ABSTRACT
Most homozygotes for an abnormal hemoglobin or thalassemia gene, or heterozygotes for two such genes, have an anemia, that decreases fitness to some extent. This decreased fitness has certainly been present for most of human history. For an autosomal locus with two alleles, when the heterozygote has an increased fitness over both homozygotes, a stable equilibrium with both alleles present can exist; it is called a balanced polymorphism. The more common abnormal hemoglobins and thalassemia are undoubtedly balanced polymorphisms, but this is not necessarily true for some of the more uncommon hemoglobins or the glucose-6-phosphate dehydrogenase (G6PD) deficiency. Although the G6PD deficiency genotypes do not have so severe a handicap, neonatal jaundice and susceptibility to infection would have selected against these genes. There is a considerable evidence that heterozygotes for the hemoglobin S gene have a greater resistance to malarial infections and perhaps have a greater fertility in a malarious environment.
