Frequency of the gene for cystic fibrosis with a view of replacement and recognition effects and reproduction by homozygotes.

The apparent frequency of the recessive autosomal allele for cystic fibrosis (cf) is too high to be satisfactorily explained by mutation equilibrium. However, higher reproductive fitness by heterozygotes (expressed by t greater than 0) or higher gametic viability of the cf allele (expressed by a greater than 0.5) can provide reasonable explanations of the present frequency. Furthermore, birth replacement in families with CF children (RPE) can account for the estimated historical values of t. Today, however, t is probably at or approaching zero as a result of discontinuation of RPE. In the future we can look for an imperceptibly slow decrease in the incidence of CF. Reproduction by CF females will not cause an increase in incidence; it will only slightly slow the rate of decrease.