Both the X and Y chromosomes have a remarkable enrichment of genes involved in gonadogenesis and gametogenesis. The small Y chromosome contains the sex determining gene SRY, as well as many genes that are critical for spermatogenesis and are expressed exclusively in the testis. The X chromosome, too, is enriched for genes involved in sex and reproduction. This biased gene content can be best understood in terms of the origin and evolution of our sex chromosomes. The Y chromosome can be seen as the relic of the ancient autosome, on which only a few genes survive by virtue of their critical male-specific role. The X is more complicated - it has evolved male-advantage genes because of its representation as a single copy in males, where it is exposed to selection for male-advantage genes.