Evolutionary conservation of sex specific DNA sequences.

A family of DNA sequences which appears to be limited to eukaryotes is concentrated on the sex-determining chromosomes of species as widely separated evolutionarily as snakes and mammals. The significance of this distribution is presently seen in terms of the function of these sequences in cycles of chromosome condensation and decondensation involved in the control of gene expression. Thus, their concentration on the sex chromosomes is interpreted in the context of the evolution of such chromosomes which, it is hypothesised, involves the superimposition of the controls of the dominant sex-determining gene(s) upon the entire linkage group. This would result in the prevention of the large majority of its genes from having effects upon the phenotype and, in consequence, lead to their mutation to functionlessness at the maximum rate.