The seasonal variation of births of offspring from couples heterozygous for cystic fibrosis.

To test whether the month of birth of patients with cystic fibrosis (CF) is seasonally distributed as reported in a Dutch survey, published Australian data were examined and no difference in the monthly variation was found between births of CF patients and their unaffected sibs. When the two groups were combined and analysed by periodic regression using successively higher Fourier terms, a significant fit to the observed frequencies was obtained with a bimodal curve. Possession of a single CF gene seems therefore sufficient for expression of the cyclic variation. In substantial agreement with the Dutch series, maximum frequencies were found to occur in March or April and in September. It is suggested that one or both parents may be subject to variable fertility mediated by a temperature-dependent mechanism. The relevance of a periodic fertility pattern in preserving the CF gene in the population by selective heterozygote advantage is discussed.