The HbS allele, which causes sickle cell anemia, occurs at a higher frequency in Africa than in the US due to a phenomenon called balancing selection. Here's why:

* Malaria: Malaria, a deadly parasitic disease spread by mosquitoes, is prevalent in many parts of Africa.

* Heterozygote Advantage: Individuals who carry one copy of the HbS allele (HbS/HbA genotype) have sickle cell trait. They don't have the full-blown disease, but their red blood cells are somewhat sickle-shaped. This trait provides resistance to malaria, as the parasite finds it difficult to multiply and survive in these cells.

* Selective Pressure: In areas with high malaria prevalence, individuals with sickle cell trait have a survival advantage over those with two normal HbA alleles (HbA/HbA genotype) who are susceptible to malaria.

* Increased Frequency: This selective advantage leads to an increased frequency of the HbS allele in populations exposed to malaria.

In the US, the situation is different:

* Malaria is not endemic: The prevalence of malaria is much lower in the US compared to Africa.

* No selective pressure: Therefore, there is no selective advantage for carrying the HbS allele.

* Lower frequency: As a result, the HbS allele occurs at a much lower frequency in the US population.

In summary: The higher frequency of the HbS allele in Africa is a result of natural selection favoring individuals with sickle cell trait, who are resistant to malaria. This advantage is not present in the US, leading to a lower frequency of the allele.