Around 85 per cent of women whose BRCA gene is first-degree-in other words, it affects only her mother or sister rather than generations of women in the family-will never develop breast cancer. Paradoxically, 85 per cent of women who have breast cancer don't have the genetic malfunction.
Studies have also demonstrated that genes play a secondary role in determining whether or not someone develops breast cancer. The Women's Health Initiative (WIH) study discovered that only women who took HRT (hormone replacement therapy), irrespective of their genetic make-up, were more likely to develop breast cancer.
(Sources: The Times, May 19, 2013; Epidemiology, 2009; 20: 752-6).