Interaction between gene loci
Epistasis is an interaction between different gene loci in which the genotype at one locus affects whether the alleles at another locus are expressed in the phenotype.
The genes involved influence the same characteristic. As a result, the phenotype depends on the combination of alleles present at both loci, rather than on either gene considered alone.
Example: feather colour
In the pigeon feather-colour example, one gene has alleles R and r.
- R results in pigment that can give grey feathers.
- r does not result in this pigment.
A second gene has alleles F and f. When the dominant F allele is present, grey feather colour is prevented even if an R allele is also present.
| Genotype pattern | Feather phenotype |
|---|---|
| F_ | White, including genotypes that also contain R |
| R_ff | Grey |
| rrff | White |
The effect of the R/r gene therefore depends on which alleles are present at the F/f locus.