What is the phenomenon called when a dominant allele does not completely mask the effects of a recessive allele?

The correct term for the phenomenon where a dominant allele does not completely mask the effects of a recessive allele is known as incomplete dominance. In situations of incomplete dominance, the phenotype of the heterozygote is an intermediate or blended form between the dominant and recessive traits. For example, if one parent has red flowers (dominant) and the other has white flowers (recessive), the offspring might display pink flowers, which is indicative of this blending effect.

In contrast to co-dominance, where both alleles in a heterozygote are fully expressed (like AB blood types), incomplete dominance results in a mix rather than a complete expression of both traits. Polygenic inheritance involves multiple genes contributing to a single trait, such as skin color or height, while multiple alleles refer to a gene having more than two allelic forms, such as the ABO blood group system. Understanding these distinctions clarifies why incomplete dominance is the correct choice for the question posed.