What type of selection is expected on the sickle-cell gene in the U.S. where malaria is rare?

The correct answer is the expectation of directional selection to eliminate the sickle-cell gene in the U.S. where malaria is rare. In this context, the sickle-cell allele is associated with health disadvantages when an individual has two copies of it, leading to sickle cell disease. In regions where malaria is prevalent, carrying one copy of the sickle-cell allele can confer a survival advantage, as it offers some protection against malaria. However, in areas where malaria is not present, there is no benefit to carrying the allele. Instead, individuals with two copies of the sickle-cell gene are more likely to experience the negative health impacts of sickle-cell disease without any compensating advantage.

Consequently, the selective pressure would favor the elimination of this allele from the population, as individuals without the allele would tend to have better overall health and reproductive success. In such a scenario, populations are likely to experience a decrease in the frequency of the sickle-cell gene, a process known as directional selection against it.

This understanding highlights the connections between environment, disease prevalence, and genetic variations in populations, illustrating the dynamic nature of natural selection based on varying conditions.