For the ABO system of blood groups, allele IA produces N acetylgalactosamyl transferase enzyme which recognizes H antigen present in RBC membrane and adds N-acetylgalactosamine to sugar part of H antigen to form A antigen. The allele I^{B }produces galactosyl transferase enzyme which adds galactose o the sugar part of H antigen to form B antigen. Allele 'i' does not produce any sugar or antigen. When both IAand IB are present in a person, both enzymes thus both antigens A and B are produced. This is because of co-dominance. These antigens determine the type of AB blood group.