When pregnant rats were fed a fatty diet, both their daughters and granddaughters proved to be at greater risk of breast cancer. In fact, even if the daughters of the first generation of rats ate healthily, their offspring -- the third generation -- were still at greater risk of disease.
According to the New Scientist:
“... a fatty diet may cause "epigenetic" DNA modifications that can be passed on to future generations.”
If this is also true for humans, it means that genetics could be only one of several reasons why a family history of breast cancer increases your risk for the disease.