced. Scientists using the newly developed microscopes imagined that they could see miniature replicas of human beings inside sperm heads. French biologist Jean-Baptiste Lamarck invoked the idea of the inheritance of acquired characters, not as an explanation for heredity but as a model for evolution. He lived at a time when the fixity of species was taken for granted, yet he maintained that this fixity was only found in a constant environment. He enunciated the law of use and disuse, which states that when certain organs become specially developed as a result of some environmental need, then that state of development is hereditary and can be passed on to progeny. He believed that in this way, over many generations, giraffes could arise from deerlike animals that had to keep stretching their necks to reach high leaves on trees.

British naturalist Alfred Russel Wallace originally postulated the theory of evolution by natural selection. However, Charles Darwins observations during his circumnavigation of the globe aboard the HMS Beagle (183136) provided evidence for natural selection and his suggestion that humans and animals shared a common ancestry. Many scientists at the time believed in a hereditary mechanism that was a version of the ancient Greek idea of pangenesis, and Darwins ideas did not appear to fit with the theory of heredity that sprang from the experiments of Mendel.

Before Gregor Mendel, theories for a hereditary mechanism were based largely on logic and speculation, not on experimentation. In his monastery garden, Mendel carried out a large number of cross-pollination experiments between variants of the garden pea, which he obtained as pure-breeding lines. He crossed peas with yellow seeds to those with green seeds and observed that the progeny seeds (the first generation, F1) were all yellow. When the F1 individuals were self-pollinated or crossed among themselves, their progeny (F2) showed a ratio of 3:1 (3/4 yellow and 1/4 green). He deduced that, since the F2 generation contained some green individuals, the determinants of greenness must have been present in the F1 generation, although they were not expressed because yellow is dominant over green. From the precise mathematical 3:1 ratio (of which he found several other examples), he deduced not only the existence of discrete hereditary units (genes) but also that the units were present in pairs in the pea plant and that the pairs separated during gamete formation. Hence, the two original lines of pea plants were proposed to be YY (yellow) and yy (green). The gametes from these were Y and y, thereby producing an F1 generation of Yy that were yellow in colour because of the dominance of Y. In the F1 generation, half the gametes were Y and the other half were y, making the F2 generation produced from random mating 1/4 Yy, 1/2 YY, and 1/4 yy, thus explaining the 3:1 ratio. The forms of the pea colour genes, Y and y, are called alleles.

Mendel also analyzed pure lines that differed in pairs of characters, such as seed colour (yellow versus green) and seed shape (round versus wrinkled). The cross of yellow round seeds with green wrinkled seeds resulted in an F1 generation that were all yellow and round, revealing the dominance of the yellow and round traits. However, the F2 generation produced by self-pollination of F1 plants showed a ratio of 9:3:3:1 (9/16 yellow round, 3/16 yellow wrinkled, 3/16 green round, and 1/16 green wrinkled; note that a 9:3:3:1 ratio is simply two 3:1 ratios combined). From this result and others like it, he deduced the independent assortment of separate gene pairs at gamete formation.

Mendels success can be attributed in part to his classic experimental approach. He chose his experimental organism well and performed many controlled experiments to collect data. From his results, he developed brilliant explanatory hypotheses and went on to test these hypotheses experimentally. Mendels methodology established a prototype for genetics that is still used today for gene discovery and understanding the genetic properties of inheritance.

Mendels genes were only hypothetical entities, factors that could be inferred to exist in order to explain his results. The 20th century saw tremendous strides in the development of the understanding of the nature of genes and how they function. Mendels publications lay unmentioned in the research literature until 1900, when the same conclusions were reached by several other investigators. Then there followed hundreds of papers showing Mendelian inheritance in a wide array of plants and animals, including humans. It seemed that Mendels ideas were of general validity. Many biologists noted that the inheritance of genes closely paralleled the inheritance of chromosomes during nuclear divisions, called meiosis, that occur in the cell divisions just prior to gamete formation.

It seemed that genes were parts of chromosomes. In 1910 this idea was strengthened through the demonstration of parallel inheritance of certain Drosophila (a type of fruit fly) genes on sex-determining chromosomes by American zoologist and geneticist Thomas Hunt Morgan. Morgan and one of his students, Alfred Henry Sturtevant, showed not only that certain genes seemed to be linked on the same chromosome but that the distance between genes on the same chromosome could be calculated by measuring the frequency at which new chromosomal combinations arose (these were proposed to be caused by chromosomal breakage and reunion, also known as crossing over). In 1916 another student of Morgans, Calvin Bridges, used fruit flies with an extra chromosome to prove beyond reasonable doubt that the only way to explain the abnormal inheritance of certain genes was if they were part of the extra chromosome. American geneticist Hermann Joseph Mller showed that new alleles (called mutations) could be produced at high frequencies by treating cells with X-rays, the first demonstration of an environmental mutagenic agent (mutations can also arise spontaneously). In 1931 American botanist Harriet Creighton and American scientist Barbara McClintock demonstrated that new allelic combinations of linked genes were correlated with physically exchanged chromosome parts.

In 1908 British physician Archibald Garrod proposed the important idea that the human disease alkaptonuria, and certain other hereditary diseases, were caused by inborn errors of metabolism, suggesting for the first time that linked genes had molecular action at the cell level. Molecular genetics did not begin in earnest until 1941 when American geneticist George Beadle and American biochemist Edward Tatum showed that the genes they were studying in the fungus Neurospora crassa acted by coding for catalytic proteins called enzymes. Subsequent studies in other organisms extended this idea to show that genes generally code for proteins. Soon afterward, American bacteriologist Oswald Avery, Canadian American geneticist Colin M. MacLeod, and American biologist Maclyn McCarty showed that bacterial genes are made of DNA, a finding that was later extended to all organisms.

A major landmark was attained in 1953 when American geneticist and biophysicist James D. Watson and British biophysicists Francis Crick and Maurice Wilkins devised a double helix model for DNA structure. This model showed that DNA was capable of self-replication by separating its complementary strands and using them as templates for the synthesis of new DNA molecules. Each of the intertwined strands of DNA was proposed to be a chain of chemical groups called nucleotides, of which there were known to be four types. Because proteins are strings of amino acids, it was proposed that a specific nucleotide sequence of DNA could contain a code for an amino acid sequence and hence protein structure. In 1955 American molecular biologist Seymour Benzer, extending earlier studies in Drosophila, showed that the mutant sites within a gene could be mapped in relation to each other. His linear map indicated that the gene itself is a linear structure.

In 1958 the strand-separation method for DNA replication (called the semiconservative method) was demonstrated experimentally for the first time by American molecular biologist Matthew Meselson and American geneticist Franklin W. Stahl. In 1961 Crick and South African biologist Sydney Brenner showed that the genetic code must be read in triplets of nucleotides, called codons. American geneticist Charles Yanofsky showed that the positions of mutant sites within a gene matched perfectly the positions of altered amino acids in the amino acid sequence of the corresponding protein. In 1966 the complete genetic code of all 64 possible triplet coding units (codons), and the specific amino acids they code for, was deduced by American biochemists Marshall Nirenberg and Har Gobind Khorana. Subsequent studies in many organisms showed that the double helical structure of DNA, the mode of its replication, and the genetic code are the same in virtually all organisms, including plants, animals, fungi, bacteria, and viruses. In 1961 French biologist Franois Jacob and French biochemist Jacques Monod established the prototypical model for gene regulation by showing that bacterial genes can be turned on (initiating transcription into RNA and protein synthesis) and off through the binding action of regulatory proteins to a region just upstream of the coding region of the gene.

Technical advances have played an important role in the advance of genetic understanding. In 1970 American microbiologists Daniel Nathans and Hamilton Othanel Smith discovered a specialized class of enzymes (called restriction enzymes) that cut DNA at specific nucleotide target sequences. That discovery allowed American biochemist Paul Berg in the early 1970s to make the first artificial recombinant DNA molecule by isolating DNA molecules from different sources, cutting them, and joining them together in a test tube. Shortly thereafter, American biochemists Herbert W. Boyer and Stanley N. Cohen came up with methods to produce recombinant plasmids (extragenomic circular DNA elements), which replicated naturally when inserted into bacterial cells. These advances allowed individual genes to be cloned (amplified to a high copy number) by splicing them into self-replicating DNA molecules, such as plasmids or viruses, and inserting these into living bacterial cells. From these methodologies arose the field of recombinant DNA technology that came to dominate molecular genetics. In 1977 two different methods were invented for determining the nucleotide sequence of DNA: one by American molecular biologists Allan Maxam and Walter Gilbert and the other by English biochemist Fred Sanger. Such technologies made it possible to examine the structure of genes directly by nucleotide sequencing, resulting in the confirmation of many of the inferences about genes originally made indirectly.

In the 1970s Canadian biochemist Michael Smith revolutionized the art of redesigning genes by devising a method for inducing specifically tailored mutations at defined sites within a gene, creating a technique known as site-directed mutagenesis. In 1983 American biochemist Kary B. Mullis invented the polymerase chain reaction, a method for rapidly detecting and amplifying a specific DNA sequence without cloning it. In the last decade of the 20th century, progress in recombinant DNA technology and in the development of automated sequencing machines led to the elucidation of complete DNA sequences of several viruses, bacteria, plants, and animals. In 2001 the complete sequence of human DNA, approximately three billion nucleotide pairs, was made public.

A time line of important milestones in the history of genetics is provided in the table.

Classical genetics, which remains the foundation for all other areas in genetics, is concerned primarily with the method by which genetic traitsclassified as dominant (always expressed), recessive (subordinate to a dominant trait), intermediate (partially expressed), or polygenic (due to multiple genes)are transmitted in plants and animals. These traits may be sex-linked (resulting from the action of a gene on the sex, or X, chromosome) or autosomal (resulting from the action of a gene on a chromosome other than a sex chromosome). Classical genetics began with Mendels study of inheritance in garden peas and continues with studies of inheritance in many different plants and animals. Today a prime reason for performing classical genetics is for gene discoverythe finding and assembling of a set of genes that affects a biological property of interest.

Cytogenetics, the microscopic study of chromosomes, blends the skills of cytologists, who study the structure and activities of cells, with those of geneticists, who study genes. Cytologists discovered chromosomes and the way in which they duplicate and separate during cell division at about the same time that geneticists began to understand the behaviour of genes at the cellular level. The close correlation between the two disciplines led to their combination.

Plant cytogenetics early became an important subdivision of cytogenetics because, as a general rule, plant chromosomes are larger than those of animals. Animal cytogenetics became important after the development of the so-called squash technique, in which entire cells are pressed flat on a piece of glass and observed through a microscope; the human chromosomes were numbered using this technique.

Today there are multiple ways to attach molecular labels to specific genes and chromosomes, as well as to specific RNAs and proteins, that make these molecules easily discernible from other components of cells, thereby greatly facilitating cytogenetics research.

Microorganisms were generally ignored by the early geneticists because they are small in size and were thought to lack variable traits and the sexual reproduction necessary for a mixing of genes from different organisms. After it was discovered that microorganisms have many different physical and physiological characteristics that are amenable to study, they became objects of great interest to geneticists because of their small size and the fact that they reproduce much more rapidly than larger organisms. Bacteria became important model organisms in genetic analysis, and many discoveries of general interest in genetics arose from their study. Bacterial genetics is the centre of cloning technology.

Viral genetics is another key part of microbial genetics. The genetics of viruses that attack bacteria were the first to be elucidated. Since then, studies and findings of viral genetics have been applied to viruses pathogenic on plants and animals, including humans. Viruses are also used as vectors (agents that carry and introduce modified genetic material into an organism) in DNA technology.

Molecular genetics is the study of the molecular structure of DNA, its cellular activities (including its replication), and its influence in determining the overall makeup of an organism. Molecular genetics relies heavily on genetic engineering (recombinant DNA technology), which can be used to modify organisms by adding foreign DNA, thereby forming transgenic organisms. Since the early 1980s, these techniques have been used extensively in basic biological research and are also fundamental to the biotechnology industry, which is devoted to the manufacture of agricultural and medical products. Transgenesis forms the basis of gene therapy, the attempt to cure genetic disease by addition of normally functioning genes from exogenous sources.

The development of the technology to sequence the DNA of whole genomes on a routine basis has given rise to the discipline of genomics, which dominates genetics research today. Genomics is the study of the structure, function, and evolutionary comparison of whole genomes. Genomics has made it possible to study gene function at a broader level, revealing sets of genes that interact to impinge on some biological property of interest to the researcher. Bioinformatics is the computer-based discipline that deals with the analysis of such large sets of biological information, especially as it applies to genomic information.

The study of genes in populations of animals, plants, and microbes provides information on past migrations, evolutionary relationships and extents of mixing among different varieties and species, and methods of adaptation to the environment. Statistical methods are used to analyze gene distributions and chromosomal variations in populations.

Population genetics is based on the mathematics of the frequencies of alleles and of genetic types in populations. For example, the Hardy-Weinberg formula, p2 + 2pq + q2 = 1, predicts the frequency of individuals with the respective homozygous dominant (AA), heterozygous (Aa), and homozygous recessive (aa) genotypes in a randomly mating population. Selection, mutation, and random changes can be incorporated into such mathematical models to explain and predict the course of evolutionary change at the population level. These methods can be used on alleles of known phenotypic effect, such as the recessive allele for albinism, or on DNA segments of any type of known or unknown function.

Human population geneticists have traced the origins and migration and invasion routes of modern humans, Homo sapiens. DNA comparisons between the present peoples on the planet have pointed to an African origin of Homo sapiens. Tracing specific forms of genes has allowed geneticists to deduce probable migration routes out of Africa to the areas colonized today. Similar studies show to what degree present populations have been mixed by recent patterns of travel.

