Final answer:
ABO blood groups are determined by multiple alleles and codominance.
Step-by-step explanation:
The ABO blood types in humans are determined by multiple alleles. The three common alleles for the ABO blood group system are IA, IB, and i. The IA allele encodes the A blood group antigen, IB encodes B, and i encodes O. The A and B alleles are both dominant to the O allele. If a heterozygous blood type A parent (IAi) and a heterozygous blood type B parent (IBi) mate, there is a one in four chance that their offspring will have the AB blood type (IAIB) where both antigens are expressed equally. Therefore, the ABO blood groups are an example of multiple alleles and codominance.