Advanced Placement (AP) Biology Practice Exam

In codominance, how are alleles expressed?

1. Only one allele is expressed often

2. Both alleles are expressed fully

3. Alleles blend together to form a new trait

4. One allele suppresses the expression of another

The correct answer is: Both alleles are expressed fully

In codominance, both alleles contribute equally to the phenotype of the organism, resulting in a situation where the traits associated with each allele are both fully visible and expressed. This is distinct from complete dominance, where one allele overwhelmingly masks the effect of another, or incomplete dominance, where the alleles blend to create a third phenotype that is intermediate between the two parental traits. For example, in the case of certain flower colors, if one allele codes for red flowers and another for white flowers, the resulting flowers may exhibit both colors distinctly rather than appearing pink or only one color. This clear expression of both traits is why codominance is characterized by the full expression of both alleles.