The abo blood groups in humans are expressed as the ia, ib, and i alleles. the ia allele encodes the a blood group antigen, ib encodes b, and i encodes o. both a and b are dominant to o. if a heterozygous blood type a parent (iai) and a heterozygous blood type b parent (ibi) mate, one quarter of their offspring will have ab blood type (iaib) in which both antigens are expressed equally. therefore, abo blood groups are an example of _______.