the X and Y chromosome were identical chromosomes. At some point
in the evolutionary process a sex differentiating gene appears on
one of the chromosomes, and genetic recombination becomes inhibited
around this gene. In humans this is the SRY gene on the Y chromosome.
of recombination spreads to cover all but the very ends of the Y
time, without benefit of genetic recombination, the Y chromosome
begins to degrade.
the Y chromosome loses active genes, each organism must develop
a method for equalizing genetic expression in males and females.
For this reason one of the X chromosomes in every female mammal
becomes inactive and curls up into a structure known as a Barr Body.
Y: THE DRIVING
of the evolution of X inactivation without speaking of the history
of the X and Y chromosomes leaves out critical context, something
like cataloguing the rarefied structures of orchids without considering
the habits of insects which evolved in concert with the flowers.
The X and Y chromosomes hardly look like a pair.
Textbooks have considered the mammalian X and Y fully differentiated
from each other.
Yet the X and the Y descended from a pair of identical
sister chromosomes. Early in the evolution of mammals, before monotremes
(echidnas and the platypus) and marsupials (opossum, kangaroos and
other Australian mammals) diverged from the placental mammals about
130 to 150 million years ago, the sex chromosomes arose. We can reconstruct
how this happened. Probably, a gene variant appeared that influenced
sex determination. In mammals the gene on the Y that triggers male
development was discovered a decade ago.
This evolutionary history applies generally when
sex chromosomes arise. A sex-affecting variant appears on a chromosome
that contains a haphazard collection of genes, most of which have
nothing to do with sex determination or differentiation.
Once a sex-determining gene appeared, recombination
became restricted around that gene, as if to ensure that it remained
confined to the male line. Eventually, the region of suppressed recombination
expanded to include nearly the whole Y. How recombination between
sex chromosomes becomes suppressed and how the suppression spreads
remain some of the most mysterious steps in sex chromosome evolution.
Now, the human X and Y chromosomes recombine only at their very tips.
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