Another aspect of genetics is the study of the influence of heredity on behaviour. Many aspects of animal behaviour are genetically determined and can therefore be treated as similar to other biological properties. This is the subject material of behaviour genetics, whose goal is to determine which genes control various aspects of behaviour in animals. Human behaviour is difficult to analyze because of the powerful effects of environmental factors, such as culture. Few cases of genetic determination of complex human behaviour are known. Genomics studies provide a useful way to explore the genetic factors involved in complex human traits such as behaviour.

Some geneticists specialize in the hereditary processes of human genetics. Most of the emphasis is on understanding and treating genetic disease and genetically influenced ill health, areas collectively known as medical genetics. One broad area of activity is laboratory research dealing with the mechanisms of human gene function and malfunction and investigating pharmaceutical and other types of treatments. Since there is a high degree of evolutionary conservation between organisms, research on model organismssuch as bacteria, fungi, and fruit flies (Drosophila)which are easier to study, often provides important insights into human gene function.

Many single-gene diseases, caused by mutant alleles of a single gene, have been discovered. Two well-characterized single-gene diseases include phenylketonuria (PKU) and Tay-Sachs disease. Other diseases, such as heart disease, schizophrenia, and depression, are thought to have more complex heredity components that involve a number of different genes. These diseases are the focus of a great deal of research that is being carried out today.

Another broad area of activity is clinical genetics, which centres on advising parents of the likelihood of their children being affected by genetic disease caused by mutant genes and abnormal chromosome structure and number. Such genetic counseling is based on examining individual and family medical records and on diagnostic procedures that can detect unexpressed, abnormal forms of genes. Counseling is carried out by physicians with a particular interest in this area or by specially trained nonphysicians.

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This article is about Icelanders as an ethnic group. For information about residents or nationals of Iceland, see Demographics of Iceland.IcelandersslendingarTotal population383,500[1]465,000Regions with significant populationsIceland 295,672[2]Canada94,205[3]United States42,716[4]Denmark8,429[5]Norway8,274[5]Sweden5,454[5]United Kingdom2,225[5]Germany1,802[5]Spain1,122[5]Australia980[6]Brazil576[5]Poland492[5]Other countries combinedc.3,000[5]LanguagesIcelandicReligionLutheranism (mainly the Church of Iceland);[7] Neo-pagan; Roman Catholic and Eastern Orthodox minorities among other faiths; secular. Historically Norse paganism, Celtic Christianity (c. 1000) and Catholicism (c. 1000 1551). See Religion in IcelandRelated ethnic groupsOther Germanic peoples, especially Norwegians, Danes, Faroese Islanders

Icelanders (Icelandic: slendingar) are a North Germanic ethnic group and nation who are native to the island nation of Iceland and speak Icelandic.[8]

Icelanders established the country of Iceland in 930 A.D. when the Althingi (Parliament) met for the first time. Iceland came under the reign of Norwegian, Swedish and Danish kings but regained full sovereignty and independence from the Danish monarchy on 1 December 1918, when the Kingdom of Iceland was established. On 17 June 1944, the monarchy was abolished and the Icelandic republic was founded. The language spoken is Icelandic, a North Germanic language, and Lutheranism is the predominant religion. Historical and DNA records indicate that around 60 to 80 percent of the male settlers were of Norse origin (primarily from Western Norway) and a similar percentage of the women were of Gaelic stock from Ireland and peripheral Scotland.[9][10]

Icelanders have had a tumultuous history. Development of the island was slow due to a lack of interest from the countries controlling it for most of its history: Norway, DenmarkNorway, and ultimately Denmark. Through this time, Iceland had relatively little contact with the outside world.[11] The island became independent in personal union with the Kingdom of Denmark in 1918. Since 1944, Iceland has been a republic, and Icelandic society has undergone a rapid modernisation process in the post-independence era.

Iceland is a geologically young land mass, having formed an estimated 20 million years ago due to volcanic eruptions on the Mid-Atlantic ridge. One of the last larger islands to remain uninhabited, the first human settlement date is generally accepted to be 874 AD, although there is some evidence to suggest human activity prior to the Norse arrival.[12]

The first Viking to sight Iceland was Gardar Svavarsson, who went off course due to harsh conditions when sailing from Norway to the Faroe Islands. His reports led to the first efforts to settle the island. Flki Vilgerarson (b. 9th century) was the first Norseman to sail to Iceland intentionally. His story is documented in the Landnmabk manuscript, and he is said to have named the island sland (Iceland). The first permanent settler in Iceland is usually considered to have been a Norwegian chieftain named Inglfur Arnarson. He settled with his family in around 874, at a place he named "Bay of Smokes", or Reykjavk in Icelandic.[13]

Following Inglfur, and also in 874, another group of Norwegians set sail across the North Atlantic Ocean with their families, livestock, slaves, and possessions, escaping the domination of the first King of Norway, Harald Fairhair. They traveled 1,000km (600mi) in their Viking longships to the island of Iceland. These people were primarily of Norwegian, Irish or Gaelic Scottish origin. The Irish and the Scottish Gaels were either slaves or servants of the Norse chiefs, according to the Icelandic sagas, or descendants of a "group of Norsemen who had settled in Scotland and Ireland and intermarried with Gaelic-speaking people".[14] Genetic evidence suggests that approximately 62% of the Icelandic maternal gene pool is derived from Ireland and Scotland, which is much higher than other Scandinavian countries, although comparable to the Faroese, while 37% is of Nordic origin.[15] About 20-25% of the Icelandic paternal gene pool is of Gaelic origin, with the rest being Nordic.[16]

The Icelandic Age of Settlement (Icelandic: Landnmsld) is considered to have lasted from 874 to 930, at which point most of the island had been claimed and the Alingi (English: Althing), the assembly of the Icelandic Commonwealth, was founded at ingvellir.[17]

In 930, on the ingvellir (English: Thingvellir) plain near Reykjavk, the chieftains and their families met and established the Alingi, Iceland's first national assembly. However, the Alingi lacked the power to enforce the laws it made. In 1262, struggles between rival chieftains left Iceland so divided that King Haakon IV of Norway was asked to step in as a final arbitrator for all disputes, as part of the Old Covenant. This is known as the Age of the Sturlungs.[18]

Iceland was under Norwegian leadership until 1380, when the Royal House of Norway died out. At this point, both Iceland and Norway came under the control of the Danish Crown. With the introduction of absolute monarchy in Denmark, the Icelanders relinquished their autonomy to the crown, including the right to initiate and consent to legislation. This meant a loss of independence for Iceland, which led to nearly 300 years of decline: perhaps largely because Denmark and its Crown did not consider Iceland to be a colony to be supported and assisted. In particular, the lack of help in defense led to constant raids by marauding pirates along the Icelandic coasts.[11]

Unlike Norway, Denmark did not need Iceland's fish and homespun wool. This created a dramatic deficit in Iceland's trade, and no new ships were built as a result. In 1602 Iceland was forbidden to trade with other countries by order of the Danish Government, and in the 18th century climatic conditions had reached an all-time low since Settlement.[11]

In 178384 Laki, a volcanic fissure in the south of the island, erupted. The eruption produced about 15km (3.6mi) of basalt lava, and the total volume of tephra emitted was 0.91km.[19] The aerosols that built up caused a cooling effect in the Northern Hemisphere. The consequences for Iceland were catastrophic, with approximately 25-33% of the population dying in the famine of 1783 and 1784. Around 80% of sheep, 50% of cattle, and 50% of horses died of fluorosis from the 8 million tons of fluorine that were released.[20] This disaster is known as the Mist Hardship (Icelandic: Muharindin).

In 179899 the Alingi was discontinued for several decades, eventually being restored in 1844. It was moved to Reykjavk, the capital, after being held at ingvellir for over nine centuries.

The 19th century brought significant improvement in the Icelanders' situation. A protest movement was led by Jn Sigursson, a statesman, historian, and authority on Icelandic literature. Inspired by the romantic and nationalist currents from mainland Europe, Jn protested strongly, through political journals and self-publications, for 'a return to national consciousness' and for political and social changes to be made to help speed up Iceland's development.[21]

In 1854, the Danish government relaxed the trade ban that had been imposed in 1602, and Iceland gradually began to rejoin Western Europe economically and socially. With this return of contact with other peoples came a reawakening of Iceland's arts, especially its literature. Twenty years later in 1874, Iceland was granted a constitution. Icelanders today recognize Jn's efforts as largely responsible for their economic and social resurgence.[21]

Iceland gained full sovereignty and independence from Denmark in 1918 after World War I. It became the Kingdom of Iceland. The King of Denmark also served as the King of Iceland but Iceland retained only formal ties with the Danish Crown. On 17 June 1944 the monarchy was abolished and a republic was established on what would have been Jn Sigursson's 133rd birthday. This ended nearly six centuries of ties with Denmark.[21]

Due to their small founding population and history of relative isolation, Icelanders have often been considered highly genetically homogeneous as compared to other European populations. For this reason, along with the extensive genealogical records for much of the population that reach back to the settlement of Iceland, Icelanders have been the focus of considerable genomics research by both biotechnology companies and academic and medical researchers.[22][23] It was, for example, possible for researchers to reconstruct much of the maternal genome of Iceland's first known black inhabitant, Hans Jonatan, from the DNA of his present-day descendants partly because the distinctively African parts of his genome were unique in Iceland until very recent times.[24]

Genetic evidence shows that most DNA lineages found among Icelanders today can be traced to the settlement of Iceland, indicating that there has been relatively little immigration since. This evidence shows that the founder population of Iceland came from Ireland, Scotland, and Scandinavia: studies of mitochondrial DNA and Y-chromosomes indicate that 62% of Icelanders' matrilineal ancestry derives from Scotland and Ireland (with most of the rest being from Scandinavia), while 75% of their patrilineal ancestry derives from Scandinavia (with most of the rest being from the Irish and British Isles).[25] Despite Iceland's historical isolation, the genetic makeup of Icelanders today is still quite different from the founding population, due to founder effects and genetic drift.[26] One study found that the mean Norse ancestry among Iceland's settlers was 56%, whereas in the current population the figure was 70%.[27]

Other studies have identified other ancestries, however. One study of mitochondrial DNA, blood groups, and isozymes revealed a more variable population than expected, comparable to the diversity of some other Europeans.[28] Another study showed that a tiny proportion of samples of contemporary Icelanders carry a more distant lineage, which belongs to the haplogroup C1e, which can possibly be traced to the settlement of the Americas around 14,000 years ago. This hints a small proportion of Icelanders have some Native American ancestry arising from Norse colonization of Greenland and North America.[29]

The first Europeans to emigrate to and settle in Greenland were Icelanders who did so under the leadership of Erik the Red in the late 10th century CE and numbered around 500 people. Isolated fjords in this harsh land offered sufficient grazing to support cattle and sheep, though the climate was too cold for cereal crops. Royal trade ships from Norway occasionally went to Greenland to trade for walrus tusks and falcons. The population eventually reached a high point of perhaps 3,000 in two communities and developed independent institutions before fading away during the 15th century.[30] A papal legation was sent there as late as 1492, the year Columbus attempted to find a shorter spice route to Asia but instead encountered the Americas.

According to the Saga of Eric the Red, Icelandic immigration to North America dates back to Vinland circa 1006. The colony was believed to be short-lived and abandoned by the 1020s. [31] European settlement of the region was not archeologically and historically confirmed as more than legend until the 1960s. The former Norse site, now known as L'Anse aux Meadows, pre-dated the arrival of Colombus in the Americas by almost 500 years.

A more recent instance of Icelandic emigration to North America occurred in 1855, when a small group settled in Spanish Fork, Utah.[32] Another Icelandic colony formed in Washington Island, Wisconsin.[33] Immigration to the United States and Canada began in earnest in the 1870s, with most migrants initially settling in the Great Lakes area. These settlers were fleeing famine and overcrowding on Iceland.[34] Today, there are sizable communities of Icelandic descent in both the United States and Canada. Gimli, in Manitoba, Canada, is home to the largest population of Icelanders outside of the main island of Iceland.[35]

From the mid-1990s, Iceland experienced rising immigration. By 2017 the population of first-generation immigrants (defined as people born abroad with both parents foreign-born and all grandparents foreign-born) stood at 35,997 (10.6% of residents), and the population of second-generation immigrants at 4,473. Correspondingly, the numbers of foreign-born people acquiring Icelandic citizenship are markedly higher than in the 1990s, standing at 703 in 2016.[36][37] Correspondingly, Icelandic identity is gradually shifting towards a more multicultural form.[38]

Icelandic, a North Germanic language, is the official language of Iceland (de facto; the laws are silent about the issue). Icelandic has inflectional grammar comparable to Latin, Ancient Greek, more closely to Old English and practically identical to Old Norse.

Old Icelandic literature can be divided into several categories. Three are best known to foreigners: Eddic poetry, skaldic poetry, and saga literature, if saga literature is understood broadly. Eddic poetry is made up of heroic and mythological poems. Poetry that praises someone is considered skaldic poetry or court poetry. Finally, saga literature is prose, ranging from pure fiction to fairly factual history.[39]

Written Icelandic has changed little since the 13th century. Because of this modern readers can understand the Icelanders' sagas. The sagas tell of events in Iceland in the 10th and early 11th centuries. They are considered to be the best-known pieces of Icelandic literature.[40]

The elder or Poetic Edda, the younger or Prose Edda, and the sagas are the major pieces of Icelandic literature. The Poetic Edda is a collection of poems and stories from the late 10th century, whereas the younger or Prose Edda is a manual of poetry that contains many stories of Norse mythology.

