Can epistasis be sex-influenced?

• A. No
• B. Only in males
• C. Yes
• D. Only in females

Answer: (C)

Epistasis can be sex-influenced because the way genes interact can be modulated by the body's hormonal and regulatory environment, which differs between males and females. When one gene masks or modifies the effect of another, that interaction can be strengthened, weakened, or even reversed depending on sex-specific factors like testosterone or estrogen levels that influence gene expression or penetrance of the modifiers.

For example, imagine a gene that enables pigment production and a second gene that modifies how that pigment is deposited. The epistatic effect of the second gene might be more pronounced in males if male hormones enhance its activity, leading to a different appearance than in females where the hormone milieu yields a weaker or different epistatic effect. This illustrates that the same genotype can yield different phenotypes in the two sexes due to sex-influenced epistasis. It’s distinct from merely having a gene on a sex chromosome; here the interaction itself varies with sex because of regulatory differences.