The population structure of two alleles similarly affected by selection, HbS and Beta-thalassemia, was studied in the same area, in Sicily. It was observed that Beta-thalassemia has uniform frequencies over the island, and that HbS shows a South-North cline. A singularity exists for HbS, in the twon of Butera, where the frequency of the S gene is six times higher than the island average. It was advanced that probably HbS entered Sicily from North Africa, when the Beta-thalassemia allele was already established in the Island. From the correlation analysis concluded that HbS came to Sicily with the frequency of HbS, it was concluded that HbS came to Sicily before the Arab invasions of the 700 's AD.