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| Evolution of the Y Chromosome |
The evolutionary ancestor of the sex chromosomes was a pair of matched, autosomal chromosomes that acquired sex-determining genes on one member of the pair. This occurred about 350 million years ago in a reptile-like ancestor. Over time additional genes with male-specific functions accumulated in this same chromosome, called proto-Y, which then lost its ability to recombine with its counterpart chromosome, called proto-X. There are four regions of the proto-X chromosome, which appear to have been involved in four different steps, resulting in the loss of recombination with proto-Y. Each of the four regions accumulated mutations in those non-recombining regions of proto-Y at four different times in evolution. Each time recombination was lost there was degradation and loss of the non-recombining region. Over time this chromosome evolved into Y, losing most of its genetic information as a result of the degradation of the non-recombining regions of the chromosome. Its partner chromosome evolved into the X chromosome. The degeneration of the Y was offset at various times by additions of autosomal genes to this chromosome (as well as to X), leading to a pattern of loss and gain of genetic material over a period of about 170 million years (Fig. 3).