GREEDY genes might cause Down鈥檚 syndrome, say researchers who have refined a
model of 鈥渟elfish鈥� chromosomes stampeding to get into an egg.
Only half of a woman鈥檚 genes get into each egg. During meiosis, the process
which creates eggs and sperm, half the genes end up in a 鈥減olar body鈥� which then
dies. The other half get into the egg, and have a chance to be passed on to
future generations.
In Proceedings of the National Academy of Sciences (vol 95, p 2361),
biologist Troy Day and mathematician Peter Taylor of Queens University in
Kingston, Ontario, describe a mathematical model which assumes that chromosomes
can cheat, trying to force themselves into the egg. As a woman reaches
menopause, the greedy chromosomes grow more desperate to get into an egg, as the
chances of being passed on rapidly diminish. Although this would be beneficial
to the individual chromosome, it may harm the woman鈥檚 ovaries.
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When they included the assumptions in a game theory model, the outcome was
that an ageing woman produces more eggs with extra chromosomes. If one of these
eggs is fertilised, it can lead to a genetic disorder such as Down鈥檚 syndrome.
鈥淭he model predicts a fairly constant percentage of trisomic pregnancies of
around 1 per cent during the first 20 years of reproductive life, followed by a
steep increase during the last 10 reproductive years up to around 6 per cent,鈥�
says Taylor, who adds that this is close to observed values.