Sickle Cell Anaemia Is a Debilitating Disease Caused by a Mutation

Multiple Choice

Sickle cell anaemia is a debilitating disease caused by a mutation in a haemoglobin gene. Homozygotes for this allele have decreased fitness and suffer haemolytic anaemia. However, the allele occurs at high frequencies in some populations because

A) heterozygotes for the mutant allele have the same fitness as homozygotes for the normal allele.
B) homozygotes for this allele have a high reproductive capacity.
C) selection does not act to reduce this allele as haemolytic anaemia is not a problem in some populations.
D) heterozygotes for this allele have increased resistance to malaria than homozygotes with the normal allele.
E) there is very little difference in fitness between people with the mutant allele and those with the normal allele.