Cystic fibrosis is a recessive genetic disease caused by a mutation in the gene that encodes the CFTR protein, which transports chloride ions in and out of cells. Some populations have a relatively high rate of cystic fibrosis carriers, possibly because the mutation protects against severe dehydration in diseases like cholera or typhoid. What concept does this best illustrate? A: Polygenic traits
B: Genetic drift
C: Pleiotrophy
D: Heterozygote advantage