Q.The harmful allele of sickle cell anemia has not been eliminated from human population. Such afflicted people derive some other benefit. Discuss.
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Start your 14-day free trial to unlock the full solution →The sickle cell allele persists because carriers (heterozygotes) are protected against malaria, a survival advantage that outweighs the cost of the disease in homozygotes.
The story of sickle cell anemia is a powerful example of how a harmful genetic condition can remain common in a population — not because nature is careless, but because the same allele offers a hidden benefit in certain environments. This is a classic case of what biologists call balanced polymorphism, where the harmful allele is maintained because the heterozygote (carrier) has a survival edge.
Let’s set the scene. Sickle cell anemia is caused by a mutation in the haemoglobin gene. Individuals who inherit two copies of the mutant allele (homozygous) suffer from severe anaemia, pain crises, and often die young. You would expect natural selection to eliminate such a deadly allele quickly. Yet, in many parts of Africa, India, and the Mediterranean, the allele frequency remains surprisingly high — sometimes as high as 20% in certain populations.
Why hasn’t it been weeded out? The answer lies in the link between the sickle cell trait and malaria.
The key point is that carriers (heterozygotes) are resistant to malaria, a deadly disease caused by the Plasmodium parasite. This resistance gives them a strong survival advantage in malaria-endemic regions.
Here’s how it works. The malaria parasite spends part of its life cycle inside red blood cells. In a carrier, the red blood cells contain both normal and sickle haemoglobin. When the parasite infects such a cell, the cell tends to sickle (collapse) more readily. This sickling disrupts the parasite’s development and triggers the immune system to destroy the infected cell. As a result, carriers suffer less severe malaria and are more likely to survive to reproductive age.
Now, think about the population-level effect. In a region where malaria is a major cause of death, the benefit of being a carrier (protection from malaria) outweighs the cost of having a few children born with sickle cell disease. The allele is preserved because carriers pass it on to the next generation. This is not a case of the allele being “good” — it is a trade-off. …
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