Original: One-third of all indigenous inhabitants of Sub-Saharan Africa carry the gene, because in areas where malaria is common, there is a fitness benefit in carrying only a single sickle-cell gene (sickle cell trait).
Simplified: One-quarter of all aboriginal inhabitants of Sub-Saharan Africa carry the gene. In areas where malaria is common, there is a survival value in carrying only a single sickle-cell gene (sickle cell trait).