The abo blood groups in humans are expressed as the ia, ib, and i alleles. the ia allele encodes the a blood group antigen, ib encodes b, and i encodes o. both a and b are dominant to o. if a heterozygous blood type a parent (iai) and a heterozygous blood type b parent (ibi) mate, one quarter of their offspring will have ab blood type (iaib) in which both antigens are expressed equally. therefore, abo blood groups are an example of:
multiple alleles and incomplete dominance
codominance and incomplete dominance
incomplete dominance only
multiple alleles and codominance

Multiple alleles and codominance

Explanation:

Co dominance may be defined as the phenomena in which both the alleles can express themselves independently in heterozygous condition. The involvement of more than two alternative form of a single gene is known as multiple allele.

The ABO blood group represent the example of codominance and multiple allelism. In case of AB blood group both the alleles I(A) and I (B) express themselves in heterozygous condition and result in the formation of AB blood group. Different alleles like I(A), I(B), I(A)I(B),i are involved in case of ABO blood group, this shows multiple allele.

Thus, the correct answer is option (4).

i believe the correct answer is d