Multiple alleles

Multiple alleles

Multiple alleles are two or more alternative forms of a gene controlling a particular characteristic, of which any two may occupy the same gene loci on homologous chromosomes.

An example of such multiple allele is provided by the alleles controlling the ABO blood group system in humans. The ABO system is controlled by three alleles generally referred to as I^A, I^B, and I^O.

The I^A allele is responsible for production of type A antigens in the person’s red blood cells, and the I^B allele for type B antigen. The I⁰ produces neither antigen.

But only two of the three alleles can be present at any one time, an individual may thus, possess any of the following six genotype; I^AI^A, I^AI⁰, I^BI^B, I^BI^O, I^AI^B, and I^OI^O.

I^A and I^B show equal dominance with respect to one another [i.e. they are codominant] but each is dominant to I^O thus;

• A person belongs to blood group A has genotype I^AI^A or I^AI⁰
• A person belongs to blood group B has genotype I^BI^B or I^BI^O
• A person belong to blood group AB has genotype I^AI^B

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