Editing Allele (section)

==Multiple alleles==
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| footer            = [[Eye color]] is an inherited trait influenced by [[Polygene|more than one gene]], including ''[[OCA2]]'' and ''[[HERC2]]''. The interaction of multiple genes—and the variation in these genes ("alleles") between individuals—help to determine a person's eye color [[phenotype]]. Eye color is influenced by [[pigment]]ation of the [[Iris (anatomy)|iris]] and the frequency-dependence of the light [[scattering]] by the [[Turbidity|turbid]] medium within the [[Stroma of iris|stroma]] of the iris.
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| image1            = Blue eye 2.jpg
| image2            = Boje oka.jpg
| image3            = Hazel eye1.png
| image4            = Human eye with limbal ring, anterior view.jpg
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[[File:ABO blood type.svg|thumb|upright=1.45|In the [[ABO blood group system]], a person with Type A blood displays A-antigens and may have a genotype I<sup>A</sup>I<sup>A</sup> or I<sup>A</sup>i. A person with Type B blood displays B-antigens and may have the genotype I<sup>B</sup>I<sup>B</sup> or I<sup>B</sup>i. A person with Type AB blood displays both A- and B-antigens and has the genotype I<sup>A</sup>I<sup>B</sup> and a person with Type O blood, displaying neither antigen, has the genotype ii.]]

A population or [[species]] of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles ([[Genetic polymorphism|polymorphism]]) present, or the proportion of heterozygotes in the population. A [[null 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 the [[ABO]] [[blood type]] [[carbohydrate]] [[antigen]]s in humans,<ref name="OnlineMendelianInheritanceinMan"/> classical genetics recognizes three alleles, I<sup>A</sup>, I<sup>B</sup>, and i, which determine compatibility of [[blood transfusion]]s. Any individual has one of six possible [[genotype]]s (I<sup>A</sup>I<sup>A</sup>, I<sup>A</sup>i, I<sup>B</sup>I<sup>B</sup>, I<sup>B</sup>i, I<sup>A</sup>I<sup>B</sup>, and ii) which produce one of four possible [[phenotype]]s: "Type A" (produced by I<sup>A</sup>I<sup>A</sup> homozygous and I<sup>A</sup>i heterozygous genotypes), "Type B" (produced by I<sup>B</sup>I<sup>B</sup> homozygous and I<sup>B</sup>i heterozygous genotypes), "Type AB" produced by I<sup>A</sup>I<sup>B</sup> 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.<ref name="Sequence variation at the human ABO locus"/> Hence an individual with "Type A" blood may be an AO heterozygote, an AA homozygote, or an AA heterozygote with two different "A" alleles.)