In some human populations, the proportion of individuals who are heterozygous for the sickle cell allele is much higher than would be expected by chance alone. Why?
A. Individuals with two harmful alleles have an advantage over individuals with two normal alleles.
B. Heterozygous individuals have an advantage over individuals with two normal alleles.
C. Individuals with two normal alleles have an advantage over heterozygous individuals.
D. Individuals with two harmful alleles have an advantage over heterozygous individuals.
Answer: B
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