Variation
AS people are more resistant to
malaria as the blood cells which contain the malaria parasites become sickle-shaped
and the parasite can't complete its development. Because AS
people are unlikely to die from malaria, their genes, A
and S are kept in the population.
The result is that in malaria regions, individuals who are
AA or SS both tend
to die more frequently while those who are AS have
a greater survival tendency.
In regions where malaria is no longer a threat, we see more
AA people as the AS
and especially the SS people have less survival
tendency.
Long & Slichter