Linked DNA polymorphisms can be used to study the evolution of structural gene mutations. Both the β(S)-[β6(Glu→Val) and β(C)-(β6(Glu→Lys)] genes are common in West Africa. We have analyzed their linkage to a polymorphic Hpa 1 site appearing 3' to the β-globin gene locus in selected populations from West Africa. A large reservoir of β(A)-genes linked to 13-kilobase Hpa 1 fragments with a frequency of 17-18% has been identified. In addition, the β(S)- and β(C)-genes in Togo are found to be tightly linked to the 13-kilobase Hpa 1 fragment, whereas 72% of the β(S)-genes in the Ivory Coast reside on the 7.6-kilobase Hpa 1 fragment. These studies are consistent with the selection and expansion of two different chromosomes bearing β(S)-genes in at least two physically close, but ethnically separate regions of West Africa, with subsequent diffusion to North, Equatorial, and East Africa.
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