What is the term for the genetic outcome that results when both alleles in a heterozygous individual are expressed equally?

The term that describes the phenomenon where both alleles in a heterozygous individual are expressed equally is co-dominance. In co-dominance, neither allele can mask the expression of the other, leading to a phenotype that displays characteristics of both alleles simultaneously. A common example of co-dominance can be seen in blood types, where individuals with one allele for type A blood and one for type B blood express both antigens, resulting in type AB blood. This distinct pattern of inheritance highlights the equality of expression in the resulting phenotype when both alleles are present.

In contrast, incomplete dominance refers to a situation where the phenotype is a blend of the two alleles, resulting in a phenotype that is intermediate but not fully representative of either allele. Dominant inheritance describes a scenario where one allele completely masks the effect of another, leading to the phenotype being dictated solely by the dominant allele. Multiple alleles refer to the existence of more than two alleles for a particular gene within a population but does not necessarily relate to how those alleles interact within an individual.