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. 