After malaria is cured, the frequency of the HbS allele should decrease.
The correct answer and explanation is:
Correct Answer:
Yes, after malaria is cured or eliminated, the frequency of the HbS allele (sickle cell allele) is expected to decrease over time.
Explanation (300 words):
The HbS allele is a genetic variant of the hemoglobin gene that causes sickle cell disease when inherited in two copies (homozygous). However, individuals who carry only one copy of the HbS allele (heterozygotes) have what’s called sickle cell trait and typically do not suffer from the disease’s severe symptoms. Interestingly, heterozygotes are more resistant to malaria caused by Plasmodium falciparum, which is a major cause of death in many parts of Africa and other malaria-endemic regions.
This heterozygote advantage is a classic example of balanced polymorphism or heterozygote advantage in evolutionary biology. The presence of malaria creates a strong selective pressure favoring individuals who carry one HbS allele because they have increased survival against malaria infection. This selective advantage maintains the HbS allele at a higher frequency in the population, despite the fact that homozygous individuals suffer from sickle cell disease, which reduces their survival.
If malaria were completely cured or eliminated, the selective pressure maintaining the HbS allele would be removed. Without the survival advantage against malaria, carrying the HbS allele would primarily be detrimental due to the risk of sickle cell disease in homozygous individuals and complications in some heterozygotes. Natural selection would then tend to reduce the frequency of the HbS allele over generations, as individuals with normal hemoglobin (HbA) would have higher overall fitness without malaria as a threat.
Therefore, after malaria eradication, the HbS allele frequency is expected to decline gradually as the balancing selection is lost. However, this decline would not be immediate since genetic changes in populations happen over multiple generations. Also, other factors such as migration, genetic drift, and social factors can influence allele frequencies.
In summary, the persistence of the HbS allele is tightly linked to the presence of malaria. Eradication of malaria removes the protective benefit of the HbS allele, leading to a predicted decrease in its frequency over time.