In codominance, what phenotype is shown by a heterozygote for blood type A and B?

• A. Both A and B antigens are expressed
• B. Only A antigen is expressed
• C. Neither A nor B antigens are expressed
• D. Only B antigen is expressed

Answer: (A)

In codominance, both alleles contribute to the phenotype. For the ABO system, the A and B alleles are codominant, so when both are present, both antigens appear on the surface of red blood cells. A person with one A allele and one B allele expresses both antigens, giving the AB blood type. This happens because neither antigen masks the other; the cell surface shows both A and B structures.

If only one antigen type is present, it means that only one allele is contributing—two A alleles give only A antigen, two B alleles give only B antigen. If neither antigen is produced, as when those alleles are not present, neither A nor B antigen appears (type O).