In ABO blood group genetics, which statement about dominance is correct?

In ABO blood group genetics, which statement about dominance is correct?

• A. IA and IB are codominant; i is recessive
• B. IA is dominant over IB
• C. IB is recessive to IA
• D. i is codominant with IA or IB

Answer: (A)

In this system, the two alleles that specify the A and B antigens (IA and IB) are codominant with each other, meaning a person who has IAIB expresses both A and B antigens on the red blood cells. The allele i, which has no antigen, is recessive to both IA and IB, so IAi or IBi give A or B phenotypes, while ii gives the O phenotype. This makes the pattern different from a simple dominant-recessive pair where one allele masks the other; instead, IA and IB both contribute when present together, and i is recessive to either.

In this system, the two alleles that specify the A and B antigens (IA and IB) are codominant with each other, meaning a person who has IAIB expresses both A and B antigens on the red blood cells. The allele i, which has no antigen, is recessive to both IA and IB, so IAi or IBi give A or B phenotypes, while ii gives the O phenotype. This makes the pattern different from a simple dominant-recessive pair where one allele masks the other; instead, IA and IB both contribute when present together, and i is recessive to either.