In codominant inheritance, how do the two alleles interact to determine phenotype?

In codominant inheritance, how do the two alleles interact to determine phenotype?

• A. One allele is completely dominant and masks the other
• B. The two alleles are equally expressed in the phenotype
• C. The phenotype is a blend of the two alleles
• D. The trait is influenced by environmental factors

Answer: (B)

In codominance, both alleles are expressed equally in the phenotype, so heterozygotes show both traits distinctly rather than one masking the other or blending into an intermediate form. A classic example is the ABO blood group system, where IA and IB are codominant; someone with IAIB has red blood cells carrying both A and B antigens. This differs from complete dominance, where one allele hides the other, and from incomplete dominance, where the phenotype is a blend. Environmental factors can influence traits in general, but codominance specifically describes how the alleles contribute to the visible characteristics.

In codominance, both alleles are expressed equally in the phenotype, so heterozygotes show both traits distinctly rather than one masking the other or blending into an intermediate form. A classic example is the ABO blood group system, where IA and IB are codominant; someone with IAIB has red blood cells carrying both A and B antigens. This differs from complete dominance, where one allele hides the other, and from incomplete dominance, where the phenotype is a blend. Environmental factors can influence traits in general, but codominance specifically describes how the alleles contribute to the visible characteristics.