Final answer:
Type AB blood expresses both A and B antigens because of codominance, not incomplete dominance. Both alleles IA and IB are fully expressed in AB blood type, exemplifying codominance and multiple alleles in the ABO blood system.
Step-by-step explanation:
People with type AB blood express both A and B antigens on the surface of their cells, and this is not an example of incomplete dominance. It is, in fact, an example of codominance, wherein two different alleles in the genotype are expressed equally in the phenotype.
In ABO blood types, the alleles IA and IB are both dominant to the allele i. Individuals with type AB blood have one IA allele and one IB allele. The presence of both antigens on red blood cells in type AB blood demonstrates that these alleles are codominant because both are expressed fully and neither masks the expression of the other.
Therefore, the correct answer to the question of whether ABO blood groups are an example of multiple alleles and incomplete dominance; codominance and incomplete dominance; incomplete dominance only; or multiple alleles and codominance would be d. multiple alleles and codominance.