Correct answer:
(2) Heterozygote advantage

Introduction

Evolution often balances the costs and benefits of genetic traits within populations. The relationship between sickle cell anemia and malaria resistance is a classic example of how a seemingly harmful gene can persist because it offers a survival advantage under certain conditions—a phenomenon known as heterozygote advantage.

What Is Heterozygote Advantage?

Heterozygote advantage occurs when individuals with two different alleles for a particular gene (heterozygotes) have higher fitness than individuals with two identical alleles (homozygotes). This form of balancing selection maintains genetic diversity in a population.

Sickle Cell Trait and Malaria: The Classic Case

• Sickle cell anemia is caused by a mutation in the beta-globin gene. Homozygous individuals (SS) suffer from severe anemia, while heterozygotes (AS) carry one normal and one mutated allele.

• In malaria-endemic regions, heterozygotes are less likely to die from malaria because the presence of some sickled cells makes it harder for the malaria parasite (Plasmodium falciparum) to thrive.

• At the same time, heterozygotes do not experience the life-threatening symptoms of sickle cell anemia, unlike those with two sickle alleles.

Why Does This Matter?

This advantage means that the sickle cell allele is maintained at higher frequencies in populations where malaria is common. It is a powerful demonstration of how natural selection can favor genetic diversity when it provides a survival benefit in a specific environment.

Other Examples of Heterozygote Advantage

• Cystic fibrosis carriers are more resistant to certain diarrheal diseases.

• Tay-Sachs carriers may have increased resistance to tuberculosis.

Conclusion

The persistence of the sickle cell allele in malaria-prone regions is a textbook example of heterozygote advantage, where carrying one copy of a mutated gene provides protection against a deadly disease without causing the full-blown genetic disorder.

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Heterozygote advantage is a striking illustration of how evolution can shape the genetic landscape of populations, sometimes turning a genetic disadvantage into a critical survival tool.