Which phenomenon describes a situation where both dominant genes are expressed?

Codominance is the phenomenon where both alleles of a gene pair in a heterozygote are fully expressed, resulting in a phenotype that is neither dominant nor recessive. This means that in individuals with two different alleles for a trait, both traits can be observed simultaneously in the organism. A classic example of codominance is seen in blood types, where individuals with both A and B alleles express both A and B antigens on the surface of their red blood cells, resulting in type AB blood.

In contrast, epistasis refers to the interaction between genes whereby one gene masks or modifies the expression of another gene. Pleiotropy describes a situation where a single gene influences multiple phenotypic traits. Incomplete dominance occurs when the phenotype of a heterozygote is intermediate between the phenotypes of the two homozygotes, rather than both being fully expressed. Therefore, codominance specifically captures the essence of a situation where both dominant genes manifest distinctly in the organism's phenotype.