Iceland embraced Christianity in c. AD 1000, in what is called the kristnitaka, and the country, while mostly secular in observance, is still predominantly Christian culturally. The Lutheran church claims some 84% of the total population.[41] While early Icelandic Christianity was more lax in its observances than traditional Catholicism, Pietism, a religious movement imported from Denmark in the 18th century, had a marked effect on the island. By discouraging all but religious leisure activities, it fostered a certain dourness, which was for a long time considered an Icelandic stereotype. At the same time, it also led to a boom in printing, and Iceland today is one of the most literate societies in the world.[21][42]

While Catholicism was supplanted by Protestantism during the Reformation, most other world religions are now represented on the island: there are small Protestant Free Churches and Catholic communities, and even a nascent Muslim community, composed of both immigrants and local converts. Perhaps unique to Iceland is the fast-growing satrarflag, a legally recognized revival of the pre-Christian Nordic religion of the original settlers. According to the Roman Catholic Diocese of Reykjavk, there were only approximately 30 Jews in Iceland as of 2001.[43] The former First Lady of Iceland Dorrit Moussaieff was an Israeli-born Bukharian Jew.

Icelandic cuisine consists mainly of fish, lamb, and dairy. Fish was once the main part of an Icelander's diet but has recently given way to meats such as beef, pork, and poultry.[20]

Iceland has many traditional foods called orramatur. These foods include smoked and salted lamb, singed sheep heads, dried fish, smoked and pickled salmon, and cured shark. Andrew Zimmern, a chef who has traveled the world on his show Bizarre Foods with Andrew Zimmern, responded to the question "What's the most disgusting thing you've ever eaten?" with the response "That would have to be the fermented shark fin I had in Iceland." Fermented shark fin is a form of orramatur.[44]

The earliest indigenous Icelandic music was the rmur, epic tales from the Viking era that were often performed a cappella. Christianity played a major role in the development of Icelandic music, with many hymns being written in the local idiom. Hallgrmur Ptursson, a poet and priest, is noted for writing many of these hymns in the 17th century. The island's relative isolation ensured that the music maintained its regional flavor. It was only in the 19th century that the first pipe organs, prevalent in European religious music, first appeared on the island.[45]

Many singers, groups, and forms of music have come from Iceland. Most Icelandic music contains vibrant folk and pop traditions. Some more recent groups and singers are Voces Thules, The Sugarcubes, Bjrk, Sigur Rs, and Of Monsters and Men.

The national anthem is " Gu vors lands" (English: "Our Country's God"), written by Matthas Jochumsson, with music by Sveinbjrn Sveinbjrnsson. The song was written in 1874, when Iceland celebrated its one thousandth anniversary of settlement on the island. It was originally published with the title A Hymn in Commemoration of Iceland's Thousand Years.[45]

Iceland's men's national football team participated in their first FIFA World Cup in 2018, after reaching the quarter finals of its first major international tournament, UEFA Euro 2016. The women's national football team has yet to reach a World Cup; its best result at a major international event was a quarterfinal finish in UEFA Women's Euro 2013. The country's first Olympic participation was in the 1912 Summer Olympics; however, they did not participate again until the 1936 Summer Olympics. Their first appearance at the Winter Games was at the 1948 Winter Olympics. In 1956, Vilhjlmur Einarsson won the Olympic silver medal for the triple jump.[46] The Icelandic national handball team has enjoyed relative success. The team received a silver medal at the 2008 Olympic Games and a 3rd place at the 2010 European Men's Handball Championship.

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1910

Thomas Hunt Morgan (1866-1945) establishes the chromosomal theory of heredity

Thomas Hunt Morgan, an embryologist who had turned to research in heredity, in 1907 began to extensively breed the common fruit fly, Drosophila melanogaster. He hoped to discover large-scale mutations that would represent the emergence of new species. As it turned out, Morgan confirmed Mendelian laws of inheritance and the hypothesis that genes are located on chromosomes. He thereby inaugurated classical experimental genetics.

These results were suggestive for hypotheses of which Morgan himself was skeptical. He was at the time critical of the Mendelian theory of inheritance, mistrusted aspects of chromosomal theory, and did not believe that Darwin's concept of natural selection could account for the emergence of new species. But Morgan's discoveries with white- and red-eyed flies led him to reconsider each of these hypotheses.

In particular, Morgan began to entertain the possibility that association of eye color and sex in fruit flies had a physical and mechanistic basis in the chromosomes. The shape of one of Drosophila's four chromosome pairs was thought to be distinctive for sex determination. Males invariably possess the XY chromosome pair (Morgan used a more cumbersome notation) while flies with the XX chromosome are female. If the factor for eye color was located exclusively on the X chromosome, Morgan realized, Mendelian rules for inheritance of dominant and recessive traits could apply.

In brief, Morgan had discovered that eye color in Drosophila expressed a sex-linked trait. All first-generation offspring of a mutant white-eyed male and a normal red-eyed female would have red eyes because every chromosome pair would contain at least one copy of the X chromosome with the dominant trait. But half the females from this union would now possess a copy of the white-eyed male's recessive X chromosome. This chromosome would be transmitted, on average, to one-half of second-generation offspringone-half of which would be male. Thus, second-generation offspring would include one-quarter with white eyesand all of these would be male.

Intensive work led Morgan to discover more mutant traitssome two dozen between 1911 and 1914. With evidence drawn from cytology he was able to refine Mendelian laws and combine them with the theoryfirst suggested by Theodor Boveri and Walter Suttonthat the chromosomes carry hereditary information. In 1915, Morgan and his colleagues published The Mechanism of Mendelian Heredity. Its major tenets:

Discrete pairs of factors located on chromosomes like beads on a string bear hereditary information. These factorsMorgan would soon call them genessegregate in germ cells and combine during reproduction, essentially as predicted by Mendelian laws. However:

Certain characteristics are sex-linkedthat is, occur together because they arise on the same chromosome that determines gender. More generally:

Other characteristics are also sometimes associated because, as paired chromosomes separate during germ cell development, genes proximate to one another tend to remain together. But sometimes, as a mechanistic consequence of reproduction, this linkage between genes is broken, allowing for new combinations of traits.

Morgan's experimental and theoretical work inaugurated research in genetics and promoted a revolution in biology. Evidence he adduced from embryology and cell theory pointed the way toward a synthesis of genetics with evolutionary theory. Morgan himself explored aspects of these developments in later work, including Evolution and Genetics published in 1925, and The Theory of the Gene in 1926. He received the Nobel Prize in Physiology or Medicine in 1933.

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Starting about 7,000 years ago, something weird seems to have happened to men: Over the next two millennia, recent studies suggest, their genetic diversity - specifically, the diversity of their Y chromosomes - collapsed. So extreme was that collapse that it was as if there was only one man left to mate for every 17 women.

Anthropologists and biologists were perplexed, but Stanford researchers now believe they've found a simple - if revealing - explanation. The collapse, they argue, was the result of generations of war between patrilineal clans, whose membership is determined by male ancestors.

The outlines of that idea came to Tian Chen Zeng, a Stanford undergraduate in sociology, after spending hours reading blog posts that speculated - unconvincingly, Zeng thought - on the origins of the "Neolithic Y-chromosome bottleneck," as the event is known. He soon shared his ideas with his high school classmate Alan Aw, also a Stanford undergraduate in mathematical and computational science.

"He was really waxing lyrical about it," Aw said, so the pair took their idea to Marcus Feldman, a professor of biology in Stanford's School of Humanities and Sciences. Zeng, Aw and Feldman published their results May 25 in Nature Communications .

"Woman Triumphant" by Rudolf Cronau. (1919). ( Public Domain )

It's not unprecedented for human genetic diversity to take a nosedive once in a while, but the Y-chromosome bottleneck, which was inferred from genetic patterns in modern humans, was an odd one. First, it was observed only in men - more precisely, it was detected only through genes on the Y chromosome, which fathers pass to their sons. Second, the bottleneck is much more recent than other biologically similar events, hinting that its origins might have something to do with changing social structures.

Certainly, the researchers point out, social structures were changing. After the onset of farming and herding around 12,000 years ago, societies grew increasingly organized around extended kinship groups, many of them patrilineal clans - a cultural fact with potentially significant biological consequences. The key is how clan members are related to each other. While women may have married into a clan, men in such clans are all related through male ancestors and therefore tend to have the same Y chromosomes. From the point of view of those chromosomes at least, it's almost as if everyone in a clan has the same father.

That only applies within one clan, however, and there could still be considerable variation between clans. To explain why even between-clan variation might have declined during the bottleneck, the researchers hypothesized that wars, if they repeatedly wiped out entire clans over time, would also wipe out a good many male lineages and their unique Y chromosomes in the process.

Cave art in Magura cave from between 10000-8000 years ago. ( Public Domain )

To test their ideas, the researchers turned to mathematical models and computer simulations in which men fought - and died - for the resources their clans needed to survive. As the team expected, wars between patrilineal clans drastically reduced Y chromosome diversity over time, while conflict between non-patrilineal clans - groups where both men and women could move between clans - did not.

Zeng, Aw and Feldman's model also accounted for the observation that among the male lineages that survived the Y-chromosome bottleneck, a few lineages underwent dramatic expansions, consistent with the patrilineal clan model, but not others.

Now the researchers are looking at applying the framework in other areas - anywhere "historical and geographical patterns of cultural interactions could explain the patterns you see in genetics," said Feldman, who is also the Burnet C. and Mildred Finley Wohlford Professor.

Feldman said the work was an unusual example of undergraduates driving research that was broad both in terms of the academic disciplines spanned - in this case, sociology, mathematics and biology - and in terms of its potential implications for understanding the role of culture in shaping human evolution. And, he said, "Working with these talented guys is a lot of fun."

Top image: Prehistoric Man Hunting Bears by Emmanuel Benner the Younger. Source: Public Domain

The article, originally titled Wars and clan structure may explain a strange biological event 7,000 years ago, was first published on Science Daily.

Stanford University. "Wars and clan structure may explain a strange biological event 7,000 years ago." ScienceDaily. ScienceDaily, 29 May 2018. http://www.sciencedaily.com/releases/2018/05/180529185356.htm

Tian Chen Zeng, Alan J. Aw, Marcus W. Feldman. Cultural hitchhiking and competition between patrilineal kin groups explain the post-Neolithic Y-chromosome bottleneck . Nature Communications , 2018; 9 (1) DOI: 10.1038/s41467-018-04375-6

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Neolithic Male Genetic Diversity Plummeted Heres Why ...

The genetic causes of mens health issues cut both ways. On the one hand, it can make you resigned to the fact that youre going to have this or that problem. On the other hand, you can just blame it on your genes!

Having a certain type of genes doesnt mean that you will definitely develop the related disease or health issue. Very few genetic markers are like that. Most inherited genes only increase the risk of you getting the health problem. Lets look more closely at the known and suspected genetic causes of mens health issues.

The most common talking points about baldness are far from proven. They are only educated guesses with certain promising correlations shown in studies, though far from conclusive. These include hair follicles health, blood circulation in the head, eating too much greasy food, etc.

In comparison, male pattern baldness is definitively linked to genetics. You are more likely to go bald if your father is bald. This is also true concerning your grandfather and uncles on your mothers side of the family. A study using over 52,000 genetic data from the UK Biobank found that among the men in the top 10% highest risk pool, 58% of them had moderate to severe hair loss. There are many more such studies.

We are happy to report that research into the genetics of erectile dysfunction is in its infancy. This is probably because most types of ED are unlikely to be caused by genes.

There is a small chance that infertility has a genetic root, and thats only if the infertility is caused by Klinefelters syndrome, Y chromosome deletions, and cystic fibrosis gene mutation.

As for prostate cancer, about 5-10% of prostate cancers are genetic, according to the Memorial Sloan Kettering Cancer Center. However, your chances of getting prostate cancer can increase 5 times if two or more of your close male relatives have it.

And thats about it. Apart from baldness, how you live your life is often more influential than the genetic causes of mens health issues.

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More than 55% of VHL-affected individuals develop only multiple renal cell cysts. The VHL-associated RCCs that occur are characteristically multifocal and bilateral and present as a combined cystic and solid mass.[66] Among individuals with VHL, the cumulative RCC risk has been reported as 24% to 45% overall. RCCs smaller than 3 cm in this disease tend to be low grade (Fuhrman nuclear grade 2) and minimally invasive,[67] and their rate of growth varies widely.[68] An investigation of 228 renal lesions in 28 patients who were followed up for at least 1 year showed that transition from a simple cyst to a solid lesion was infrequent.[66] Complex cystic and solid lesions contained neoplastic tissue that uniformly enlarged. These data may be used to help predict the progression of renal lesions in VHL. Figure 1 depicts bilateral renal tumors in a patient with VHL.

Enlarge Figure 1. von Hippel-Lindau diseaseassociated renal cell cancers are characteristically multifocal and bilateral and present as a combined cystic and solid mass. Red arrow indicates a lesion with a solid and cystic component, and white arrow indicates a predominantly solid lesion.

Tumors larger than 3 cm may increase in grade as they grow, and metastasis may occur.[68,69] RCCs often remain asymptomatic for long intervals.

