Cystic fibrosis (CF) is caused by a genetic mutation resulting in defective proteins in secretory cells, mainly in the epithelial lining of the respiratory tract. The one in every 2,000 Caucasian babies who has the disease is homozygous for the recessive mutant. Although medical treatment is becoming more effective, in the past most children with CF died before their teens. About 20 Caucasians in 2,000 are carries of the trait, having one mutant and one normal allele, but they do not develop this disease. According to the rules of population genetics, the frequency of the homozygous recessive genotype should be rarer than it is. What is one possible explanation for the unusually high frequency of this allele in Caucasian populations?