The ABO blood groups in humans are expressed as the I A , I B , and i alleles. The I A allele encodes the A blood group antigen, I B encodes B, and i encodes O. Both A and B are dominant to O. If a heterozygous blood type A parent (I A i) and a heterozygous blood type B parent (I B i) mate, one quarter of their offspring will have AB blood type (I A I B ) in which both antigens are expressed equally. Therefore, ABO blood groups are an example of: a. multiple alleles and incomplete dominance b. codominance and incomplete dominance c. incomplete dominance only d. multiple alleles and codominance

ABO blood group is controlled by three alleles instead of the usual two. Hence, it is an example of multiple alleles. These three alleles are IA, IB and i. IAIA genotype leads to production of A antigen hence A blood group. IBIB genotype leads to production of B antigen hence B blood group. When they are present together (IAIB) both A and B antigens are produced resulting in AB blood group. Hence ABO blood group is also an example of codominance.

Both IA and IB are dominant over the i allele. So IAi and IBi will produce A and B blood group respectively. ii genotype leads to O genotype.