Sex ratio and heterozygote advantage in cystic fibrosis families.

The families of aunts and uncles of cystic fibrosis (CF) patients were compared with one another and with controls in order to test claims of increased fertility and sex ratio in families containing the CF allele and to identify the more fertile sex. Uncles' families were found to contain a significant excess of sons, particularly at primiparous births, as well as a significantly increased sex ratio. Reanalysis of published records of CF sibships confirmed the effect of parity on sex ratio. The families of uncles were also exceptional in exceeding the size intended, although their mean size was not raised significantly. It is suggested that these effects could be related to an interaction between CF-related, paternally-derived antigens and the maternal immune system.