epistasis
Epistasis is when one gene overrides or modifies what another gene does — a kind of genetic veto. Picture a light fixture: one gene controls the bulb's color, but another controls the master power switch. If the power switch gene is off, the color gene's setting no longer matters; the light stays dark either way.
More precisely, an epistatic gene at one locus masks or alters the effect of a gene at a different locus. This is distinct from dominance, which is about the two alleles of the same gene; epistasis is an interaction between different genes acting on the same trait.
Many traits depend on a chain of steps, each run by a different gene, so a block early in the chain can hide everything downstream. Because the genes interact rather than act independently, epistasis distorts the simple ratios Mendel observed — a dihybrid cross can yield modified ratios such as 9:3:4 or 9:7 instead of the textbook 9:3:3:1.
In Labrador retrievers, one gene sets coat color (black or brown), but a second gene can switch pigment deposition off entirely, producing a yellow dog regardless of the first gene.
A second gene vetoes the first — the yellow coat masks whatever color was specified.
Keep epistasis (one gene masking a different gene) apart from dominance (one allele masking the other allele of the same gene). Different loci versus same locus is the dividing line.