Just one example: About 12 percent of women in the general population will develop breast cancer sometime during their lives. By contrast, according to the most recent estimates, 55 to 65 percent of women who inherit a harmful BRCA1 mutation and around 45 percent of women who inherit a harmful BRCA2 mutation will develop breast cancer by age 70 years. These estimations are a bit in flux, I've heard much higher ones in the past.

(http://www.cancer.gov/about-cancer/causes-prevention/genetic...)