Fig. 3.

Traits encoded by genes on minor sex chromosomes are not subject to intra-locus sexual conflict, and therefore are more able to respond to sex-specific selection. Autosomal genes are expressed in both males and females, and male-specific selection (blue arrow) may be counter to female-specific selection (red arrow), resulting in unresolved conflict over optimal trait values (a). This is resolved for genes on minor sex chromosomes, as male-specific selection on Y-linked traits (b) is unconstrained by expression in females, and similarly, female-specific selection on W-linked traits (c) is unconstrained by expression in males. In these latter two panels, the trait responds to sex-specific selection by moving to male (Y chromosome) or female (W chromosome) optimum