Final answer:
Individuals with the HbAHbS genotype have resistance to malaria because the hemoglobin S variant makes their red blood cells inhospitable to malaria parasites. This trait, while disadvantageous when having two copies causing sickle cell anemia, provides a protective advantage against malaria in heterozygous carriers, and thus remains in the population in areas where malaria is prevalent.
Step-by-step explanation:
Individuals with the HbAHbS genotype, also known as heterozygotes for the sickle cell trait, do not develop malaria because the presence of hemoglobin S (HbS) confers a certain resistance to the malaria parasite. In areas such as sub-Saharan Africa, this genetic adaptation has provided a survival advantage in environments where malaria is endemic. The HbS allele causes the red blood cells to sickle under certain conditions, making them less hospitable to the parasites that cause malaria. Consequently, these individuals have naturally selected protection against this deadly tropical disease.
The advantage of carrying one sickle cell gene is illustrated by the example of two sisters in rural Zambia, Africa. Luwi, who carries the gene, is at a lower risk of contracting malaria, while Sena, who does not carry the gene, lacks this protective effect. This evolutionary favorability keeps the S allele present in the gene pool despite the potential health risks associated with having two copies and suffering from sickle cell anemia.