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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).
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