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Alleles are alternative forms of a gene, and they are responsible for differences in phenotypic expression of a given trait (e.g., brown eyes versus green eyes). A gene for which at least two alleles exist is said to be polymorphic. Instances in which a particular gene may exist in three or more allelic forms are known as multiple allele conditions. It is important to note that while multiple alleles occur and are maintained within a population, any individual possesses only two such alleles (at equivalent loci on homologous chromosomes).

Examples Of Multiple Alleles

Two human examples of multiple-allele genes are the gene of the ABO blood group system, and the human-leukocyte-associated antigen (HLA) genes. (https://cdn.jwplayer.com/previews/nXuiOHB6)

The ABO system (https://www.encyclopedia.com/medicine/anatomy-and-physiology/anatomy-and-physiology/abo-system) in humans is controlled by three alleles, usually referred to as I ^A , I ^B , and I ^O (the "I" stands for isohaemagglutinin). I ^A and I ^B are codominant and produce type A and type B antigens, respectively, which migrate to the surface of red blood cells, while I ^O is the recessive allele and produces no antigen.

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