In ABO system, which concept explains AB phenotype in heterozygotes?

• A. Incomplete dominance
• B. Pleiotropy
• C. Codominance
• D. Complete dominance

Answer: (C)

Codominance is when two alleles are both expressed in the phenotype. In the ABO blood group, the IA and IB alleles are codominant, so a person with IAIB expresses both A and B antigens on their red blood cells, giving AB blood type. If only one functional allele is present (IAi or IBi), only that antigen is produced, giving type A or type B. If neither functional allele is present (ii), neither antigen is produced, giving type O. This differs from incomplete dominance, where the heterozygote shows an intermediate trait, and from complete dominance, where one allele masks the other. Pleiotropy isn’t involved here.