Final answer:
The S allele for sickle cell anemia remains common because heterozygous individuals have a survival advantage against malaria, keeping the allele in the gene pool. If malaria were eliminated, the allele frequency would likely decrease as the advantage would disappear, and only the deleterious effects of the disease would remain.
Step-by-step explanation:
The frequency of the S allele, which causes sickle cell anemia, is higher in populations where malaria is most common due to a protective effect. Individuals who are heterozygous for the S allele (carry one copy of the mutation and one normal allele) have a resistance to malaria. This survival advantage means they are more likely to survive and pass on the S allele to offspring. Even though people who are homozygous for the S allele (carry two copies of the mutation) suffer from the severe consequences of sickle cell anemia, the benefits of heterozygous resistance to malaria in high-risk regions maintain the allele's presence in the gene pool.
If malaria were eliminated in these populations, the selective pressure that maintains the high frequency of the S allele would be removed. Consequently, over time, one could expect the prevalence of the S allele to decrease because the survival advantage it confers would no longer exist, and individuals with the homozygous condition would experience reduced fitness without any compensatory benefit.