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 are expected to have the AB blood type (IAIB) 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


Ans: d) multiple alleles and codominance

Biology & Microbiology

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