Epistatic interactions between genetic disorders of hemoglobin can explain why the sickle-cell gene is uncommon in the Mediterranean.

Several human genetic disorders of hemoglobin have risen in frequency because of the protection they offer against death from malaria, sickle-cell anemia being a canonical example. Here we address the issue of why this highly protective mutant, present at high frequencies in subSaharan Africa, is uncommon in Mediterranean populations that instead harbor a diverse range of thalassemic hemoglobin disorders. We demonstrate that these contrasting profiles of malaria-protective alleles can arise and be stably maintained by two well-documented phenomena: an alleviation of the clinical severity of alpha- and beta-thalassemia in compound thalassemic genotypes and a cancellation of malaria protection when alpha-thalassemia and the sickle-cell trait are coinherited. The complex distribution of globin mutants across Africa and the Mediterranean can therefore be explained by their specific intracellular interactions.