Any of a set of three or more alleles, or alternative states of a gene, only two of which can be present in a diploid organism.
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An allele (UK /ˈæliːl/ or US /əˈliːl/), or allel, is one of a number of alternative forms of the same gene or same genetic locus (generally a group of genes). It is the alternative form of a gene for a character producing different effects. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. However, many variations at the genetic level result in little or no observable variation.
Most multicellular organisms have two sets of chromosomes, that ...view middle of the document...
 An individual with "Type A" blood may be an AO heterozygote, an AA homozygote, or an AA heterozygote with two different ″A″ alleles.
The word "allele" is a short form of allelomorph (″other form″), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes. It derives from the Greek prefix ἀλλήλ- [″allel-″], meaning "reciprocal" or "each other", which itself is related to the Greek adjective ἄλλος (allos; cognate with Latin ″alius″), meaning "other".
Dominant and recessive alleles
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 genotypes the phenotype of the heterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele involved is said to be dominant to the other, which is said to be recessive to the former. The degree and pattern of dominance varies among loci. For a further discussion see Dominance (genetics). This type of interaction was first formally described by Gregor Mendel. However, many traits defy this simple categorization and the phenotypes are modeled by 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 dominant, common, and normal, in contrast to "mutant" alleles regarded as recessive, rare, and frequently deleterious. It was commonly 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
Allele and genotype frequencies
The frequency of alleles in a diploid population can be used to predict the frequencies of the corresponding genotypes (see Hardy-Weinberg principle). For a simple model, with two alleles:
where ″p″ is the frequency of one allele and ″q″ is the frequency of the alternative allele, which necessarily sum to unity. Then, ″p″2 is the fraction of the population homozygous for the first allele, 2 ″pq″ is the fraction of heterozygotes, and ″q″2 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 is ″p″2 + 2 ″pq″, and the fraction with the recessive phenotype is ″q″2.
With three alleles:
In the case...