Which ABO blood type demonstrates codominance in inheritance?

• A. Type O
• B. Type A
• C. Type B
• D. Type AB

Answer: (D)

Codominance happens when both alleles contribute to the trait and are both expressed. In the ABO system, the A allele (IA) and the B allele (IB) are codominant to each other. When someone inherits IA from one parent and IB from the other, their red blood cells show both A and B antigens, giving Type AB blood. This is the clearest example of codominance because you can see both traits (A and B antigens) expressed together. If someone has two recessive i alleles, they lack A and B antigens and become Type O. If IA pairs with IA or i, only A antigens are present (Type A); if IB pairs with IB or i, only B antigens are present (Type B).