Patients can also develop pancreatic cysts, cystadenomas, and pancreatic NETs.[2] Pancreatic cysts and cystadenomas are not malignant, but pancreatic NETs possess malignant characteristics and are typically resected if they are 3 cm or larger (2 cm if located in the head of the pancreas).[70] A review of the natural history of pancreatic NETs shows that these tumors may demonstrate nonlinear growth characteristics.[71]

Retinal manifestations, first reported more than a century ago, were one of the first recognized aspects of VHL. Retinal hemangioblastomas (also known as capillary retinal angiomas) are one of the most frequent manifestations of VHL and are present in more than 50% of patients.[72] Retinal involvement is one of the earliest manifestations of VHL, with a mean age at onset of 25 years.[1,2] These tumors are the first manifestation of VHL in nearly 80% of affected individuals and may occur in children as young as 1 year.[2,73,74]

Retinal hemangioblastomas occur most frequently in the periphery of the retina but can occur in other locations such as the optic nerve, a location much more difficult to treat. Retinal hemangioblastomas appear as a bright orange spherical tumor supplied by a tortuous vascular supply. Nearly 50% of patients have bilateral retinal hemangioblastomas.[72] The median number of lesions per affected eye is approximately six.[75] Other retinal lesions in VHL can include retinal vascular hamartomas, flat vascular tumors located in the superficial aspect of the retina.[76]

Longitudinal studies are important for the understanding of the natural history of these tumors. Left untreated, retinal hemangioblastomas can be a major source of morbidity in VHL, with approximately 8% of patients [72] having blindness caused by various mechanisms, including secondary maculopathy, contributing to retinal detachment, or possibly directly causing retinal neurodegeneration.[77] Patients with symptomatic lesions generally have larger and more numerous retinal hemangioblastomas. Long-term follow-up studies demonstrate that most lesions grow slowly and that new lesions do not develop frequently.[75,78]

Hemangioblastomas are the most common disease manifestation in patients with VHL, affecting more than 70% of individuals. A prospective study assessed the natural history of hemangioblastomas.[79] The mean age at onset of CNS hemangioblastomas is 29.1 years (range, 773 y).[80] After a mean follow-up of 7 years, 72% of the 225 patients studied developed new lesions.[81] Fifty-one percent of existing hemangioblastomas remained stable. The remaining lesions exhibited heterogeneous growth rates, with cerebellar and brainstem lesions growing faster than those in the spinal cord or cauda equina. Approximately 12% of hemangioblastomas developed either peritumoral or intratumoral cysts, and 6.4% were symptomatic and required treatment. Increased tumor burden or total tumor number detected was associated with male sex, longer follow-up, and genotype (all P < .01). Partial germline deletions were associated with more tumors per patient than were missense variants (P < .01). Younger patients developed more tumors per year. Hemangioblastoma growth rate was higher in men than in women (P < .01). Figures 2 and 3 depict cerebellar and spinal hemangioblastomas, respectively, in patients with VHL.

Enlarge Figure 2. Hemangioblastomas are the most common disease manifestation in patients with von Hippel-Lindau disease. The left panel shows a sagittal view of brainstem and cerebellar lesions. The middle panel shows an axial view of a brainstem lesion. The right panel shows a cerebellar lesion (red arrow) with a dominant cystic component (white arrow).

Enlarge Figure 3. Hemangioblastomas are the most common disease manifestation in patients with von Hippel-Lindau disease. Multiple spinal cord hemangioblastomas are shown.

The rate of pheochromocytoma formation in the VHL patient population is 25% to 30%.[82,83] Of patients with VHL-associated pheochromocytomas, 44% developed disease in both adrenal glands.[84] The rate of malignant transformation is very low. Levels of plasma and urine normetanephrine are typically elevated in patients with VHL,[85] and approximately two-thirds will experience physical manifestations such as hypertension, tachycardia, and palpitations.[82] Patients with a partial loss of VHL function (Type 2 disease) are at higher risk of pheochromocytoma than are VHL patients with a complete loss of VHL function (Type 1 disease); the latter develop pheochromocytoma very rarely.[13,14,82,86] The rate of VHL germline pathogenic variants in nonsyndromic pheochromocytomas and paragangliomas was very low in a cohort of 182 patients, with only 1 of 182 patients ultimately diagnosed with VHL.[87]

Paragangliomas are rare in VHL patients but can occur in the head and neck or abdomen.[88] A review of VHL patients who developed pheochromocytomas and/or paragangliomas revealed that 90% of patients manifested pheochromocytomas and 19% presented with a paraganglioma.[84]

The mean age at diagnosis of VHL-related pheochromocytomas and paragangliomas is approximately 30 years,[83,89] and patients with multiple tumors were diagnosed more than a decade earlier than patients with solitary lesions in one series (19 vs. 34 y; P < .001).[89] Diagnosis of pheochromocytoma was made in patients as young as 5 years in one cohort,[83] providing a rationale for early testing. All 21 pediatric patients with pheochromocytomas in this 273-patient cohort had elevated plasma normetanephrines.[83]

VHL patients may develop multiple serous cystadenomas, pancreatic NETs, and simple pancreatic cysts.[1] VHL patients do not have an increased risk of pancreatic adenocarcinoma. Serous cystadenomas are benign tumors and warrant no intervention. Simple pancreatic cysts can be numerous and rarely cause symptomatic biliary duct obstruction. Endocrine function is nearly always maintained; occasionally, however, patients with extensive cystic disease requiring pancreatic surgery may ultimately require pancreatic exocrine supplementation.

Pancreatic NETs are usually nonfunctional but can metastasize (to lymph nodes and the liver). The risk of pancreatic NET metastasis was analyzed in a large cohort of patients, in which the mean age at diagnosis of a pancreatic NET was 38 years (range, 1668 y).[90] The risk of metastasis was lower in patients with small primary lesions (3 cm), in patients without an exon 3 pathogenic variant, and in patients whose tumor had a slow doubling time (>500 days). Nonfunctional pancreatic NETs can be followed by imaging surveillance with intervention when tumors reach 3 cm. Lesions in the head of the pancreas can be considered for surgery at a smaller size to limit operative complexity.

ELSTs are adenomatous tumors arising from the endolymphatic duct or sac within the posterior part of the petrous bone.[91] ELSTs are rare in the sporadic setting, but are apparent on imaging in 11% to 16% of patients with VHL. Although these tumors do not metastasize, they are locally invasive, eroding through the petrous bone and the inner ear structures.[91,92] Approximately 30% of VHL patients with ELSTs have bilateral lesions.[91,93]

ELSTs are an important cause of morbidity in VHL patients. ELSTs evident on imaging are associated with a variety of symptoms, including hearing loss (95% of patients), tinnitus (92%), vestibular symptoms (such as vertigo or disequilibrium) (62%), aural fullness (29%), and facial paresis (8%).[91,92] In approximately half of patients, symptoms (particularly hearing loss) can occur suddenly, probably as a result of acute intralabyrinthine hemorrhage.[92] Hearing loss or vestibular dysfunction in VHL patients can also present in the absence of radiologically evident ELSTs (approximately 60% of all symptomatic patients) and is believed to be a consequence of microscopic ELSTs.[91]

Hearing loss related to ELSTs is typically irreversible; serial imaging to enable early detection of ELSTs in asymptomatic patients and resection of radiologically evident lesions are important components in the management of VHL patients.[94,95] Surgical resection by retrolabyrinthine posterior petrosectomy is usually curative and can prevent onset or worsening of hearing loss and improve vestibular symptoms.[92,94]

Tumors of the broad ligament can occur in females with VHL and are known as papillary cystadenomas. These tumors are extremely rare, and fewer than 20 have been reported in the literature.[96] Papillary cystadenomas are histologically identical to epididymal cystadenomas commonly observed in males with VHL.[97] One important difference is that papillary cystadenomas are almost exclusively observed in patients with VHL, whereas epididymal cystadenomas in men can occur sporadically.[98] These tumors are frequently cystic, and although they become large, they generally have a fairly indolent behavior.

More than one-third of all cases of epididymal cystadenomas reported in the literature and most cases of bilateral cystadenomas have been reported in patients with VHL.[99] Among symptomatic patients, the most common presentation is a painless, slow-growing scrotal swelling. The differential diagnoses of epididymal tumors include adenomatoid tumor (which is the most common tumor in this site), metastatic ccRCC, and papillary mesothelioma.[100]

In a small series, histological analysis did not reveal features typically associated with malignancy, such as mitotic figures, nuclear pleomorphism, and necrosis. Lesions were strongly positive for CK7 and negative for RCC. Carbonic anhydrase IX (CAIX) was positive in all tumors. PAX8 was positive in most cases. These features were reminiscent of clear cell papillary RCC, a relatively benign form of RCC without known metastatic potential.[97]

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Genetics of Kidney Cancer (Renal Cell Cancer) (PDQ ...

Homosexual behavior in animals is sexual behavior among non-human species that is interpreted as homosexual or bisexual. This may include same-sex sexual activity, courtship, affection, pair bonding, and parenting among same-sex animal pairs.[1][2][3][4] Research indicates that various forms of this are found in every major geographic region and every major animal group. The sexual behavior of non-human animals takes many different forms, even within the same species, though homosexual behavior is best known from social species.

Scientists perceive homosexual behavior in animals to different degrees. The motivations for and implications of these behaviors have yet to be fully understood, since most species have yet to be fully studied.[5] According to Bruce Bagemihl, the animal kingdom engages in homosexual behavior "with much greater sexual diversity including homosexual, bisexual and nonreproductive sex than the scientific community and society at large have previously been willing to accept."[6] Bagemihl adds, however, that this is "necessarily an account of human interpretations of these phenomena".[7] Simon LeVay introduced caveat that "[a]lthough homosexual behavior is very common in the animal world, it seems to be very uncommon that individual animals have a long-lasting predisposition to engage in such behavior to the exclusion of heterosexual activities. Thus, a homosexual orientation, if one can speak of such thing in animals, seems to be a rarity."[8] One species in which exclusive homosexual orientation occurs, however, is that of domesticated sheep (Ovis aries).[9][10] "About 10% of rams (males), refuse to mate with ewes (females) but do readily mate with other rams."[10]

According to Bagemihl (1999), same-sex behavior (comprising courtship, sexual, pair-bonding, and parental activities) has been documented in over 450 species of animals worldwide.[11]

The term homosexual was coined by Karl-Maria Kertbeny in 1868 to describe same-sex sexual attraction and sexual behavior in humans.[12] Its use in animal studies has been controversial for two main reasons: animal sexuality and motivating factors have been and remain poorly understood, and the term has strong cultural implications in western society that are irrelevant for species other than humans.[13] Thus homosexual behavior has been given a number of terms over the years. According to Bruce Bagemihl, when describing animals, the term homosexual is preferred over gay, lesbian, and other terms currently in use, as these are seen as even more bound to human homosexuality.[14]

Bailey et al. says: "Homosexual: in animals, this has been used to refer to same-sex behavior that is not sexual in character (e.g. homosexual tandem running in termites), same-sex courtship or copulatory behavior occurring over a short period of time (e.g. homosexual mounting in cockroaches and rams) or long-term pair bonds between same-sex partners that might involve any combination of courting, copulating, parenting and affectional behaviors (e.g. homosexual pair bonds in gulls). In humans, the term is used to describe individual sexual behaviors as well as long-term relationships, but in some usages connotes a gay or lesbian social identity. Scientific writing would benefit from reserving this anthropomorphic term for humans and not using it to describe behavior in other animals, because of its deeply rooted context in human society".[15]

Animal preference and motivation is always inferred from behavior. In wild animals, researchers will as a rule not be able to map the entire life of an individual, and must infer from frequency of single observations of behavior. The correct usage of the term homosexual is that an animal exhibits homosexual behavior or even same-sex sexual behavior; however, this article conforms to the usage by modern research,[14][16][17][18][pageneeded][19]applying the term homosexuality to all sexual behavior (copulation, genital stimulation, mating games and sexual display behavior) between animals of the same sex. In most instances, it is presumed that the homosexual behavior is but part of the animal's overall sexual behavioral repertoire, making the animal "bisexual" rather than "homosexual" as the terms are commonly understood in humans.[18][pageneeded], but cases of homosexual preference and exclusive homosexual pairs are known.[20]

The observation of homosexual behavior in animals can be seen as both an argument for and against the acceptance of homosexuality in humans, and has been used especially against the claim that it is a peccatum contra naturam ("sin against nature"). For instance, homosexuality in animals was cited by the American Psychiatric Association and other groups in their amici curiae brief to the United States Supreme Court in Lawrence v. Texas, which ultimately struck down the sodomy laws of 14 states.[21][22]

A majority of the research available concerning homosexual behavior in animals lacks specification between animals that exclusively exhibit same-sex tendencies and those that participate in heterosexual and homosexual mating activities interchangeably. This lack of distinction has led to differing opinions and conflicting interpretations of collected data amongst scientists and researchers. For instance, Bruce Bagemihl, author of the book Biological Exuberence: Animal Homosexuality and Natural Diversity, emphasizes that there are no anatomical or endocrinological differences between exclusively homosexual and exclusively heterosexual animal pairs.[23][pageneeded] However, if the definition of "homosexual behavior" is made to include animals that participate in both same-sex and opposite-sex mating activities, hormonal differences have been documented among key sex hormones, such as testosterone and estradiol, when compared to those who participate solely in heterosexual mating.[24]

Many of the animals used in laboratory-based studies of homosexuality do not appear to spontaneously exhibit these tendencies often in the wild. Such behavior is often elicited and exaggerated by the researcher during experimentation through the destruction of a portion of brain tissue, or by exposing the animal to high levels of steroid hormones prenatally.[25][pageneeded] Information gathered from these studies is limited when applied to spontaneously occurring same-sex behavior in animals outside of the laboratory.[25]

Homosexual behaviour in animals has been discussed since classical antiquity. The earliest written mention of animal homosexuality appears to date back to 2,300 years ago, when Aristotle (384322 BC) described copulation between pigeons, partridges and quails of the same sex.[26] The Hieroglyphics of Horapollo, written in the 4th century AD by the Egyptian writer Horapollo, mentions "hermaphroditism" in hyenas and homosexuality in partridges.[26] The first review of animal homosexuality was written by the zoologist Ferdinand Karsch-Haack in 1900.[26]

Until recent times, the presence of same-sex sexual behavior was not "officially" observed on a large scale, possibly due to observer bias caused by social attitudes to same-sex sexual behavior,[27] innocent confusion, lack of interest, distaste, scientists fearing loss of their grants or even from a fear of "being ridiculed by their colleagues".[28][29] Georgetown University biologist Janet Mann states "Scientists who study the topic are often accused of trying to forward an agenda, and their work can come under greater scrutiny than that of their colleagues who study other topics."[30] They also noted "Not every sexual act has a reproductive function ... that's true of humans and non-humans."[30] It appears to be widespread amongst social birds and mammals, particularly the sea mammals and the primates. The true extent of homosexuality in animals is not known. While studies have demonstrated homosexual behavior in a number of species, Petter Bckman, the scientific advisor of the exhibition Against Nature? in 2007, speculated that the true extent of the phenomenon may be much larger than was then recognized:

No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at all, such as sea urchins and aphis. Moreover, a part of the animal kingdom is hermaphroditic, truly bisexual. For them, homosexuality is not an issue.[28]

An example of overlooking homosexual behavior is noted by Bagemihl describing mating giraffes where nine out of ten pairings occur between males:

Every male that sniffed a female was reported as sex, while anal intercourse with orgasm between males was only "revolving around" dominance, competition or greetings.[31]

Some researchers believe this behavior to have its origin in male social organization and social dominance, similar to the dominance traits shown in prison sexuality. Others, particularly Bagemihl, Joan Roughgarden, Thierry Lod[32] and Paul Vasey suggest the social function of sex (both homosexual and heterosexual) is not necessarily connected to dominance, but serves to strengthen alliances and social ties within a flock. Others have argued that social organization theory is inadequate because it cannot account for some homosexual behaviors, for example, penguin species where male individuals mate for life and refuse to pair with females when given the chance.[33][34] While reports on many such mating scenarios are still only anecdotal, a growing body of scientific work confirms that permanent homosexuality occurs not only in species with permanent pair bonds,[19] but also in non-monogamous species like sheep.

