The multistage model of sex chromosome evolution. The mammalian X and Y chromosomes are thought to derive from a common initial autosomal pair. By a gradual process of genetic instability, which may have been related to failure in the recombination process, the chromosomes have begun to differ from each other. The first area to acquire a sex-specific role is considered to be the locus around the major sex determinant gene, i.e. SRY. Thus, in evolutionary lower mammals with a more conserved chromosomal content, such as monotremes, X and Y retain homology in all their length but for the SRY region. Subsequent stages of X-Y recombination failure have led to other, transient forms of X-Y structure, such as those observed in marsupials and primates. The greatest level of heterogeny is considered to be that found in modern humans.