Organisms of many species are specialized into male and female varieties, each known as a sex.[1]Sexual reproduction involves the combining and mixing of genetic traits: specialized cells known as gametes combine to form offspring that inherit traits from each parent. Gametes can be identical in form and function (known as isogamy), but in many cases an asymmetry has evolved such that two sex-specific types of gametes (heterogametes) exist (known as anisogamy). By definition, male gametes are small, motile, and optimized to transport their genetic information over a distance, while female gametes are large, non-motile and contain the nutrients necessary for the early development of the young organism. Among humans and other mammals, males typically carry XY chromosomes, whereas females typically carry XX chromosomes, which are a part of the XY sex-determination system.

The gametes produced by an organism determine its sex: males produce male gametes (spermatozoa, or sperm, in animals; pollen in plants) while females produce female gametes (ova, or egg cells); individual organisms which produce both male and female gametes are termed hermaphroditic. Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures[clarification needed] the sexes experience.

Sexual reproduction first probably evolved about a billion years ago within ancestral single-celled eukaryotes.[2] The reason for the evolution of sex, and the reason(s) it has survived to the present, are still matters of debate. Some of the many plausible theories include: that sex creates variation among offspring, sex helps in the spread of advantageous traits, and that sex helps in the removal of disadvantageous traits.

Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria. In addition to animals, plants, and fungi, other eukaryotes (e.g. the malaria parasite) also engage in sexual reproduction. Some bacteria use conjugation to transfer genetic material between cells; while not the same as sexual reproduction, this also results in the mixture of genetic traits.

The defining characteristic of sexual reproduction in eukaryotes is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, no third gamete is known in multicellular animals.[3][4][5]

While the evolution of sex dates to the prokaryote or early eukaryote stage,[citation needed] the origin of chromosomal sex determination may have been fairly early in eukaryotes.[citation needed] The ZW sex-determination system is shared by birds, some fish and some crustaceans. Most mammals, but also some insects (Drosophila) and plants (Ginkgo) use XY sex-determination.[citation needed]X0 sex-determination is found in certain insects.

No genes are shared between the avian ZW and mammal XY chromosomes,[6] and from a comparison between chicken and human, the Z chromosome appeared similar to the autosomal chromosome 9 in human, rather than X or Y, suggesting that the ZW and XY sex-determination systems do not share an origin, but that the sex chromosomes are derived from autosomal chromosomes of the common ancestor of birds and mammals. A paper from 2004 compared the chicken Z chromosome with platypus X chromosomes and suggested that the two systems are related.[7]

Sexual reproduction in eukaryotes is a process whereby organisms form offspring that combine genetic traits from both parents. Chromosomes are passed on from one generation to the next in this process. Each cell in the offspring has half the chromosomes of the mother and half of the father.[8] Genetic traits are contained within the deoxyribonucleic acid (DNA) of chromosomesby combining one of each type of chromosomes from each parent, an organism is formed containing a doubled set of chromosomes. This double-chromosome stage is called "diploid", while the single-chromosome stage is "haploid". Diploid organisms can, in turn, form haploid cells (gametes) that randomly contain one of each of the chromosome pairs, via meiosis.[9] Meiosis also involves a stage of chromosomal crossover, in which regions of DNA are exchanged between matched types of chromosomes, to form a new pair of mixed chromosomes. Crossing over and fertilization (the recombining of single sets of chromosomes to make a new diploid) result in the new organism containing a different set of genetic traits from either parent.

In many organisms, the haploid stage has been reduced to just gametes specialized to recombine and form a new diploid organism; in others, the gametes are capable of undergoing cell division to produce multicellular haploid organisms. In either case, gametes may be externally similar, particularly in size (isogamy), or may have evolved an asymmetry such that the gametes are different in size and other aspects (anisogamy).[10] By convention, the larger gamete (called an ovum, or egg cell) is considered female, while the smaller gamete (called a spermatozoon, or sperm cell) is considered male. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male. An individual that produces both types of gametes is a hermaphrodite; in some cases hermaphrodites are able to self-fertilize and produce offspring on their own, without a second organism.[11]

Most sexually reproducing animals spend their lives as diploid organisms, with the haploid stage reduced to single cell gametes.[12] The gametes of animals have male and female formsspermatozoa and egg cells. These gametes combine to form embryos which develop into a new organism.

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