Final answer:
The hemoglobin S allele is most common in regions with high malaria prevalence, such as sub-Saharan Africa, the Middle East, and India. The allele persists because heterozygotes (AS) have a survival advantage over those without this allele in malaria-endemic areas.
Step-by-step explanation:
The prevalence of the hemoglobin S (HbS) allele is especially high in parts of the world where malaria is common, such as sub-Saharan Africa, the Middle East, and India. The presence of one hemoglobin S allele (known as the AS genotype) provides resistance to malaria, which is a deadly tropical disease prevalent in these regions. This protective effect means that heterozygotes (individuals with one HbS allele and one normal allele) are more likely to survive and reproduce in malaria-endemic areas, thus maintaining the S allele within the population despite the risks associated with having two HbS alleles (the SS genotype), which leads to sickle cell disease.