I believe you pass on some sensibility in that area, like the kids will be more prone to get the disease, not that they will actually develop it. Major influence if both parents have it, even more if their parents had it.

For example, if your grandparents had lung cancer, your parents have lung cancer, it would be wise if you didn't smoke or inhale toxic chemicals on a regular basis.

Edit for clarification: by inhaling toxic chemicals on a regular basis I mean work in places where said chemicals are present: factories, paint shops, gas stations etc