The sickle-cell allele of hemoglobin $(S)$ has a relatively high frequency in Central Africa $(0.12),$ even though individuals homozygous for this allele usually die young. Why has this allele persisted in the population, when you would expect there to be strong selection against it?a. Heterozygous individuals are resistant to malaria.b. Homozygous individuals are resistant to malaria.c. Heterozygous females are more fertile than homozygotes.d. Homozygous females are more fertile than heterozygotes.

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