Linus Pauling: This shows where the sickle cell gene is found. And, you know, it turns out that
the sickle cell gene is found in these areas because they are malarial, and the sickle
cell gene protects against malaria. In some regions in central Africa, almost everyone
is a sickle cell heterozygote. You can see what happens: the heterozygotes marry one
another, a quarter of their children are normal and die from malaria, and a quarter
are sickle cell homozygotes and die of sickle cell disease, but half of them survive.
Well, it’s only recently that a fifty-percent yield in children has been considered
unsatisfactory, so this was a good step.
This is the reason that the gene spread so fast - the heterozygotes were protected.
You only needed to get one of these genes to survive against malaria. If it had been
the homozygotes, if it were a feebler mutation such that the homozygotes were protected,
it wouldn’t spread because you wouldn’t have any homozygotes. And you would have to
have in-breeding for awhile before you got any homozygotes.
Probably mutations of this sort take place in two steps; another ten-thousand years,
probably, would have seen the sickle cell gene disappearing and another gene taking
its place with the homozygotes protected against malaria, everybody protected against
malaria. This gene has its incidence such that you could calculate that it would only
take about a thousand years for the gene to spread through a population, forty generations;
a much larger number of viable survivors for people carrying the gene than those not
carrying the gene.