In codominance, how are the alleles expressed in the phenotype? (Using ABO blood groups as an example.)

• A. Only one allele is expressed in a heterozygote.
• B. Both alleles are fully expressed in the phenotype.
• C. A heterozygote shows a blending phenotype.
• D. The phenotype is intermediate between two alleles.

Answer: (B)

In codominance, both alleles contribute to the phenotype, so a heterozygote shows both traits expressed. In the ABO system, IA and IB are codominant to each other while i is recessive. An IAIB individual has red blood cells that display both A and B antigens, so both alleles are fully expressed in the phenotype. This differs from complete dominance, where one allele masks the other, and from incomplete dominance, where the heterozygote has an intermediate or blended phenotype.