The enhancer-like sequence 3' to the (A)γ gene is polymorphic in human populations

Cloning and sequencing of the enhancer 3' of the (A)γ globin gene of a particular low (G)γ and HbF sickle cell anemia (SCA) patient unexpectedly revealed three base changes (T → C, C → A, and A → G at sites +2285, +2460, and +2676) previously associated with the Seattle-type HPFH, thus leading the authors to suspect that the three mutations were polymorphic. The determination of the incidence of the mutations among various ethnic groups allowed the authors to conclude that this is a widely spread polymorphism, thus excluding any role of these base changes in the determination of the heredity persistence of fetal hemoglobin (HPFH) phenotype. The origin of these three mutations is not clear because they appear linked, and the same bases (C, A, G) are found in homologous position in the 3' of the normal (G)γ gene. As C, A, G at positions +2285, +2460, and +2676 are found with a 100% frequency in African SS patients and presumably among normal Africans (to explain the extremely high frequency among normal American blacks), it is likely that this was the sequence preceding the division of races. The presence of T, C, and A at the same positions apparently occurred after the divergence between blacks and the other races, that is, within the last 1 million years.