How are ABO blood groups genetically transmitted?

The ABO blood group is an example of codominance and multiple alleles. Unlike other characteristics which are determined by 2 alleles, blood group is determined by 3 alleles. The allele I^A corresponds to blood group A (genotype I^A I^A ) and the allele I^B corresponds to blood group B (genotype I^B I^B) . Both these are dominant so if I^A and I^Bare present together they form blood group AB (genotype I^AI^B ). The allele i is recessive to both I^A and I^B and hence if you have genotype IAi you will have blood group A and if you have genotype IAi you will have blood group A and if you have the genotype I^B i you will have blood group B. However, if you have the genotype ii then you are homozygous for i, and you will have blood group O.