Which term describes most blood group genes?

• A. Dominant
• B. Recessive
• C. Codominant
• D. Incomplete dominance

Answer: (C)

Most blood group genes express codominance, where both alleles contribute to what's shown on the cell surface. For the ABO system, the two alleles encode different transferases that add distinct sugars to the H antigen on red blood cells. When both alleles are present, both enzymes work and both A and B antigens appear on the cell surface, giving the AB blood type. This simultaneous expression is the hallmark of codominance, unlike complete dominance (one allele masks the other) or incomplete dominance (an intermediate phenotype). The O allele is recessive to both, so lacking A or B antigens occurs only when two O alleles are present. Hence codominance is the correct description.