What genetic phenomenon occurs when both alleles in a pair are expressed in the phenotype?

The correct answer is codominance, a genetic phenomenon where both alleles in a pair are fully expressed in the phenotype, resulting in a distinct expression of both traits. In a codominant situation, neither allele is dominant over the other, leading to a phenotype that shows characteristics of both alleles simultaneously. A classic example of this is seen in certain breeds of flowers where one allele may encode for red petals and the other for white petals. When both alleles are present, the resulting phenotype displays both red and white patches, rather than blending them as with other inheritance patterns.

In contrast to codominance, polygenic inheritance involves multiple genes contributing to a single phenotype, leading to a continuous range of traits, such as human height or skin color. Incomplete dominance refers to a blending of traits where the phenotype is an intermediate expression of the two alleles, rather than both being distinctly visible, as seen in certain flower colors. Complete dominance occurs when one allele completely masks the expression of the other, indicating that only the dominant phenotype is observable. Thus, in codominance, the distinct and simultaneous expression of both alleles sets it apart from these other modes of inheritance.