A 28-year-old man from rural Nigeria presents with a history of recurrent malaria episodes despite living in a malaria-endemic region. Genetic testing reveals he is heterozygous for the sickle cell mutation (HbAS). The diagram shows four mechanisms by which genetic polymorphisms persist in populations. The mechanism marked **A** explains why the sickle cell allele remains at high frequency (5–15%) in sub-Saharan Africa despite causing severe disease in homozygotes. Which of the following best describes the population-genetic principle represented by structure **A**?

Explanation

## Why Heterozygote advantage (overdominance) is right The mechanism marked **A** represents heterozygote advantage (balanced polymorphism), the textbook explanation for why sickle cell trait persists at high frequency in malaria-endemic regions. HbAS heterozygotes have superior fitness compared to both homozygotes: HbAA individuals are vulnerable to severe Plasmodium falciparum malaria (which causes high childhood mortality in endemic areas), while HbSS homozygotes suffer from sickle cell disease (historically <50% reproductive fitness). HbAS individuals gain ~50–90% protection against severe malaria through multiple mechanisms (HbS polymerization in parasitized red cells triggering splenic clearance, reduced parasite growth, enhanced innate immunity, and altered cytoadherence), while remaining largely asymptomatic. This creates a balanced polymorphism where both the normal (A) and sickle (S) alleles are maintained at frequencies far higher than expected by mutation alone. The Hardy-Weinberg equilibrium equation for balanced polymorphism (q = s_AA / (s_AA + s_SS)) predicts that when malaria mortality in HbAA and sickle disease mortality in HbSS are both significant, the HbAS genotype dominates the population. This is the classic textbook example of heterozygote advantage in human genetics (Allison AC, Br Med J 1954; Williams TN et al., Lancet 2016). ## Why each distractor is wrong - **Founder effect with random genetic drift**: While founder effects do occur in population genetics, they explain initial allele frequency changes in small populations by chance, not the maintenance of high allele frequencies in large endemic populations. Drift alone cannot explain why the sickle allele remains at 5–15% in millions of Africans; this requires active selection (heterozygote advantage). - **Consanguinity-driven homozygote increase**: Consanguinity increases the frequency of homozygotes (both HbAA and HbSS) relative to heterozygotes, but it does not explain why the sickle allele itself remains common. In fact, if consanguinity were the primary driver, we would expect HbSS disease to be even more prevalent, which contradicts the observation that HbAS trait is maintained at high frequency while HbSS disease remains relatively rare. - **New mutation hotspot in the HBB gene**: The sickle mutation (GAG→GTG at codon 6) is a single, well-characterized nucleotide substitution, not a hotspot. Mutation rates alone (typically 10⁻⁸ per base pair per generation) are far too low to maintain an allele at 5–15% frequency; selection pressure (heterozygote advantage) is essential. **High-Yield:** Sickle cell trait persists in malaria-endemic regions because HbAS heterozygotes have superior fitness — protected from malaria but not severely affected by sickling — a classic example of balanced polymorphism maintained by heterozygote advantage. [cite: Allison AC, Br Med J 1954; Williams TN et al., Lancet 2016; Robbins Basic Pathology 11th ed., Ch. 12]