. The four blood groups of humans are determined by combination of different alleles. The alleles I^A for A group, I^B for B blood group are both dominant. Therefore person with alleles I^A and I^B have the blood group AB as both the genes I^A and I^B are co-dominant. The gene i^o when homozygous (i^o i^o) gives the blood group O. Genotype and phenotype of blood groups in humans are shown in table

Genotypes and Phenotypes of human blood groups