The history of the Y chromosome diverges from that of the X chromosome. That is, a random mutation gives rise to an allele that confers sexual characteristics regardless of temperature, as previously occurred in many vertebrates. 184 Y genes were already part of the ancestral sex chromosomes (X) about 300 million years ago. This sexual characteristic will prove to be successful and will be favored by evolution. Nevertheless, the human Y chromosome, for example, currently retains only 19 of the original 600 genes, but has been particularly stable lately: in the last 25 million years, it has lost only one gene.
