Cystic fibrosis (CF) is a Mendelian recessive disorder of humans caused by defec
ID: 57808 • Letter: C
Question
Cystic fibrosis (CF) is a Mendelian recessive disorder of humans caused by defects in ion transport. Until the 1950’s, when antibiotics were first used to treat CF patients, most newborns with CF died at an early age. Yet CF is relatively common in Caucasians, with a frequency at birth of 1/2500, which implies that the frequency of the CF-causing mutation is about 0.02—a surprisingly high frequency of an allele that is lethal to homozygotes. There is no agreement on the reason for this high frequency.
a) Suppose that an allele that causes CF is maintained by mutation-selection balance. What would be the mutation rate necessary for that allele to have a frequency of 0.02?
b) Suppose that an allele that causes CF is maintained by heterozygote advantage. In order for the equilibrium frequency to be 0.02, what would the difference between the viabilities of the homozygote and the heterozygote have to be?
Explanation / Answer
a. 1/5000
b. Heterozygotes will be carriers and homozygous will be lethal.
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