Evidence Indicating that the X and the Y or the Z and the W were Originally an Homologous Pair of Ordinary Chromosomes

Abstract

Today, the X and Y of mammals and the Z and W of birds are totally different from each other in size as well as genetic content. Yet, it can be shown that the two were originally an homologous pair of ordinary chromosomes or autosomes. A broad review of the sex-determining mechanism of various vertebrates enables us to reconstruct, step-by-step, this process of sex chromosome differentiation.