Final answer:
The ABO blood type system, determined by the alleles IA, IB, and i, results in four possible phenotypes: A, B, AB, and O, due to the codominance of IA and IB alleles, and both being dominant over i. correct answer is option D.
Step-by-step explanation:
The ABO blood type system in humans is determined by three alleles: IA, IB, and i. The alleles IA and IB are codominant, meaning that both can be expressed equally when an individual has both alleles (as in blood type AB). They are also both dominant over allele i, which encodes for the absence of these antigens, resulting in blood type O when present in a homozygous state (ii).
Given this, when a heterozygous blood type A parent (iAi) mates with a heterozygous blood type B parent (iBi), their offspring could have any of the four possible blood types: A, B, AB, or O. Thus, the combination of these alleles produces four different phenotypes: blood type A (IAIA or IAi), blood type B (IBIB or IBi), blood type AB (IAIB), and blood type O (ii).