Codominance is an interaction between two allele variants of a gene, which results in a phenotypic expression of both versions. For example, if a gene for fur color has white and black alleles, then the codominant expression of the gene results in a phenotype of a grey color. The main reason is that both genes possess an equal degree of dominance in relation to each other, which means neither is recessive.
However, epistasis refers to an interaction between two different genes, where one gene impacts the expression of another gene. For instance, a person’s gene A is responsible for hair color and might carry an allele for blonde hair, but another gene B is responsible for hair growth and might carry an allele for baldness. In this case, a person will not have blonde hair due to baldness. Therefore, gene B impacts the phenotypic expression of gene A.