What best describes epistasis in sticklebacks?

• A. Epistasis is the interaction of two genes where one gene's effect depends on another, producing non-additive effects on a trait.
• B. Epistasis refers to the independent additive effects of many genes on a trait.
• C. Epistasis describes gene mutations that always reduce fitness.
• D. Epistasis is the evolution of armor traits driven by predator pressure.

Answer: (A)

Epistasis is when the effect of one gene on a trait depends on the allele present at another gene, creating non-additive outcomes. In sticklebacks, armor plating is influenced by multiple genes, and one gene’s impact on plate number can be amplified, reduced, or even masked by the genotype at a second gene. So you can’t just add up each gene’s individual effect to predict the phenotype; the interaction between genes changes the result. This contrasts with a purely additive model where each gene contributes independently, and with ideas about mutations that always reduce fitness or about predator-driven evolution, which describe different concepts than gene-gene interaction.