One report on sheep cited below states:

Approximately 8% of rams exhibit sexual preferences [that is, even when given a choice] for male partners (male-oriented rams) in contrast to most rams, which prefer female partners (female-oriented rams). We identified a cell group within the medial preoptic area/anterior hypothalamus of age-matched adult sheep that was significantly larger in adult rams than in ewes...[35]

In fact, apparent homosexual individuals are known from all of the traditional domestic species, from sheep, cattle and horses to cats, dogs and budgerigars.[36][pageneeded]

A definite physiological explanation or reason for homosexual activity in animal species has not been agreed upon by researchers in the field. Numerous scholars are of the opinion that varying levels (either higher or lower) of the sex hormones in the animal,[37] in addition to the size of the animal's gonads,[24] play a direct role in the sexual behavior and preference exhibited by that animal. Others firmly argue no evidence to support these claims exists when comparing animals of a specific species exhibiting homosexual behavior exclusively and those that do not. Ultimately, empirical support from comprehensive endocrinological studies exist for both interpretations.[37][38] Researchers found no evidence of differences in the measurements of the gonads, or the levels of the sex hormones of exclusively homosexual western gulls and ring-billed gulls.[39] However, when analyzing these differences in bisexual rams, males were found to have lower levels of testosterone and estradiol in their blood, as well as smaller gonads than their heterosexual counterpart.[citation needed]

Additional studies pertaining to hormone involvement in homosexual behavior indicate that when administering treatments of testosterone and estradiol to female heterosexual animals, the elevated hormone levels increase the likelihood of homosexual behavior. Additionally, boosting the levels of sex hormones during an animal's pregnancy appears to increase the likelihood of it birthing a homosexual offspring.[37]

Researchers found that disabling the fucose mutarotase (FucM) gene in laboratory mice which influences the levels of estrogen to which the brain is exposed caused the female mice to behave as if they were male as they grew up. "The mutant female mouse underwent a slightly altered developmental programme in the brain to resemble the male brain in terms of sexual preference" said Professor Chankyu Park of the Korea Advanced Institute of Science and Technology in Daejon, South Korea, who led the research. His most recent findings have been published in the BMC Genetics journal on July 7, 2010.[40][41] Another study found that by manipulating a gene in fruit flies (Drosophila), homosexual behavior appeared to have been induced. However, in addition to homosexual behavior, several abnormal behaviors were also exhibited apparently due to this mutation.[42]

In March 2011, research showed that serotonin is involved in the mechanism of sexual orientation of mice.[43][44] A study conducted on fruit flies found that inhibiting the dopamine neurotransmitter inhibited lab-induced homosexual behavior.[45]

An estimated one-quarter of all black swans pairings are of males. They steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs. The males spent time in each other's society, guarded the common territory, performed greeting ceremonies before each other, and (in the reproductive period) pre-marital rituals, and if one of the birds tried to sit on the other, an intense fight began.[1][2] More of their cygnets survive to adulthood than those of different-sex pairs, possibly due to their superior ability to defend large portions of land. The same reasoning has been applied to male flamingo pairs raising chicks.[46][47]

Female albatross, on the north-western tip of the island of Oahu, Hawaii, form pairs for co-growing offspring. On the observed island, the number of females considerably exceeds the number of males (59% N=102/172), so 31% of females, after mating with males, create partnerships for hatching and feeding chicks. Compared to male-female couples female partnerships have a lower hatching rate (41% vs 87%) and lower overall reproductive success (31% vs. 67%).[48]

Research has shown that the environmental pollutant methylmercury can increase the prevalence of homosexual behavior in male American white ibis. The study involved exposing chicks in varying dosages to the chemical and measuring the degree of homosexual behavior in adulthood. The results discovered was that as the dosage was increased the likelihood of homosexual behavior also increased. The endocrine blocking feature of mercury has been suggested as a possible cause of sexual disruption in other bird species.[49][50]

Mallards form male-female pairs only until the female lays eggs, at which time the male leaves the female. Mallards have rates of male-male sexual activity that are unusually high for birds, in some cases, as high as 19% of all pairs in a population.[36][pageneeded] Kees Moeliker of the Natural History Museum Rotterdam has observed one male mallard engage in homosexual necrophilia.[51]

Penguins have been observed to engage in homosexual behaviour since at least as early as 1911. George Murray Levick, who documented this behaviour in Adlie penguins at Cape Adare, described it as "depraved". The report was considered too shocking for public release at the time, and was suppressed. The only copies that were made available privately to researchers were translated into Greek, to prevent this knowledge becoming more widely known. The report was unearthed only a century later, and published in Polar Record in June 2012.[52]

In early February 2004 the New York Times reported that Roy and Silo, a male pair of chinstrap penguins in the Central Park Zoo in New York City had successfully hatched and fostered a female chick from a fertile egg they had been given to incubate.[21] Other penguins in New York zoos have also been reported to have formed same-sex pairs.[53][54]

In Odense Zoo in Denmark, a pair of male king penguins adopted an egg that had been abandoned by a female, proceeding to incubate it and raise the chick.[55][56]Zoos in Japan and Germany have also documented homosexual male penguin couples.[33][34] The couples have been shown to build nests together and use a stone as a substitute for an egg. Researchers at Rikkyo University in Tokyo found 20 homosexual pairs at 16 major aquariums and zoos in Japan.

The Bremerhaven Zoo in Germany attempted to encourage reproduction of endangered Humboldt penguins by importing females from Sweden and separating three male pairs, but this was unsuccessful. The zoo's director said that the relationships were "too strong" between the homosexual pairs.[57] German gay groups protested at this attempt to break up the male-male pairs[58] but the zoo's director was reported as saying "We don't know whether the three male pairs are really homosexual or whether they have just bonded because of a shortage of females ... nobody here wants to forcibly separate homosexual couples."[59]

A pair of male Magellanic penguins who had shared a burrow for six years at the San Francisco Zoo and raised a surrogate chick, split when the male of a pair in the next burrow died and the female sought a new mate.[60]

Buddy and Pedro, a pair of male African penguins, were separated by the Toronto Zoo to mate with female penguins.[61][62] Buddy has since paired off with a female.[62]

Suki and Chupchikoni are two female African penguins that pair bonded at the Ramat Gan Safari in Israel. Chupchikoni was assumed to be male until her blood was tested.[63]

In 2014 Jumbs and Hurricane, two Humboldt penguins at Wingham Wildlife Park became the center of international media attention as two male penguins who had pair bonded a number of years earlier and then successfully hatched and reared an egg given to them as surrogate parents after the mother abandoned it halfway through incubation.[64]

In 1998 two male griffon vultures named Dashik and Yehuda, at the Jerusalem Biblical Zoo, engaged in "open and energetic sex" and built a nest. The keepers provided the couple with an artificial egg, which the two parents took turns incubating; and 45 days later, the zoo replaced the egg with a baby vulture. The two male vultures raised the chick together.[65] A few years later, however, Yehuda became interested in a female vulture that was brought into the aviary. Dashik became depressed, and was eventually moved to the zoological research garden at Tel Aviv University where he too set up a nest with a female vulture.[66]

Two male vultures at the Allwetter Zoo in Muenster built a nest together, although they were picked on and their nest materials were often stolen by other vultures. They were eventually separated to try to promote breeding by placing one of them with female vultures, despite the protests of German homosexual groups.[67]

Both male and female pigeons sometimes exhibit homosexual behavior. In addition to sexual behavior, same-sex pigeon pairs will build nests, and hens will lay (infertile) eggs and attempt to incubate them.[citation needed]

The Amazon river dolphin or boto has been reported to form up in bands of 35 individuals engaging in sexual activity. The groups usually comprise young males and sometimes one or two females. Sex is often performed in non-reproductive ways, using snout, flippers and genital rubbing, without regard to gender.[68] In captivity, they have been observed to sometimes perform homosexual and heterosexual penetration of the blowhole, a hole homologous with the nostril of other mammals, making this the only known example of nasal sex in the animal kingdom.[68][69] The males will sometimes also perform sex with males from the tucuxi species, a type of small porpoise.[68]

Courtship, mounting, and full anal penetration between bulls has been noted to occur among American bison. The Mandan nation Okipa festival concludes with a ceremonial enactment of this behavior, to "ensure the return of the buffalo in the coming season".[70] Also, mounting of one female by another (known as "bulling") is extremely common among cattle. The behaviour is hormone driven and synchronizes with the emergence of estrus (heat), particularly in the presence of a bull.

More than 20 species of bat have been documented to engage in homosexual behavior.[26][71] Bat species that have been observed engaging in homosexual behavior in the wild include:[26]

Bat species that have been observed engaging in homosexual behavior in captivity include the Comoro flying fox (Pteropus livingstonii), the Rodrigues flying fox (Pteropus rodricensis) and the common vampire bat (Desmodus rotundus).[26]

Homosexual behavior in bats has been categorized into 6 groups: mutual homosexual grooming and licking, homosexual masturbation, homosexual play, homosexual mounting, coercive sex, and cross-species homosexual sex.[26][71]

In the wild, the grey-headed flying fox (Pteropus poliocephalus) engages in allogrooming wherein one partner licks and gently bites the chest and wing membrane of the other partner. Both sexes display this form of mutual homosexual grooming and it is more common in males. Males often have erect penises while they are mutually grooming each other. Like opposite-sex grooming partners, same-sex grooming partners continuously utter a pre-copulation call, which is described as a "pulsed grating call," while engaged in this activity.[26][71]

In wild Bonin flying foxes (Pteropus pselaphon), males perform fellatio or 'male-male genital licking' on other males. Malemale genital licking events occur repeatedly several times in the same pair, and reciprocal genital licking also occurs. The male-male genital licking in these bats is considered a sexual behavior. Allogrooming in Bonin flying foxes has never been observed, hence the male-male genital licking in this species does not seem to be a by-product of allogrooming, but rather a behavior of directly licking the male genital area, independent of allogrooming.[71] In captivity, same-sex genital licking has been observed among males of the Comoro flying fox (Pteropus livingstonii) as well as among males of the common vampire bat (Desmodus rotundus).[26][71]

In wild Indian flying foxes (Pteropus giganteus), males often mount one another, with erections and thrusting, while play-wrestling.[26] Males of the long-fingered bat (Myotis capaccinii) have been observed in the same position of male-female mounting, with one gripping the back of the others fur. A similar behavior was also observed in the common bent-wing bat (Miniopterus schreibersii).[26]

In wild little brown bats (Myotis lucifugus), males often mount other males (and females) during late autumn and winter, when many of the mounted individuals are torpid.[26] 35% of matings during this period are homosexual.[72] These coercive copulations usually include ejaculation and the mounted bat often makes a typical copulation call consisting of a long squawk.[26] Similarly, in hibernacula of the common noctule (Nyctalus noctula), active males were observed to wake up from lethargy on a warm day and engage in mating with lethargic males and (active or lethargic) females. The lethargic males, like females, called out loudly and presented their buccal glands with opened mouth during copulation.[26]

Vesey-Fitzgerald (1949) observed homosexual behaviours in all 12 British bat species known at the time: Homosexuality is common in the spring in all species, and, since the males are in full possession of their powers, I suspect throughout the summer...I have even seen homosexuality between Natterer's and Daubenton's bats (Myotis nattereri and M. daubentonii)."[26]

Dolphins of several species engage in homosexual acts, though it is best studied in the bottlenose dolphins.[36][pageneeded] Sexual encounters between females take the shape of "beak-genital propulsion", where one female inserts her beak in the genital opening of the other while swimming gently forward.[73] Between males, homosexual behaviour includes rubbing of genitals against each other, which sometimes leads to the males swimming belly to belly, inserting the penis in the others genital slit and sometimes anus.[74]

Janet Mann, Georgetown University professor of biology and psychology, argues that the strong personal behavior among male dolphin calves is about bond formation and benefits the species in an evolutionary context.[75] She cites studies showing that these dolphins later in life as adults are in a sense bisexual, and the male bonds forged earlier in life work together for protection as well as locating females to reproduce with. Confrontations between flocks of bottlenose dolphins and the related species Atlantic spotted dolphin will sometimes lead to cross-species homosexual behaviour between the males rather than combat.[76]

African and Asian males will engage in same-sex bonding and mounting. Such encounters are often associated with affectionate interactions, such as kissing, trunk intertwining, and placing trunks in each other's mouths. Male elephants, who often live apart from the general herd, often form "companionships", consisting of an older individual and one or sometimes two younger males with sexual behavior being an important part of the social dynamic. Unlike heterosexual relations, which are always of a fleeting nature, the relationships between males may last for years. The encounters are analogous to heterosexual bouts, one male often extending his trunk along the other's back and pushing forward with his tusks to signify his intention to mount. Same-sex relations are common and frequent in both sexes, with Asiatic elephants in captivity devoting roughly 45% of sexual encounters to same-sex activity.[77]

Male giraffes have been observed to engage in remarkably high frequencies of homosexual behavior. After aggressive "necking", it is common for two male giraffes to caress and court each other, leading up to mounting and climax. Such interactions between males have been found to be more frequent than heterosexual coupling.[78] In one study, up to 94% of observed mounting incidents took place between two males. The proportion of same sex activities varied between 30 and 75%, and at any given time one in twenty males were engaged in non-combative necking behavior with another male. Only 1% of same-sex mounting incidents occurred between females.[79]

Olympic marmot (left) and Hoary marmot (right).

