Most alleles observed result in little or no change in the function or amount of thegene product(s) they code or regulate for. However, sometimes different alleles can result in different observablephenotypic traits, such as differentpigmentation. A notable example of this isGregor Mendel's discovery that the white and purple flower colors inpea plants were the result of a single gene with two alleles.
Nearly allmulticellular organisms have two sets ofchromosomes at some point in theirbiological life cycle; that is, they arediploid. For a given locus, if the two chromosomes contain the same allele, they, and the organism, arehomozygous with respect to that allele. If the alleles are different, they, and the organism, areheterozygous with respect to those alleles.
Popular definitions of 'allele' typically refer only to different alleles within genes. For example, theABO blood grouping is controlled by theABO gene, which has six common alleles (variants). Inpopulation genetics, nearly every living human'sphenotype for the ABO gene is some combination of just these six alleles.[5][6]
The word "allele" is a short form of "allelomorph" ("other form", a word coined by British geneticistsWilliam Bateson andEdith Rebecca Saunders) in the 1900s,[7][8] which was used in the early days ofgenetics to describe variant forms of agene detected in differentphenotypes and identified to cause the differences between them. It derives from theGreek prefix ἀλληλο-,allelo-, meaning "mutual", "reciprocal", or "each other", which itself is related to the Greek adjective ἄλλος,allos (cognate withLatinalius), 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 asdominant orrecessive, according to which of the two homozygous phenotypes theheterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele expressed is the one that leads to the "dominant" phenotype,[9][10] and the other allele is said to be "recessive". The degree and pattern of dominance varies among loci. This type of interaction was first formally-described byGregor Mendel. However, many traits defy this simple categorization and the phenotypes are modelled byco-dominance andpolygenic inheritance.[11]
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 asgenetic 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 allelep).[12]
Eye color is an inherited trait influenced bymore than one gene, includingOCA2 andHERC2. The interaction of multiple genes—and the variation in these genes ("alleles") between individuals—help to determine a person's eye colorphenotype. Eye color is influenced bypigmentation of theiris and the frequency-dependence of the lightscattering by theturbid medium within thestroma of the iris.
In theABO blood group system, a person with Type A blood displays A-antigens and may have a genotype IAIA or IAi. A person with Type B blood displays B-antigens and may have the genotype IBIB or IBi. A person with Type AB blood displays both A- and B-antigens and has the genotype IAIB and a person with Type O blood, displaying neither antigen, has the genotype ii.
A population orspecies of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles (polymorphism) present, or the proportion of heterozygotes in the population. Anull allele is a gene variant that lacks the gene's normal function because it either is not expressed, or the expressed protein is inactive.
For example, at the gene locus for theABOblood typecarbohydrateantigens in humans,[13] classical genetics recognizes three alleles, IA, IB, and i, which determine compatibility ofblood transfusions. Any individual has one of six possiblegenotypes (IAIA, IAi, IBIB, IBi, IAIB, and ii) which produce one of four possiblephenotypes: "Type A" (produced by IAIA homozygous and IAi heterozygous genotypes), "Type B" (produced by IBIB homozygous and IBi heterozygous genotypes), "Type AB" produced by IAIB heterozygous genotype, and "Type O" produced by ii homozygous genotype. (It is now known that each of the A, B, and O alleles is actually a class of multiple alleles with different DNA sequences that produce proteins with identical properties: more than 70 alleles are known at the ABO locus.[14] Hence an individual with "Type A" blood may be an AO heterozygote, an AA homozygote, or an AA heterozygote with two different "A" alleles.)
The frequency of alleles in a diploid population can be used to predict the frequencies of the corresponding genotypes (seeHardy–Weinberg principle). For a simple model, with two alleles;
wherep is the frequency of one allele andq is the frequency of the alternative allele, which necessarily sum to unity. Then,p2 is the fraction of the population homozygous for the first allele, 2pq is the fraction of heterozygotes, andq2 is the fraction homozygous for the alternative allele. If the first allele is dominant to the second then the fraction of the population that will show the dominant phenotype isp2 + 2pq, and the fraction with the recessive phenotype isq2.
With three alleles:
and
In the case of multiple alleles at a diploid locus, the number of possible genotypes (G) with a number of alleles (a) is given by the expression:
Whileheritable traits are typically studied in terms of genetic alleles,epigenetic marks such asDNA methylation can be inherited at specific genomic regions in certain species, a process termedtransgenerational epigenetic inheritance. The termepiallele is used to distinguish these heritable marks from traditional alleles, which are defined bynucleotide sequence.[15] A specific class of epiallele, themetastable epialleles, has been discovered in mice and in humans which is characterized by stochastic (probabilistic) establishment of epigenetic state that can be mitotically inherited.[16][17]
The term "idiomorph", from Greek 'morphos' (form) and 'idio' (singular, unique), was introduced in 1990 in place of "allele" to denote sequences at the same locus in different strains that have no sequence similarity and probably do not share a common phylogenetic relationship. It is used mainly in the genetic research ofmycology.[18][19]
^Ogasawara K, Bannai M, Saitou N, Yabe R, Nakata K, Takenaka M, Fujisawa K, Uchikawa M, Ishikawa Y, Juji T, Tokunaga K (June 1996). "Extensive polymorphism of ABO blood group gene: three major lineages of the alleles for the common ABO phenotypes".Human Genetics.97 (6):777–83.doi:10.1007/BF02346189.PMID8641696.S2CID12076999.
^Bateson, W. and Saunders, E. R. (1902) "The facts of heredity in the light of Mendel’s discovery." Reports to the Evolution Committee of the Royal Society,I. pp. 125–160
^Hartl, Daniel L.; Elizabeth W. Jones (2005).Essential genetics: A genomics perspective (4th ed.). Jones & Bartlett Publishers. p. 600.ISBN978-0-7637-3527-2.
^"Allele".Genome.gov.Archived from the original on 28 June 2021. Retrieved3 July 2021.
^B. A. Pierce (2020).Genetics A Conceptual Approach (7 ed.). Macmillan. p. 60.ISBN978-1-319-21680-1.
^Victor A. McKusick; Cassandra L. Kniffin; Paul J. Converse; Ada Hamosh (10 November 2009)."ABO Glycosyltransferase; ABO".Online Mendelian Inheritance in Man. National Library of Medicine.Archived from the original on 24 September 2008. Retrieved24 March 2010.
^Daxinger, Lucia; Whitelaw, Emma (31 January 2012). "Understanding transgenerational epigenetic inheritance via the gametes in mammals".Nature Reviews Genetics.13 (3):153–62.doi:10.1038/nrg3188.PMID22290458.S2CID8654616.
^Rakyan, Vardhman K; Blewitt, Marnie E; Druker, Riki; Preis, Jost I; Whitelaw, Emma (July 2002). "Metastable epialleles in mammals".Trends in Genetics.18 (7):348–351.doi:10.1016/S0168-9525(02)02709-9.PMID12127774.
^Waterland, RA; Dolinoy, DC; Lin, JR; Smith, CA; Shi, X; Tahiliani, KG (September 2006). "Maternal methyl supplements increase offspring DNA methylation at Axin Fused".Genesis.44 (9):401–6.doi:10.1002/dvg.20230.PMID16868943.S2CID36938621.
