What is it called when both alleles in a heterozygote are expressed equally?

The correct choice refers to a situation known as codominance. In codominance, when an organism is heterozygous, it produces both alleles equally and displays traits from both. An example of this could be seen in certain flower colors, where a plant with one allele for red flowers and another for white flowers would display both red and white flowers simultaneously, rather than blending the colors or expressing just one over the other.

In this scenario, the alleles do not mask each other's expression, leading to a phenotype that shows a clear expression of both traits rather than a mix or one trait dominating. This is distinct from incomplete dominance, where the phenotype is a blend of the two traits, and from complete dominance, where one allele completely overshadows the other. A recessive trait, on the other hand, is one that only manifests in the absence of the dominant allele, which does not apply in this context either. Codominance provides a clear example of how both alleles contribute to the observable characteristics of the organism.