In codominant inheritance, ABO example shows both alleles are equally expressed.

In codominant inheritance, ABO example shows both alleles are equally expressed.

• A. The two alleles are equally expressed in the phenotype
• B. Only the dominant allele is expressed
• C. The phenotype is a blend of the alleles
• D. The trait is determined solely by the environment

Answer: (A)

Codominance means both alleles contribute to the phenotype without one masking the other. In the ABO system, the IA and IB alleles are codominant. When an individual has IAIB, their red blood cells display both A and B antigens on the surface, so both alleles are expressed in the phenotype. If one allele is IA with the other being i, only the A antigen is produced, and with IB with i, only the B antigen is produced; ii gives neither antigen. This shows that the two alleles are equally contributing in the heterozygote, rather than one being dominant or the traits blending. Environment does not determine this trait.

Codominance means both alleles contribute to the phenotype without one masking the other. In the ABO system, the IA and IB alleles are codominant. When an individual has IAIB, their red blood cells display both A and B antigens on the surface, so both alleles are expressed in the phenotype. If one allele is IA with the other being i, only the A antigen is produced, and with IB with i, only the B antigen is produced; ii gives neither antigen. This shows that the two alleles are equally contributing in the heterozygote, rather than one being dominant or the traits blending. Environment does not determine this trait.