In organisms with codominant blood group genes, what is observed when an individual is heterozygous for a blood group antigen?

• A. Expression of only dominant antigen
• B. No antigen expression
• C. Expression of both antigens
• D. Expression of recessive antigen only

Answer: (C)

In codominance, two different alleles are both expressed, so neither is masked by the other. For blood group genes, this means that if a person has two different antigen-encoding alleles, both antigens appear on the red blood cell surface. A classic example is having IA and IB alleles, which produces A and B antigens on the cells, giving an AB blood type. So when an individual is heterozygous for a codominant antigen, you observe the expression of both antigens. The other patterns—expressing only a single “dominant” antigen, or no antigen at all, or only a recessive antigen—would not fit codominance.