Final answer:
The HbS allele increased in frequency in West African populations due to sickle-cell anemia, which provides some protection against malaria. This allele originated in Africa and spread to the United States during the slave trade.
Step-by-step explanation:
The HbS allele increased in frequency in West African populations due to sickle-cell anemia. Mutant genes for globins can result in hemoglobin disorders like sickle cell anemia, which originated in Africa and spread to the United States during the slave trade. The allele for sickle-cell anemia provides some protection against malaria, which is prevalent in West Africa, leading to a higher frequency of the HbS allele in these populations.