Fig. 2.

Schematic of ancestral versus derived dosage of sex-linked genes. Ancestrally, it is expected that all genes were expressed in two copies in females (from both X chromosomes) and in two copies in males (from the ancestral X and Y chromosomes). The derived condition, resulting from loss of gene content and expression in males on the Y chromosome, is expression of many sex-linked genes in a single copy in males, and so inactivation evolved to silence one copy of the gene in females (on the inactive X chromosome, X_I) and resulted in expression of only one copy of the sex-linked gene in females (from the active X chromosome, X_A). Filled rectangles represent expressed genes, whereas empty white rectangles represent silenced (inactivated) genes.