Final answer:
The sickle-cell gene persists in human populations because it confers a selective advantage in the form of protection against malaria in regions where the disease is endemic, despite the health risks associated with sickle-cell anemia for individuals with two copies of the gene.
Step-by-step explanation:
The sickle-cell gene continues to exist in human populations primarily because it offers protection against malaria. This trait is advantageous in areas where malaria is prevalent, such as parts of Africa. Carriers of the sickle-cell gene, who have one copy of the mutated gene, experience a protective effect against malaria. Conversely, individuals homozygous for the sickle-cell gene suffer from sickle-cell anemia and do not benefit from any malaria immunity. The existence of the gene is due to a classic example of balanced polymorphism, where the advantage of heterozygote protection in high-malaria areas outweighs the disadvantages of the disease in homozygotes.
For instance, in the case of two sisters from rural Zambia, Africa—Luwi, a carrier of the gene, and Sena, who is not—the former is protected from malaria following a mosquito bite. Sena, lacking the gene, contracts malaria and tragically succumbs to the disease. As a result, Luwi is more likely to survive and pass the gene on to her offspring, maintaining the presence of the sickle-cell gene within the population.