The sickle-cell allele is maintained at high frequency in malaria- endemic regions because
- A
Homozygotes have higher fitness than heterozygotes in the presence of malaria (homozygous advantage / directional selection)
- Bcheck_circle
Heterozygotes have higher fitness than either homozygote in the presence of malaria (heterozygous advantage / balancing selection)
- C
Recurrent mutation reintroduces the sickle allele faster than selection removes it (mutation-selection balance)
- D
Random genetic drift maintains the allele at high frequency despite negative selection (drift overpowering selection)
Explanation
Heterozygotes resist malaria and don't have anemia; homozygotes have anemia. Equilibrium is reached at allele frequencies determined by selection on each genotype.