In the genetic inheritance of blood groups, which pattern best describes how alleles at a locus are expressed in heterozygotes?

• A. Dominant-recessive
• B. Recessive only
• C. Additive
• D. Codominant

Answer: (D)

Codominance means that when two different alleles are present in a heterozygote, both traits are fully expressed. In the ABO blood group system, the IA and IB alleles are codominant: an individual who is IAIB expresses both A and B antigens on their red blood cells, giving AB blood type. This differs from a dominant-recessive pattern, where one allele’s trait would mask the other in a heterozygote; IAIB would not simply be A or B, it would be AB. Recessive-only would require two copies of the recessive allele to show a trait, which isn’t the case here. Additive would imply a blend or intermediate phenotype, which also doesn’t describe how A and B antigens are expressed. So the pattern that best describes heterozygote expression at this locus is codominance.