Molecular Analysis of the β-Globin Gene Cluster in the Niokholo Mandenka Population Reveals a Recent Origin of the βS Senegal Mutation

A large and ethnically well-defined Mandenka sample from eastern Senegal was analyzed for the polymorphism of the β-globin gene cluster on chromosome 11. Five RFLP sites of the 5′ region were investigated in 193 individuals revealing the presence of 10 different haplotypes. The frequency of the sickle-cell anemia causing mutation (βS) in the Mandenka estimated from this sample is 11.7%. This mutation was found strictly associated with the single Senegal haplotype. Approximately 600 bp of the upstream region of the β-globin gene were sequenced for a subset of 94 chromosomes, showing the presence of four transversions, five transitions, and a composite microsatellite polymorphism. The sequence of 22 βS chromosomes was also identical to the previously defined Senegal haplotype, suggesting that this mutation is very recent. Monte Carlo simulations (allowing for a specific balancing selection model, a logistic growth of the population, and variable initial frequencies of the Senegal haplotype) were used to estimate the age of the βS mutation. Resulting maximum-likelihood estimates are 45–70 generations (1,350–2,100 years) for very different demographic scenarios. Smallest confidence intervals (25–690 generations) are obtained under the hypothesis that the Mandenka population is large (Ne >5,000) and stationary or that it has undergone a rapid demographic expansion to a current size of >5,000 reproducing individuals, which is quite likely in view of the great diversity found on βA chromosomes