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
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.