Question

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 Caucasian in 2,000 are carriers of the trait, having one mutant and one normal allele, but they do not develop the disease. According to 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 population?

Answer 1

There are three major factor that causes an unusually high frequency for a lethal recessive allelle. The first and the most important is heterozygote advantage to Tuberculosis, which means that they have atleast partial immunity tobthis infectious disease. The second reason is the Founder effect which leads to additonal increse in the allele frequency. The idea for this is based on the observation that when human population goes through narrow bottlenecks then a resulting loss of genetic variation occurs. When the population re-establishes itself any ‘alleles’ present in high numbers amongst the ‘founders’ will be present at much higher frequencies than prior to the bottleneck.

Finally, continuous addition by newer and newer mutations also promotes high frequency.

You may read the following article: http://www.currentpediatrics.com/articles/why-does-cystic-fibrosis-display-the-prevalence-and-distribution-observed-in-human-populations.html