Biology 1st Edition by Jung Choi, Jean DeSaix, Yael Avissar, Vladimir Jurukovski, Robert Wise,Connie Rye, OpenStax

Exercise 24

The ABO blood groups in humans are expressed as the I A , I B , and i alleles. The I A allele encodes the A blood group antigen, I B encodes B, and i encodes O. Both A and B are dominant to O. If a heterozygous blood type A parent (I A i ) and a heterozygous blood type B parent (I B i ) mate, one quarter of their offspring will have AB blood type (I A I B ) in which both antigens are expressed equally. Therefore, ABO blood groups are an example of:
A) multiple alleles and incomplete dominance
B) codominance and incomplete dominance
C) incomplete dominance only
D) multiple alleles and codominance