Protection Afforded by Sickle-Cell Trait (Hb AS): What Happens when Malarial Selection Pressures Are Alleviated?
2001
A study of reproductive outcome in Mobile, AL was
conducted among a large maternal cohort with sickle-cell disease (Hb SS),
sickle-cell trait (Hb AS), and no hemoglobinopathies (Hb AA). It was found
that mean gravidity and live births among Hb AS women were significantly
higher than among Hb AA women. These findings were surprising since it is
generally held that once malarial pressure is alleviated, any reproductive
advantage that might be conferred by Hb AS would disappear and fertility
levels would reach levels similar to or slightly less than that of Hb AA
women. A search of the literature was subsequently conducted and a large
cohort study of an African-derived population was found in the United
Kingdom. Results from this study also showed that parity was significantly
higher among Hb AS women compared to Hb AA women. If survivorship is
similar among Hb AS and Hb SS women, findings from these two studies raise
doubts whether directional selection is occurring against the Hb S allele
in nonmalarial environments. Balancing selection may still be occurring.
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