Homosexual behavior is quite common in wild marmots.[80] In Olympic marmots (Marmota olympus) and Hoary Marmots (Marmota caligata), females often mount other females as well as engage in other affectionate and sexual behaviors with females of the same species.[80] They display a high frequency of these behaviors especially when they are in heat.[80][81] A homosexual encounter often begins with a greeting interaction in which one female nuzzles her nose on the other females cheek or mouth, or both females touch noses or mouths. Additionally, a female may gently chew on the ear or neck of her partner, who responds by raising her tail. The first female may sniff the other's genital region or nuzzle that region with her mouth. She may then proceed to mount the other female, during which the mounting female gently grasps the mounted female's dorsal neck fur in her jaws while thrusting. The mounted female arches her back and holds her tail to one side to facilitate their sexual interaction.[80][82]

Both male and female lions have been seen to interact homosexually.[83][84] Male lions pair-bond for a number of days and initiate homosexual activity with affectionate nuzzling and caressing, leading to mounting and thrusting. About 8% of mountings have been observed to occur with other males. Pairings between females are held to be fairly common in captivity but have not been observed in the wild.

European polecats Mustela putorius were found to engage homosexually with non-sibling animals. Exclusive homosexuality with mounting and anal penetration in this solitary species serves no apparent adaptive function.[85][pageneeded]

Bonobos, which have a matriarchal society, unusual among apes, are a fully bisexual speciesboth males and females engage in heterosexual and homosexual behavior, being noted for femalefemale homosexuality in particular, including[86] between juveniles and adults. Roughly 60% of all bonobo sexual activity occurs between two or more females. While the homosexual bonding system in bonobos represents the highest frequency of homosexuality known in any primate species, homosexuality has been reported for all great apes (a group which includes humans), as well as a number of other primate species.[87][88][89][pageneeded][90][86][91][92][93][94]

Dutch primatologist Frans de Waal on observing and filming bonobos noted that there were two reasons to believe sexual activity is the bonobo's answer to avoiding conflict. Anything that arouses the interest of more than one bonobo at a time, not just food, tends to result in sexual contact. If two bonobos approach a cardboard box thrown into their enclosure, they will briefly mount each other before playing with the box. Such situations lead to squabbles in most other species. But bonobos are quite tolerant, perhaps because they use sex to divert attention and to defuse tension.

Bonobo sex often occurs in aggressive contexts totally unrelated to food. A jealous male might chase another away from a female, after which the two males reunite and engage in scrotal rubbing. Or after a female hits a juvenile, the latter's mother may lunge at the aggressor, an action that is immediately followed by genital rubbing between the two adults.[95]

With the Japanese macaque, also known as the "snow monkey", same-sex relations are frequent, though rates vary between troops. Females will form "consortships" characterized by affectionate social and sexual activities. In some troops up to one quarter of the females form such bonds, which vary in duration from a few days to a few weeks. Often, strong and lasting friendships result from such pairings. Males also have same-sex relations, typically with multiple partners of the same age. Affectionate and playful activities are associated with such relations.[96]

Homosexual behavior forms part of the natural repertoire of sexual or sociosexual behavior of orangutans. Male homosexual behavior occurs both in the wild and in captivity, and it occurs in both adolescent and mature individuals. Homosexual behavior in orangutans is not an artifact of captivity or contact with humans.[97]

Among monkeys[clarification needed], Lionel Tiger and Robin Fox conducted a study on how Depo-Provera contraceptives lead to decreased male attraction to females.[98]

Ovis aries has attracted much attention due to the fact that around 810% of rams have an exclusive homosexual orientation.[9][99][100][101][102] Furthermore, around 1822% of rams are bisexual.[100]

An October 2003 study by Dr. Charles E. Roselli et al. (Oregon Health and Science University) states that homosexuality in male sheep (found in 8% of rams) is associated with a region in the rams' brains which the authors call the "ovine Sexually Dimorphic Nucleus" (oSDN) which is half the size of the corresponding region in heterosexual male sheep.[35] Scientists found that, "The oSDN in rams that preferred females was significantly larger and contained more neurons than in male-oriented rams and ewes. In addition, the oSDN of the female-oriented rams expressed higher levels of aromatase, a substance that converts testosterone to estradiol, a form of estrogen which is believed to facilitate typical male sexual behaviors. Aromatase expression was no different between male-oriented rams and ewes."

"The dense cluster of neurons that comprise the oSDN express cytochrome P450 aromatase. Aromatase mRNA levels in the oSDN were significantly greater in female-oriented rams than in ewes, whereas male-oriented rams exhibited intermediate levels of expression." These results suggest that "... naturally occurring variations in sexual partner preferences may be related to differences in brain anatomy and its capacity for estrogen synthesis."[35] As noted before, given the potential unagressiveness of the male population in question, the differing aromatase levels may also have been evidence of aggression levels, not sexuality. It should also be noted that the results of this study have not been confirmed by other studies.

The Merck Manual of Veterinary Medicine appears to consider homosexuality among sheep as a routine occurrence and an issue to be dealt with as a problem of animal husbandry.[103]

Homosexual courtship and sexual activity routinely occur among rams of wild sheep species, such as Bighorn sheep (Ovis canadensis), Thinhorn sheep (Ovis dalli), mouflons and urials (Ovis orientalis).[104] Usually a higher ranking older male courts a younger male using a sequence of stylized movements. To initiate homosexual courtship, a courting male approaches the other male with his head and neck lowered and extended far forward in what is called the 'low-stretch' posture. He may combine this with the 'twist,' in which the courting male sharply rotates his head and points his muzzle toward the other male, often while flicking his tongue and making grumbling sounds. The courting male also often performs a 'foreleg kick,' in which he snaps his front leg up against the other males belly or between his hind legs. He also occasionally sniffs and nuzzles the other males genital area and may perform the flehmen response. Thinhorn rams additionally lick the penis of the male they are courting. In response, the male being courted may rub his cheeks and forehead on the courting males face, nibble and lick him, rub his horns on the courting males neck, chest, or shoulders, and develop an erection. Males of another wild sheep species, the Asiatic Mouflons, perform similar courtship behaviors towards fellow males.[104]

Sexual activity between wild males typically involves mounting and anal intercourse. In Thinhorn sheep, genital licking also occurs. During mounting, the larger male usually mounts the smaller male by rearing up on his hind legs and placing his front legs on his partners flanks. The mounting male usually has an erect penis and accomplishes full anal penetration while performing pelvic thrusts that may lead to ejaculation. The mounted male arches his back to facilitate the copulation. Homosexual courtship and sexual activity can also take place in groups composed of three to ten wild rams clustered together in a circle. These non-aggressive groups are called 'huddles' and involve rams rubbing, licking, nuzzling, horning, and mounting each other. Female Mountain sheep also engage in occasional courtship activities with one another and in sexual activities such as licking each others genitals and mounting.[104]

The family structure of the spotted hyena is matriarchal, and dominance relationships with strong sexual elements are routinely observed between related females. Due largely to the female spotted hyena's unique urogenital system, which looks more like a penis rather than a vagina, early naturalists thought hyenas were hermaphroditic males who commonly practiced homosexuality.[105][not in citation given] Early writings such as Ovid's Metamorphoses and the Physiologus suggested that the hyena continually changed its sex and nature from male to female and back again. In Paedagogus, Clement of Alexandria noted that the hyena (along with the hare) was "quite obsessed with sexual intercourse". Many Europeans associated the hyena with sexual deformity, prostitution, deviant sexual behavior, and even witchcraft.

The reality behind the confusing reports is the sexually aggressive behavior between the females, including mounting between females. Research has shown that "in contrast to most other female mammals, female Crocuta are male-like in appearance, larger than males, and substantially more aggressive,"[106] and they have "been masculinized without being defeminized".[105][not in citation given]

Study of this unique genitalia and aggressive behavior in the female hyena has led to the understanding that more aggressive females are better able to compete for resources, including food and mating partners.[105][107] Research has shown that "elevated levels of testosterone in utero"[108] contribute to extra aggressiveness; both males and females mount members of both the same and opposite sex,[108][109] who in turn are possibly acting more submissive because of lower levels of testosterone in utero.[106]

Parthenogenesis. Several species of whiptail lizard (especially in the genus Aspidoscelis) consist only of females that have the ability to reproduce through parthenogenesis.[110] Females engage in sexual behavior to stimulate ovulation, with their behavior following their hormonal cycles; during low levels of estrogen, these (female) lizards engage in "masculine" sexual roles. Those animals with currently high estrogen levels assume "feminine" sexual roles. Some parthenogenetic lizards that perform the courtship ritual have greater fertility than those kept in isolation due to an increase in hormones triggered by the sexual behaviors. So, even though asexual whiptail lizards populations lack males, sexual stimuli still increase reproductive success. From an evolutionary standpoint, these females are passing their full genetic code to all of their offspring (rather than the 50% of genes that would be passed in sexual reproduction). Certain species of gecko also reproduce by parthenogenesis.[111]

"True" homosexuality in lizards. Some species of sexually reproducing geckos have been found to display homosexual behavior, e.g the day geckos Phelsuma laticauda and Phelsuma cepediana.[112]

Jonathan, the world's oldest tortoise (an Aldabra giant tortoise), had been mating with another tortoise named Frederica since 1991. In 2017, it was discovered that Frederica was actually probably male all along, and was renamed Frederic.[113]

There is evidence of same-sex sexual behavior in at least 110 species of insects and arachnids.[114] Scharf et al. says: "Males are more frequently involved in same-sex sexual (SSS) behavior in the laboratory than in the field, and isolation, high density, and exposure to female pheromones increase its prevalence. SSS behavior is often shorter than the equivalent heterosexual behavior. Most cases can be explained via mistaken identification by the active (courting/mounting) male. Passive males often resist courting/mating attempts".[114]

Scharf et al. continues: "SSS behavior has been reported in most insect orders, and Bagemihl (1999) provides a list of ~100 species of insects demonstrating such behavior. Yet, this list lacks detailed descriptions, and a more comprehensive summary of its prevalence in invertebrates, as well as ethology, causes, implications, and evolution of this behavior, remains lacking".[114]

Male homosexuality has been inferred in several species of dragonflies (the order Odonata). The cloacal pinchers of male damselflies and dragonflies inflict characteristic head damage to females during sex. A survey of 11 species of damsel and dragonflies[115][116] has revealed such mating damages in 20 to 80% of the males too, indicating a fairly high occurrence of sexual coupling between males.

Male Drosophila melanogaster flies bearing two copies of a mutant allele in the fruitless gene court and attempt to mate exclusively with other males.[20] The genetic basis of animal homosexuality has been studied in the fly Drosophila melanogaster.[117] Here, multiple genes have been identified that can cause homosexual courtship and mating.[118] These genes are thought to control behavior through pheromones as well as altering the structure of the animal's brains.[119][120] These studies have also investigated the influence of environment on the likelihood of flies displaying homosexual behavior.[121][122]

Male bed bugs (Cimex lectularius) are sexually attracted to any newly fed individual and this results in homosexual mounting. This occurs in heterosexual mounting by the traumatic insemination in which the male pierces the female abdomen with his needle-like penis. In homosexual mating this risks abdominal injuries as males lack the female counteradaptive spermalege structure. Males produce alarm pheromones to reduce such homosexual mating.

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Homosexual behavior in animals - Wikipedia

A stallion is a male horse that has not been gelded (castrated).Stallions follow the conformation and phenotype of their breed, but within that standard, the presence of hormones such as testosterone may give stallions a thicker, "cresty" neck, as well as a somewhat more muscular physique as compared to female horses, known as mares, and castrated males, called geldings.

Temperament varies widely based on genetics, and training, but because of their instincts as herd animals, they may be prone to aggressive behavior, particularly toward other stallions, and thus require careful management by knowledgeable handlers. However, with proper training and management, stallions are effective equine athletes at the highest levels of many disciplines, including horse racing, horse shows, and international Olympic competition.

The term "stallion" dates from the era of Henry VII, who passed a number of laws relating to the breeding and export of horses in an attempt to improve the British stock, under which it was forbidden to allow uncastrated male horses to be turned out in fields or on the commons; they had to be "kept within bounds and tied in stalls." (The term "stallion" for an uncastrated male horse dates from this time; stallion = stalled one.)[1] "Stallion" is also used to refer to males of other equids, including zebras and donkeys.

Contrary to popular myths, many stallions do not live with a harem of mares. Nor, in natural settings, do they fight each other to the death in competition for mares. Being social animals, stallions who are not able to find or win a harem of mares usually band together in stallions-only "bachelor" groups which are composed of stallions of all ages. Even with a band of mares, the stallion is not the leader of a herd but defends and protects the herd from predators and other stallions. The leadership role in a herd is held by a mare, known colloquially as the "lead mare" or "boss mare." The mare determines the movement of the herd as it travels to obtain food, water, and shelter. She also determines the route the herd takes when fleeing from danger. When the herd is in motion, the dominant stallion herds the straggling members closer to the group and acts as a "rear guard" between the herd and a potential source of danger. When the herd is at rest, all members share the responsibility of keeping watch for danger. The stallion is usually on the edge of the group, to defend the herd if needed.

