What is Multiple Alleles ?
Obviously, diploid organisms, such as humans can have only two different alleles for a certain gene locus. However, new alleles are continuously being produced by mutations, so that within a species there may be multiple alleles for every gene.
The ABO blood type in humans is a familiar example of multiple alleles for a given trait. These genes control the synthesis of glycoproteins that protrude from the surface of red blood cells. Alleles for types A and B (IA and IB) are codominant. The allele IO for type O blood produces no glycoproteins at all, so both type A and type B are dominant over type O.
Individuals are either AA or AO, BB or BO, AB, or O. Alleles AO are classified type A, and BO are type B. Type O is always OO. Type B individuals have proteins called type A antibodies that cause Type A blood to clump in their blood. Type A individuals have type B antibodies, and Type O individuals have both type A and type B antibodies. Because they have no antibodies to either type A or type B glycoproteins and can receive blood donations from each of the blood groups, persons with type AB blood are called universal recipients. Persons with type O blood that does not contain glycoprotein antigens are universal donors.