Final answer:
Incomplete dominance involves an intermediate phenotype in heterozygotes, while epistasis occurs when an allele of one gene mask or suppresses the expression of an allele at another locus. Both genetic concepts alter the expected phenotypic outcomes but do so in different ways.
Step-by-step explanation:
The most basic distinction between incomplete dominance and epistasis is that incomplete dominance refers to a genetic situation where a heterozygote expresses an intermediate phenotype between the two alleles, whereas epistasis involves an antagonistic interaction between genes, where the presence of a certain allele (epistatic) masks or suppresses the expression of another allele (hypostatic) at a different genetic locus.
In incomplete dominance, the phenotype is a blend of the two alleles. An example of this would be the flower color in snapdragons, where the heterozygote results in a pink flower as an intermediate between red and white alleles.
Epistasis, on the other hand, can completely override the expected phenotypic expression of another gene. For instance, in Labrador retrievers, the coat color is determined by two genes: one gene determines the pigment (black or chocolate) and another gene influences whether the pigment will be deposited in the fur. The second gene can mask the expression of the first, leading to a yellow coat regardless of the black or chocolate alleles.