This article is about the general scientific term. For the scientific journal, see Genetics (journal).

Genetics (from Ancient Greek genetikos, "genitive" and that from genesis, "origin"),[1][2][3] a discipline of biology, is the science of genes, heredity, and variation in living organisms.[4][5]

Genetics concerns the process of trait inheritance from parents to offspring, including the molecular structure and function of genes, gene behavior in the context of a cell or organism (e.g. dominance and epigenetics), gene distribution, and variation and change in populations (such as through Genome-Wide Association Studies). Given that genes are universal to living organisms, genetics can be applied to the study of all living systems; including bacteria, plants, animals, and humans. The observation that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding[citation needed]. The modern science of genetics, seeking to understand this process, began with the work of Gregor Mendel in the mid-19th century.[6]

Mendel observed that organisms inherit traits by way of discrete 'units of inheritance.' This term, still used today, is a somewhat ambiguous definition of a gene. A more modern working definition of a gene is a portion (or sequence) of DNA that codes for a known cellular function. This portion of DNA is variable, it may be small or large, have a few subregions or many subregions. The word 'Gene' refers to portions of DNA that are required for a single cellular process or single function, more than the word refers to a single tangible item. A quick idiom that is often used (but not always true) is 'one gene, one protein' meaning a singular gene codes for a singular protein type in a cell. Another analogy is that a 'gene' is like a 'sentence' and 'letters' are like 'nucleotides.' A series of nucleotides can be put together without forming a gene (non-coding regions of DNA), like a string of letters can be put together without forming a sentence (babble). Nonetheless, all sentences must have letters, like all genes must have a nucleotides.

The sequence of nucleotides in a gene is read and translated by a cell to produce a chain of amino acids which in turn spontaneously fold into proteins. The order of amino acids in a protein corresponds to the order of nucleotides in the gene. This relationship between nucleotide sequence and amino acid sequence is known as the genetic code. The amino acids in a protein determine how it folds into its unique three-dimensional shape; a structure that is ultimately responsible for the proteins function. Proteins carry out many of the functions needed for cells to live. A change to the DNA in a gene can change a protein's amino acid sequence, thereby changing its shape and function, rendering the protein ineffective or even malignant (see: sickle cell anemia). When a gene change occurs, it is referred to as a mutation.

Although genetics plays a large role in the appearance and behavior of organisms, it is a combination of genetics with the organisms' experiences (aka. environment) that determines the ultimate outcome. Genes may be activated or inactivated, which is determined by a cell's or organism's environment, intracellularly and/or extracellularly. For example, while genes play a role in determining an organism's size, the nutrition and health it experiences after inception also have a large effect.

Although the science of genetics began with the applied and theoretical work of Gregor Mendel in the mid-19th century, other theories of inheritance preceded Mendel. A popular theory during Mendel's time was the concept of blending inheritance: the idea that individuals inherit a smooth blend of traits from their parents.[citation needed] Mendel's work provided examples where traits were definitely not blended after hybridization, showing that traits are produced by combinations of distinct genes rather than a continuous blend. Blending of traits in the progeny is now explained by the action of multiple genes with quantitative effects. Another theory that had some support at that time was the inheritance of acquired characteristics: the belief that individuals inherit traits strengthened by their parents. This theory (commonly associated with Jean-Baptiste Lamarck) is now known to be wrongthe experiences of individuals do not affect the genes they pass to their children,[7] although evidence in the field of epigenetics has revived some aspects of Lamarck's theory.[8] Other theories included the pangenesis of Charles Darwin (which had both acquired and inherited aspects) and Francis Galton's reformulation of pangenesis as both particulate and inherited.[9]

Modern genetics started with Gregor Johann Mendel, a German-Czech Augustinian monk and scientist who studied the nature of inheritance in plants. In his paper "Versuche ber Pflanzenhybriden" ("Experiments on Plant Hybridization"), presented in 1865 to the Naturforschender Verein (Society for Research in Nature) in Brnn, Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically.[10] Although this pattern of inheritance could only be observed for a few traits, Mendel's work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios.

The importance of Mendel's work did not gain wide understanding until the 1890s, after his death, when other scientists working on similar problems re-discovered his research. William Bateson, a proponent of Mendel's work, coined the word genetics in 1905.[11][12] (The adjective genetic, derived from the Greek word genesis, "origin", predates the noun and was first used in a biological sense in 1860.)[13] Bateson popularized the usage of the word genetics to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London, England, in 1906.[14]

After the rediscovery of Mendel's work, scientists tried to determine which molecules in the cell were responsible for inheritance. In 1911, Thomas Hunt Morgan argued that genes are on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies.[15] In 1913, his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome.[16]

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