Correct answer:
(3) Both alleles would remain in population

Introduction

Genetic diversity is vital for the adaptability and health of populations. One of the most fascinating mechanisms that preserves this diversity is heterozygote advantage. In this evolutionary scenario, individuals with two different alleles (heterozygotes) have a higher fitness than those with two identical alleles (homozygotes). This leads to the stable maintenance of both alleles in the population, preventing the loss of genetic variants.

What Is Heterozygote Advantage?

Heterozygote advantage occurs when the heterozygous genotype (e.g., Aa) has a higher relative fitness than either the homozygous dominant (AA) or homozygous recessive (aa) genotypes. This means that natural selection favors individuals who carry both versions of a gene, rather than just one.

Key Features

• Both alleles are maintained: Neither the dominant nor the recessive allele is lost from the gene pool.

• Genetic diversity is preserved: This leads to a balanced polymorphism, where multiple alleles coexist in the population.

• Stable equilibrium: The population reaches a genetic equilibrium where the frequencies of both alleles remain constant over time.

How Does It Work?

When heterozygotes are the most fit, natural selection increases their frequency. However, as the frequency of one allele rises, the chance of producing homozygotes also increases, which are less fit. This negative feedback keeps both alleles present in the population at stable frequencies.

Classic Example: Sickle Cell Trait

In humans, the sickle cell allele illustrates heterozygote advantage. Individuals with one sickle cell allele and one normal allele (heterozygotes) are resistant to malaria and do not develop sickle cell anemia. This advantage maintains both the normal and sickle cell alleles in populations where malaria is common.

Evolutionary Significance

• Maintains adaptability: Populations with greater genetic diversity are better equipped to survive changing environments and disease pressures.

• Prevents allele loss: Unlike other forms of selection, heterozygote advantage ensures that neither allele is driven to extinction.

Conclusion

When heterozygotes are more fit than either homozygote, both alleles remain in the population. This powerful evolutionary mechanism, known as heterozygote advantage, is a cornerstone of genetic diversity and population resilience.

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Heterozygote advantage shows how nature can maintain diversity, ensuring populations are robust and adaptable in the face of environmental challenges.