Multiple Alleles | ABO blood group system | Mutations | Genetics

Multiple alleles:
Three or more alternative forms of a gene (allele) that can occupy the same locus However, only two of the alleles can be present in a single organism. For example, the ABO system of blood group is controlled by three alleles, only two of which are present in an individual.
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.
The ABO system in humans is controlled by three alleles, usually referred to as IA, IB, and IO (the "I" stands for isohaemagglutinin). IA and IB are codominant and produce type A and type B antigens, respectively, which migrate to the surface of red blood cells, while IO is the recessive allele and produces no antigen. The blood mdifferent possible genotypes are summarized in the following table.
GenotypeBlood GroupIA IAAIA IOAIB IBBIB IOBIA IBABIO IOO
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