Allele

An allele is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.

Alleles can differ at a single position through single-nucleotide polymorphisms, but they can also have insertions and deletions of up to several thousand base pairs. Most alleles result in little or no change in the characteristics of an individual organism but sometimes different alleles can result in different observable phenotypic traits such as antibiotic resistance in bacteria, developmental mutations in fruit flies, and genetic diseases in humans.

Nearly all multicellular organisms have two sets of chromosomes at some point in their biological life cycle; that is, they are diploid. For a given locus, if the two chromosomes contain the same allele, they, and the organism, are homozygous with respect to that allele. If the alleles are different, they, and the organism, are heterozygous with respect to those alleles. A notable example of this is Gregor Mendel's discovery that the white and purple flower colors in pea plants were the result of a single gene with two alleles. Mendel’s discovery of what are now known as alleles resulted in three laws that help understand how alleles are passed on to progeny. The law of dominance states that alleles that are inherited can be either dominant or recessive, and that the dominant allele masks the effect of the recessive allele. The law of segregation describes how the 2 parental alleles separate during the formation of gametes, so that diploid progeny end up with 2 alleles, one from each parent, for every inherited gene. The law of independent assortment takes place during metaphase I of meiosis when the homologous chromosomes line up at the center of the cell randomly and independently of each other. This results in gametes that have unique combinations of alleles. These laws summarize Mendelian inheritance which describes how alleles are transmitted.

Popular definitions of 'allele' typically refer only to different alleles within genes. For example, the ABO blood grouping is controlled by the ABO gene, which has six common alleles (variants). In population genetics, nearly every living human's phenotype for the ABO gene is some combination of just these six alleles.

Etymology

The word "allele" is a short form of "allelomorph" ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders in the early 1900s), which was used in the early days of genetics to describe variant forms of a gene detected in different phenotypes and identified to cause the differences between them. It derives from the Greek prefix ἀλληλο-, allelo-, meaning "mutual", "reciprocal", or "each other", which itself is related to the Greek adjective ἄλλος, allos (cognate with Latin alius), meaning "other".

Alleles that lead to dominant or recessive phenotypes

In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive, according to which of the two homozygous phenotypes the heterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele expressed is the one that leads to the "dominant" phenotype, and the other allele is said to be "recessive". In the case of a heterozygote, the dominant allele will mask the effect of the recessive allele, giving a dominant phenotype. The phenotype of recessive alleles are observable when an organism is homozygous recessive for a gene. The degree and pattern of dominance varies among loci. This type of interaction was first formally-described by Gregor Mendel. However, many traits defy this simple categorization and the phenotypes are modelled by co-dominance and polygenic inheritance.

The term "wild type" allele is sometimes used to describe an allele that is thought to contribute to the typical phenotypic character as seen in "wild" populations of organisms, such as fruit flies (Drosophila melanogaster). Such a "wild type" allele was historically regarded as leading to a dominant (overpowering – always expressed), common, and normal phenotype, in contrast to "mutant" alleles that lead to recessive, rare, and frequently deleterious phenotypes. It was formerly thought that most individuals were homozygous for the "wild type" allele at most gene loci, and that any alternative "mutant" allele was found in homozygous form in a small minority of "affected" individuals, often as genetic diseases, and more frequently in heterozygous form in "carriers" for the mutant allele. It is now appreciated that most or all gene loci are highly polymorphic, with multiple alleles, whose frequencies vary from population to population, and that a great deal of genetic variation is hidden in the form of alleles that do not produce obvious phenotypic differences. Wild type alleles are often denoted by a superscript plus sign (i.e., p⁺ for an allele p).

Multiple alleles