Dominance: The alleles IA and IB both are dominant over allele i as IA and IB form antigens A and B, respectively, but i does not form any antigen.Multiple allelism is the phenomenon of occurrence of a gene in more than two allelic forms on the same locus. In ABO blood group in humans, one gene I has three alleles IA, IB and IO/i.
Co-dominance is the phenomena in which both alleles express themselves when present together. We inherit any two alleles for the blood group. When the genotype is IAIB the individual has AB blood group since both IA and IB equally influence the formation of antigens A and B.
