In genetics, what term refers to the expression of both alleles in a heterozygous genotype?

The term that refers to the expression of both alleles in a heterozygous genotype is co-dominance. In a co-dominant situation, both alleles contribute equally and visibly to the organism's phenotype. This means that if an individual carries two different alleles for a specific trait, both traits will be fully expressed.

For example, in the case of certain blood types, an individual with one allele for type A blood and one allele for type B blood will have type AB blood, showcasing the full expression of both alleles. This is distinct from other genetic concepts, where one allele may mask the expression of another (as in complete dominance), or where blending occurs (as in incomplete dominance). Co-dominance is characterized by the simultaneous and distinct expression of both alleles, highlighting the diversity found in genetic expression.

Polygenic inheritance, on the other hand, involves multiple genes contributing to a single trait, not the interaction of two differing alleles of a single gene.