ABO blood group system
In the ABO blood group system, there are three alleles: "i", "IA", and "IB". As both "IA" and "IB" are dominant over "i", only "ii" people have type O blood. Individuals with "IAIA" or "IAi" have type A blood, and individuals with "IBIB" or "IBi" have type B. Those with "IAIB" have type AB.