Familial adenomatous polyposis (FAP) and gender. Does gender influence the genetic transmission of FAP?

Familial adenomatous polyposis (FAP) is an autosomal dominant syndrome with a penetrance close to 100% at the age of 40 years. The incidence is thought to be equal among both sexes, but we noticed an excess of males undergoing primary surgery for FAP at our institution. The aim of the study is to investigate the hypothesis that FAP patients produce an excess of affected male offspring. We identified all families with known APC mutation in the polyposis registry at St Mark's from its foundation until October 2009. We analysed their pedigrees with respect to gender of the affected individuals with progeny and to the gender and mutation status of their offspring. Only individuals with complete data regarding their offspring (gender and mutation status) were included. We identified 666 (324 males and 342 females) affected individuals with progeny. We analysed the progeny of 368 (182 males, 186 females) affected individuals with complete data on all offspring: 235 (27.5%) affected males, 212 (24.8%) affected females, 207 (24.3%) unaffected males and 200 (23.4%) unaffected females. The overall ratio of affected/unaffected and male/female offspring did not differ from the expected 50%. Further sub-analysis by gender of parents did not show any statistically significant difference in gender and mutation status of offspring. In addition the mean number of children per affected parent did not depend on gender (males 2.34; females 2.30). This study shows that gender does not influence the genetic transmission of FAP. The excess of males undergoing primary surgery at our institution is probably a result of referral bias.