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).