There is usually one dominant mature stallion for every mixed-sex herd of horses. The dominant stallion in the herd will tolerate both sexes of horses while young, but once they become sexually mature, often as yearlings or two-year-olds, the stallion will drive both colts and fillies from the herd. Colts may present competition for the stallion, but studies suggest that driving off young horses of both sexes may also be an instinctive behavior that minimizes the risk of inbreeding within the herd, as most young are the offspring of the dominant stallion in the group. In some cases, a single younger mature male may be tolerated on the fringes of the herd. One theory is that this young male is considered a potential successor, as in time the younger stallion will eventually drive out the older herd stallion.

Fillies usually soon join a different band with a dominant stallion different from the one that sired them. Colts or young stallions without mares of their own usually form small, all-male, "bachelor bands" in the wild. Living in a group gives these stallions the social and protective benefits of living in a herd. A bachelor herd may also contain older stallions who have lost their herd in a challenge.[2]

Other stallions may directly challenge a herd stallion, or may simply attempt to "steal" mares and form a new, smaller herd. In either case, if the two stallions meet, there rarely is a true fight; more often there will be bluffing behavior and the weaker horse will back off. Even if a fight for dominance occurs, rarely do opponents hurt each other in the wild because the weaker combatant has a chance to flee. Fights between stallions in captivity may result in serious injuries; fences and other forms of confinement make it more difficult for the losing animal to safely escape. In the wild, feral stallions have been known to steal or mate with domesticated mares.

The stallion's reproductive system is responsible for his sexual behavior and secondary sex characteristics (such as a large crest).The external genitalia comprise:

The internal genitalia comprise the accessory sex glands, which include the vesicular glands, the prostate gland and the bulbourethral glands. These contribute fluid to the semen at ejaculation, but are not strictly necessary for fertility.[3][9]

Domesticated stallions are trained and managed in a variety of ways, depending on the region of the world, the owner's philosophy, and the individual stallion's temperament. In all cases, however, stallions have an inborn tendency to attempt to dominate both other horses and human handlers, and will be affected to some degree by proximity to other horses, especially mares in heat. They must be trained to behave with respect toward humans at all times or else their natural aggressiveness, particularly a tendency to bite, may pose a danger of serious injury.[2]

For this reason, regardless of management style, stallions must be treated as individuals and should only be handled by people who are experienced with horses and thus recognize and correct inappropriate behavior before it becomes a danger.[10] While some breeds are of a more gentle temperament than others, and individual stallions may be well-behaved enough to even be handled by inexperienced people for short periods of time, common sense must always be used. Even the most gentle stallion has natural instincts that may overcome human training. As a general rule, children should not handle stallions, particularly in a breeding environment.

Management of stallions usually follows one of the following models: confinement or "isolation" management, where the stallion is kept alone, or in management systems variously called "natural", "herd", or "pasture" management where the stallion is allowed to be with other horses. In the "harem" model, the stallion is allowed to run loose with mares akin to that of a feral or semi-feral herd. In the"bachelor herd" model, stallions are kept in a male-only group of stallions, or, in some cases, with stallions and geldings. Sometime stallions may periodically be managed in multiple systems, depending on the season of the year.

The advantage of natural types of management is that the stallion is allowed to behave "like a horse" and may exhibit fewer stable vices. In a harem model, the mares may "cycle" or achieve estrus more readily. Proponents of natural management also assert that mares are more likely to "settle" (become pregnant) in a natural herd setting. Some stallion managers keep a stallion with a mare herd year-round, others will only turn a stallion out with mares during the breeding season.[11]

In some places, young domesticated stallions are allowed to live separately in a "bachelor herd" while growing up, kept out of sight, sound or smell of mares. A Swiss study demonstrated that even mature breeding stallions kept well away from other horses could live peacefully together in a herd setting if proper precautions were taken while the initial herd hierarchy was established.[12]

As an example, in the New Forest, England, breeding stallions run out on the open Forest for about two to three months each year with the mares and youngstock. On being taken off the Forest, many of them stay together in bachelor herds for most of the rest of the year.[13][14][15] New Forest stallions, when not in their breeding work, take part on the annual round-ups, working alongside mares and geldings, and compete successfully in many disciplines.[16][17]

There are drawbacks to natural management, however. One is that the breeding date, and hence foaling date, of any given mare will be uncertain. Another problem is the risk of injury to the stallion or mare in the process of natural breeding, or the risk of injury while a hierarchy is established within an all-male herd. Some stallions become very anxious or temperamental in a herd setting and may lose considerable weight, sometimes to the point of a health risk. Some may become highly protective of their mares and thus more aggressive and dangerous to handle. There is also a greater risk that the stallion may escape from a pasture or be stolen. Stallions may break down fences between adjoining fields to fight another stallion or mate with the "wrong" herd of mares, thus putting the pedigree of ensuing foals in question.[18]

The other general method of managing stallions is to confine them individually, sometimes in a small pen or corral with a tall fence, other times in a stable, or, in certain places, in a small field (or paddock) with a strong fence. The advantages to individual confinement include less of a risk of injury to the stallion or to other horses, controlled periods for breeding mares, greater certainty of what mares are bred when, less risk of escape or theft, and ease of access by humans. Some stallions are of such a temperament, or develop vicious behavior due to improper socialization or poor handling, that they must be confined and cannot be kept in a natural setting, either because they behave in a dangerous manner toward other horses, or because they are dangerous to humans when loose.

The drawbacks to confinement vary with the details of the actual method used, but stallions kept out of a herd setting require a careful balance of nutrition and exercise for optimal health and fertility. Lack of exercise can be a serious concern; stallions without sufficient exercise may not only become fat, which may reduce both health and fertility, but also may become aggressive or develop stable vices due to pent-up energy. Some stallions within sight or sound of other horses may become aggressive or noisy, calling or challenging other horses. This sometimes is addressed by keeping stallions in complete isolation from other animals.

However, complete isolation has significant drawbacks; stallions may develop additional behavior problems with aggression due to frustration and pent-up energy. As a general rule, a stallion that has been isolated from the time of weaning or sexual maturity will have a more difficult time adapting to a herd environment than one allowed to live close to other animals. However, as horses are instinctively social creatures, even stallions are believed to benefit from being allowed social interaction with other horses, though proper management and cautions are needed.[12]

Some managers attempt to compromise between the two methods by providing stallions daily turnout by themselves in a field where they can see, smell, and hear other horses. They may be stabled in a barn where there are bars or a grille between stalls where they can look out and see other animals. In some cases, a stallion may be kept with or next to a gelding or a nonhorse companion animal such as a goat, a gelded donkey, a cat, or other creature.

Properly trained stallions can live and work close to mares and to one another. Examples include the Lipizzan stallions of the Spanish Riding School in Vienna, Austria, where the entire group of stallions live part-time in a bachelor herd as young colts, then are stabled, train, perform, and travel worldwide as adults with few if any management problems. However, even stallions who are unfamiliar with each other can work safely in reasonable proximity if properly trained; the vast majority of Thoroughbred horses on the racetrack are stallions, as are many equine athletes in other forms of competition. Stallions are often shown together in the same ring at horse shows, particularly in halter classes where their conformation is evaluated. In horse show performance competition, stallions and mares often compete in the same arena with one another, particularly in Western and English "pleasure"-type classes where horses are worked as a group. Overall, stallions can be trained to keep focused on work and maybe brilliant performers if properly handled.[19]

A breeding stallion is more apt to present challenging behavior to a human handler than one who has not bred mares, and stallions may be more difficult to handle in spring and summer, during the breeding season, than during the fall and winter. However, some stallions are used for both equestrian uses and for breeding at the same general time of year. Though compromises may need to be made in expectations for both athletic performance and fertility rate, well-trained stallions with good temperaments can be taught that breeding behavior is only allowed in a certain area, or with certain cues, equipment, or with a particular handler.[20][21] However, some stallions lack the temperament to focus on work if also breeding mares in the same general time period, and therefore are taken out of competition either temporarily or permanently to be used for breeding. When permitted by a breed registry, use of artificial insemination is another technique that may reduce behavior problems in stallions.

Attitudes toward stallions vary between different parts of the world. In some parts of the world, the practice of gelding is not widespread and stallions are common. In other places, most males are gelded and only a few stallions are kept as breeding stock.Horse breeders who produce purebred bloodstock often recommend that no more than the top 10 percent of all males be allowed to reproduce, to continually improve a given breed of horse.

People sometimes have inaccurate beliefs about stallions, both positive and negative. Some beliefs are that stallions are always mean and vicious or uncontrollable, other beliefs are that misbehaving stallions should be allowed to misbehave because they are being "natural", "spirited" or "noble." In some cases, fed by movies and fictional depictions of horses in literature, some people believe a stallion can bond to a single human individual to the exclusion of all others. However, like many other misconceptions, there is only partial truth to these beliefs. Some, though not all stallions can be vicious or hard to handle, occasionally due to genetics, but usually due to improper training. Others are very well-trained and have excellent manners. Misbehaving stallions may look pretty or be exhibiting instinctive behavior, but it can still become dangerous if not corrected. Some stallions do behave better for some people than others, but that can be true of some mares and geldings, as well.

In some parts of Asia and the Middle East, the riding of stallions is widespread, especially among male riders. The gelding of stallions is unusual, viewed culturally as either unnecessary or unnatural. In areas where gelding is not widely practised, stallions are still not needed in numbers as great as mares, and so many will be culled, either sold for horsemeat or simply sold to traders who will take them outside the area. Of those that remain, many will not be used for breeding purposes.

In Europe, Australia, and the Americas, keeping stallions is less common, primarily confined to purebred animals that are usually trained and placed into competition to test their quality as future breeding stock. The majority of stallions are gelded at an early age and then trained for use as everyday working or riding animals.

If a stallion is not to be used for breeding, gelding the male horse will allow it to live full-time in a herd with both males and females, reduce aggressive or disruptive behavior, and allow the horse to be around other animals without being seriously distracted.[22] If a horse is not to be used for breeding, it can be gelded prior to reaching sexual maturity. A horse gelded young may grow taller[22] and behave better if this is done.[23] Older stallions that are sterile or otherwise no longer used for breeding may also be gelded and will exhibit calmer behavior, even if previously used for breeding. However, they are more likely to continue stallion-like behaviors than horses gelded at a younger age, especially if they have been used as a breeding stallion. Modern surgical techniques allow castration to be performed on a horse of almost any age with relatively few risks.[24]

In most cases, particularly in modern industrialized cultures, a male horse that is not of sufficient quality to be used for breeding will have a happier life without having to deal with the instinctive, hormone-driven behaviors that come with being left intact. Geldings are safer to handle and present fewer management problems.[23] They are also more widely accepted. Many boarding stables will refuse clients with stallions or charge considerably more money to keep them. Some types of equestrian activity, such as events involving children, or clubs that sponsor purely recreational events such as trail riding, may not permit stallions to participate.[citation needed]

However, just as some pet owners may have conflicting emotions about neutering a male dog or cat, some stallion owners may be unsure about gelding a stallion. One branch of the animal rights community maintains that castration is mutilation and damaging to the animal's psyche.[25]

A ridgling or "rig" is a cryptorchid, a stallion which has one or both testicles undescended. If both testicles are not descended, the horse may appear to be a gelding, but will still behave like a stallion. A gelding that displays stallion-like behaviors is sometimes called a "false rig".[26] In many cases, ridglings are infertile, or have fertility levels that are significantly reduced. The condition is most easily corrected by gelding the horse. A more complex and costly surgical procedure can sometimes correct the condition and restore the animal's fertility, though it is only cost-effective for a horse that has very high potential as a breeding stallion. This surgery generally removes the non-descended testicle, leaving the descended testicle, and creating a horse known as a monorchid stallion. Keeping cryptorchids or surgically-created monorchids as breeding stallions is controversial, as the condition is at least partially genetic and some handlers claim that cryptorchids tend to have greater levels of behavioral problems than normal stallions.[27][28]

Term for a male horse that has not been castrated

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Stallion - Wikipedia

-A curious adult from CaliforniaAugust 6, 2004What a fun question! This sort of thing has been bothering me too lately. The usual statistic is that all people are 99.9% the same. But is that true for men and women?And what about our similarity to other animals? We are really only about 80% the same as a mouse at the genetic level so men and women are clearly more similar to each other than to mice. But what about chimpanzees? If people really are 98.7% the same as a chimpanzee, are male chimpanzees closer genetically to men than men are to women? As you know, men have an X and a Y chromosome and women have two X chromosomes. So besides the usual 0.1% (or 3.2 million base pair) difference between people, men and women differ by the presence of the Y chromosome.The Y chromosome is a tiny thing; it is about 59 million base pairs long and has only 78 genes. If we look at base pairs, the difference between men and women would be 59 million divided by 3.2 billion or about 1.8%. This translates to men and women being 98.2% the same.Men and women are actually a bit more similar as the Y chromosome has about 5% of its DNA sequences in common with the X chromosome. This would change the number to 98.4% the same.If the 98.7% number for chimp-human similarity is right, then by this measure, men and women are less alike than are female chimps and women. (More recent data suggests that chimps may be 95% instead of 98.7% the same, but this is still up in the air.) Now if we look at the gene level instead of at the base pair level, men and women become much more similar. If we assume 30,000 total genes, then men and women are about 99.7% the same instead of 98.4%. (I haven't been able to find a good number for how many genes chimpanzees and humans share.)So is the bottom line that men and male chimps have more in common than men and women? Of course not. If we take a closer look, we see some of the dangers of looking at raw percentages instead of individual changes.Another way to think about this is the 55 million or so differences between men and women are all concentrated on one chromosome and 78 genes. For chimps, the 42-150 million differences are spread out all over the chromosomes over many, many more genes.In other words, while the quantity of changes may be the same, the quality is different. Even though we share most of our genes with a chimpanzee, lots of the chimp's genes have changed in ways not seen in people. These changes make a chimp a chimp and a human a human.Some of the products of these changed genes in a chimp now do different things, or do things differently, do them in different places, do them more strongly or weakly, or even do nothing at all. It only takes a single DNA change to make a gene stop working and there are millions and millions of differences between you and a chimp. What all of this means is that in essence, chimps have many more "different" genes than the 78 different ones between men and women even though the % difference at the DNA level may be comparable. So, even if it may not seem like it sometimes, your brother has more in common with you than with a chimp.

