ABO blood group in humans is controlled by three multiple alleles IA,IB and i. There are two types of antigens present in the surface coating of red blood cells - A and B.The IA and IB alleles produce enzyme glycosyltransferase for synthesis of sugars. The sugars are attached to lipids and produce glycolipids. These glycolipids then associate with membrane of RBC to form blood group antigens. Allele i does not produce any enzyme/antigen. Both IA and IB are codominant (i.e., able to express themselves in presence of each other) but are dominant over i.