Final answer:
The ABO blood group phenotype is an example of multiple alleles and codominance.
Step-by-step explanation:
The ABO blood group phenotype is an example of multiple alleles and codominance. The ABO blood group system is controlled by a single gene on chromosome 9 with three common alleles: IA (or A), IB (or B), and i (or O). Both IA and IB alleles are dominant to the i allele, and the IA and IB alleles are codominant to each other.