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Understanding Genetics - genetics.thetech.org

Genetictraitscan be passed from parent to child in different ways. As you will see, people can carry agenebut not be affected directly by it themselves. These patterns help to explain why a condition can seem to skip a generation or be more common in boys than in girls. Making a family health portrait, as described inHow Do I Collect My Family History?, can help to uncover these patterns.

Ourgenesare grouped into collections calledchromosomes. Most people have 46 chromosomes, in 23 pairs. One of the pairs is the sex chromosomes, called X and Y. Your sex chromosomes carry the genes that make you male or female. Women have two X chromosomes, and men have an X and a Y. The rest of your chromosomes are calledautosomalchromosomes. Let's see what happens when you have a gene that does not work the way it is supposed to on these chromosomes.

Autosomal Inheritance Patterns

Autosomal dominant

Autosomal dominant means that only one copy of the gene that does not work correctly is needed for someone to have the condition.

If one parent has an autosomal dominant condition, they have one functional copyof the gene and one copy that does not work properly. If the other parent has two copies of the gene that work correctly:

Autosomal dominant conditions, such as Huntingtons disease, affect males and females equally.

Autosomal recessive means that a person needs two copies of a gene that do not work properly to have the condition. In this pattern, people with one working copy of the gene and one copy of the gene that does not function correctly are called carriers. Carriers do not have any signs or symptoms of the condition, but they can still pass on the gene that does not function properly to their children. Usually, parents of children with anautosomal recessivecondition are carriers.

If both parents are carriers of a condition:

Autosomal recessive conditions, such as cystic fibrosis, affect males and females equally.

Your sex chromosomes carry the genes that make you a male or female. A female has two X chromosomes. A male has oneX chromosomeand oneY chromosome. If a gene for a condition is carried on the sex chromosomes, we say it is X-linked. X-linked patterns are not as simple as autosomal patterns, because they show up differently in males and females.

X-linked dominantinheritanceoccurs when a gene that does not work correctly on a single X-chromosome results in a condition. Conditions caused by X-linked dominance are rare, and the same condition can vary considerably in severity, especially among women.

The odds of passing down a condition that is X-linked dominant are different depending on whether the mother or father has the gene that does not function properly and on the sex of the child.

If a father has the condition:

If a mother has one working copy of the gene and one copy of the gene that does not work correctly:

Males are often more seriously affected than females by disorders inherited through X-linked dominance. Sometimes, even if a female inherits the gene change on one of her X chromosomes, she will not show symptoms or her symptoms will be less severe. It is thought that if a female has a working copy of the gene on one X-chromosome in addition to the altered copy on the other X-chromosome, the effects of the condition may be dampened. This has led some scientists to suggest that X-linked inheritance should not be described in terms of dominant and recessive, but rather simply be explained as X-linked inheritance.

Incontinentia pigmentiis an X-linked dominantdisorderthat affects multiple systems, but especially the skin.

X-linked recessive means that if there is one working copy of the gene, a person will not have the condition. The gene for these conditions is on the X chromosome. X-linked recessive conditions affect males more often than females. If a male has a copy of the gene that does not function the way it should on his only X chromosome, then he will be affected by the condition.

Some forms of hemophilia are X-linked recessive conditions.

If a father has an X-linked recessive condition:

If a female has two copies of the gene that do not function correctly, then she will be affected by the condition. If she has a working copy on one X chromosome and a copy of the gene that does not work the way it should on her other X chromosome, then she is called a carrier. Carriers are not affected by the condition, but they can still pass the gene that does not work correctly on to their children.

If a mother has an X-linked recessive condition, then she has two copies of the gene that do not function properly:

If a mother is a carrier of an X-linked recessive condition, she has one functional copy of the gene and one copy that does not function correctly:

If the mother is a carrier and the father has the condition, then there is a 1 in 2 chance (50%) that a daughter would be affected. She would always get the gene that does not work properly from her father, but she might get a working gene from her mother.

Most of our genes are stored in our chromosomes, which sit in each cells headquartersthe nucleus. We also have some genes in small structures in the cell called mitochondria. Mitochondria are sometimes called the power plants of the cell: they work on molecules to make them ready to give us the energy we need for our body functions. The mitochondrial genes always pass from the mother to the child. Fathers get their mitochondrial genes from their mothers, and do not pass them to their children.

Mitochondrial inheritance, also called maternal inheritance, refers to genes in the mitochondria. Although these conditions affect both males and females, only mothers pass mitochondria on to their children.

Diabetes mellitus and deafness, a rare form of diabetes, follows the mitochondrial inheritance pattern.

Check outGenetics Home Referencefor more about genetic conditions and inheritance.

See more here:
Main Inheritance Patterns | Genes in Life

WANTED! Gay Men with a Gay Brother, reads the banner. Its held aloft by Dr Alan Sanders and a group of colleagues from NorthShore University near Chicago who are attending a gay pride festival. Theyre recruiting volunteers for a groundbreaking study that sets out to answer fundamental questions about who we are.

Were trying to locate genes that may influence variation in male sexual orientation, Sanders says. Volunteers from over 700 families responded. Researchers asked them questions about their sexuality, the size and structure of their families, and took DNA samples. Sanders is now analysing that data and the results could tell us once and for all whether theres such a thing as a gay gene.

The people participating in our study are interested in contributing to this kind of scientific knowledge and want to understand at least part of how they came to be the way they are, Sanders says.

The search for gay genes goes back to 1993, when a US team led by Dr Dean Hamer described a region of DNA located on the X chromosome called Xq28. The region also goes by another name: GAY-1, a genetic marker linked to male homosexuality.

The discovery caused Hamer to be attacked from all sides. Conservative, right-wing people hated it because they felt that it was saying that being gay is like being black, that it was in-born, that it would somehow excuse gay people or give them more rights, says Hamer. On the other hand, gay people hated it too because, at that time, there were fears that the discovery would be misused to abort gay babies and wipe gay people off the face of the Earth.

Although these fears remain, in recent years the search for gay genes has become more accepted by the gay community, in no small part because a biological explanation wouldundermine arguments that being gay is a social or lifestyle choice. Conservative attitudes remain unchanged, however. They continue to be vehemently opposed to any notion that homosexuality is something natural, says Hamer.

Despite their objections, theres a lot of evidence that homosexuality has a biological basis. While there hasnt been much research on lesbians, there has been on gay men. For instance, identical twin brothers (siblings derived from the same fertilised egg) are more likely to both be gay than fraternal twins (twins that develop from separate eggs). The fact that identical twins have the same DNA and fraternal twins share 50 per cent suggests that male homosexuality is hereditary.

It was scrutinising family trees to see how homosexuality is inherited that led Hamer to the discovery of Xq28. Now chief of the gene structure and regulation section at the US National Cancer Institute, his study revealed a curious pattern: gay men tended to have more gay uncles and gay male cousins on their mothers side of the family than on their fathers.

For geneticists thats fascinating because it suggests it could be due to X chromosome linkage those types of traits tend to run on the female side for males, says Hamer. This is because males inherit their X chromosome from their mother.

To track down the DNA region linked to the gay trait, Hamer used a technique called linkage mapping, an approach that lets geneticists find a gene even when they dont know what it does or where its located. Linkage mapping works because close relatives like brothers share not only a particular trait, such as homosexuality, but also the genes underlying the trait. When comparing bits of DNA from two brothers, the sequences will, on average, be the same 50 per cent of the time. So, if you study many pairs of gay brothers and find a DNA region thats the same in more than 50 per cent of cases, its likely to be linked to homosexuality. In this case, Hamer compared the X chromosomes from 40 pairs of gay brothers, and Xq28 stood out.

Inheriting the gay version of Xq28 wont necessarily make you homosexual. Our studies showed that it significantly increased the odds of being gay, but it was not determinative, says Hamer. Many people who are gay dont have any history of homosexuality in their families. He points out that some heterosexual men in his 1993 study also had the so-called gay gene. A subsequent study in 1999 failed to replicate Hamers results and other researchers are sceptical that Xq28 is linked to homosexuality at all.

Many scientists believe that exposure to hormones during pregnancy heavily influences sexuality. Hormones are chemical messengers, released by certain cells to affect the growth and development of other cells in the body. During pre-natal development, for example, the sex organs in a foetus can recognise testosterone, which will switch on genes to make it male.

Aside from a few superficial differences (among them penis and ring-finger length both longer in homosexuals), gay and straight mens bodies appear the same. The exception is homosexual mens brains, which show remarkable similarities to the brains of heterosexual women, suggesting that sexual orientation depends on the effect hormones have on the developing brain.

But these two factors only go so far in explaining how homosexuality develops. People assume that all of the biological influence on sexual orientation is either genes or hormones, says sexologist Ray Blanchard from the University of Toronto. They might account for the lions share of variance in sexual orientation, but it looks like theres some other bit that requires a third biological mechanism.

In 1996 Blanchard and Professor Tony Bogaert revealed a peculiar phenomenon: the more older brothers a boy has, the greater their chances of being homosexual. This fraternal birth order effect meant that each subsequent brother increases the odds of being gay by 33 per cent. An only child has a two per cent chance, but with 10 brothers the odds are over 20 per cent. But why the increasing odds? Blanchard believes its related to how a mothers body protects itself when pregnant with a son.

Theres only one system in the mother that would have the memory to know how many male foetuses shes previously carried: the immune system, says Professor Blanchard. According to his theory, a mothers immune system keeps track of the number of sons shes already had, producing antibodies to protect her against male-specific proteins entering her bloodstream, which often occurs during childbirth. As the mothers level of immunisation increases with each son, so too do the chances of variation from typical sexual orientation as, in theory, the mothers antibodies could cross the placenta and neutralise proteins that her son needs for normal sexual development.

Many of these male-specific proteins are found on the Y chromosome, DNA thats foreign to females. A lot of male-specific proteins are preferentially expressed in the testes and have a crucial role in sperm development, says Blanchard. Some are expressed in the foetal brain for reasons that no-one has established, but you wouldnt expect them to be expressed without a reason.

Blanchard believes that homosexuality is 100 per cent biological, and estimates that the fraternal birth order effect accounts for 15-30 per cent of gay men in the population. So what explains the rest?

Professor Andrea Camperio Ciani at the University of Padova in Italy has tested various hypotheses by studying 100 families of gay men. Not only did he replicate Blanchards birth order effect, he also detected inheritance of homosexuality on the mothers side, supporting Hamers idea of a gay gene on chromosome X. The maternal inheritance effect seems most important too.

Genetics explains 20-25 per cent for the moment, says Camperio Ciani. The rest is unknown. A part is environment; a part can be other genetic elements that we cannot perceive with our study. In principle, the genetic component might even be the Xq28 region.

Regardless of which regions of DNA are linked to homosexuality, the very existence of gay genes creates a Darwinian paradox. How would genes that cause homosexuality pass from one generation to the next, given that gay people reproduce less than heterosexuals? Natural selection opposes anything that might cause even a small reduction in the number of offspring you produce, so a gay trait would soon disappear from the gene pool. If you carry a trait that reduces your fecundity [the number of offspring you produce] by 10 per cent, in seven to eight generations your trait and all your descendents disappear, says Camperio Ciani.

The paradox was finally resolved by his 15-year-old daughter. After Camperio Ciani described the observed patterns in pedigrees of homosexuality the effects of maternal inheritance and birth order his daughter suggested that he re-check his data to see if the female relatives of gay men had more children on the mothers side. When Camperio Ciani went back to the lab, thats exactly what he found. Mothers and aunts on the maternal line of homosexuals had around one-fifth to one-fourth more kids than the heterosexual comparison, and also than the paternal line.

He thinks that the evolution of homosexuality is driven by a process called sexually antagonistic selection. Its where a genetic factor confers an advantage when expressed in one sex, but incurs an evolutionary cost in the other. In this instance, the gay genes dont exist to make men homosexual, instead theyre a consequence of fertility factors that help women reproduce.

Nipples are another example of a sexually antagonistic trait: theyre needed for feeding babies, but developing nipples in men is a waste of the bodys resources and allow errors leading to breast cancer.

Even if Camperio Cianis fecundity factors are the same as Hamers gay genes, it doesnt tell us what the specific genes actually do. Hamer speculates the genes might boost the size or connections from parts of the brain used in reproduction such as the hypothalamus to make people more libidinous.

Alan Sanderss study at NorthShore University could finally reveal the identity and function of gay genes. Sanders, director of the Behavior Genetics Unit, is comparing DNA from gay brothers to find shared genes that underlie sexual orientation. Hes initially using linkage mapping to find candidate regions. The large sample size over 700 families provides huge statistical power for detecting regions significantly linked to homosexuality. Sanders will then use sequences from databases like the Human Genome Project to pinpoint which genes are in these regions.

So what happens if gay genes are found? While they may confirm the idea that homosexuality has a biological basis, many people fear that the results could be used to discriminate against gay people. It is a valid concern, says Sanders. People we talked to at gay pride festivals have designer-baby kind of worries a genetic test employed in a pre-natal way, or for employment and insurance discrimination, maybe in the military too. Its not just an issue in sexual orientation, but intelligence or disease screening .

A test for gay genes also has a flipside: homosexual couples might exploit reproductive technology to have gay kids. This has been a huge debate in other areas, like deaf parents wanting to have deaf children, says Hamer, who has fathered a daughter with a woman from a lesbian couple. One of them said, If I had my choice, Id select the sexual orientation of my child. But this is all theoretical for now, as its not actually happening yet.

Genes that influence our sexual orientation further fuel the debate over what makes us who we are. For Hamer at least, sexual orientation is determined at birth. Its mostly biological, he says. The way a person acts is altered by culture, society and individual choice, but thats a different issue than the underlying deep-seated orientation.

Link:
Gay genetics | Science Focus