Page "Allele" Paragraph 3

For example, at the gene locus for the ABO blood type carbohydrate antigens in humans, classical genetics recognizes three alleles, I < sup > A </ sup >, I < sup > B </ sup >, and I < sup > O </ sup >, that determine compatibility of blood transfusions.

Any individual has one of six possible genotypes ( AA, AO, BB, BO, AB, and OO ) that produce one of four possible phenotypes: " A " ( produced by AA homozygous and AO heterozygous genotypes ), " B " ( produced by BB homozygous and BO heterozygous genotypes ), " AB " heterozygotes, and " O " homozygotes.

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.

An individual with " Type A " blood may be an AO heterozygote, an AA homozygote, or an A ' A heterozygote with two different ' A ' alleles.