A possible reason for the smallness of the human Y chromosome
In humans the gamete contributed by the father determines the sex of
the child. If the gamete has a Y chromosome the child is male. If the
gamete has an X chromosome the child is a female.
It would be in the selfish interest of the Y chromosome to get itself
passed to much more than 50 per cent of the next generation. It is
perhaps possible that more than once in the course of evolution of
humans (and their non-human ancestors) mutations in the Y chromosome
have resulted in genes that discriminate against the production of X-
bearing gametes in spermatogenesis or sperm maturation. The most recent
of such events might (for instance) have occurred in Africa 60,000
Such mutations would give the Y chromosome a tremendous evolutionary
advantage. It might ensure that (for instance) four out of six of a
man's offspring were males. This mutated chromosome could spread
quickly through a widespread established population, generation by
generation (assuming that a short-displacement wavelike movement of
people, lasting a long time, was always possible).
However, no other chromosomes would accompany it far on its journey. It
would be a bit like the gene for 'hornlessness', which can be
introduced into a herd of horned black Angus cattle by a cross with a
hornless red Shorthorn bull and, under the artificial selection of the
cattle-breeder, be spread throughout the herd. In about 20 years the
herd consists of hornless black Angus cattle. It would be incorrect for
a stranger to assume that the black cattle he or she sees in the field
were of a breed that had been hornless for hundreds of years.
Under natural selection a human population in which the males had
the 'anti-X gamete' Y chromosome would suffer the disadvantages of a
surplus of males and a scarcity of females. In the long run natural
selection would favour subsequent mutants of the 'anti-X gamete' Y
chromosome in which those mutant genes that discriminated against the
production of X-bearing gametes in spermatogenesis or sperm maturation
were deleted or rendered inactive.
The result after many millenia of evolution of humans (and their
evolutionary precursors) would be a human Y chromosome from which many
genes had been deleted and in which many of the remaining DNA sequences
were permanently inactivated. It would also be a Y chromosome that
could give a misleading picture of the chronology and routes of human
migration from Africa.
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