Recent publications suggesting a skewed ratio of female to male births and a non-dominant transmission of mutant alleles to female offspring of BRCA mutation carriers have highlighted important potential implications when counselling affected individuals.^1,2 As the cancers seen in the hereditary breast and ovarian cancer syndrome (HBOC) are predominantly breast and ovarian cancers, one would expect that families with an increased number of female offspring would be more likely to have family histories suggestive of HBOC—specifically a number of breast or ovarian cancers, or both, in two or more generations. Families with more female offspring may also be more likely to seek genetic counselling. Many women with breast cancer who have female children undergo genetic counselling and testing because of concerns for the risks to their children. For these reasons, these studies are difficult to do, and despite the authors’ best efforts are subject to ascertainment bias. It is imperative that results regarding sex ratios in this population be interpreted carefully.

The ideal study would require a more population based approach, with testing carried out on a cohort of individuals and their offspring without attention to their family history. Various issues make this approach impractical, including but not limited to the cost of testing and the nature of genetic testing, in that it is not a “routine” type of laboratory test that can be done without any understanding of the implications of a positive test and a follow up plan in place to deal with individuals who are found to have a mutation.

In the first paper to address this issue, by de la Hoya and colleagues, the sex ratio of offspring was studied in 68 Spanish families who had been previously screened for mutations in BRCA1 and BRCA2.¹ These families were all tested on the …