There are many different cancers that have a genetic basis. For breast cancer, the gene with the most evidence for breast cancer is BRCA1 (pronounced brak-ah one). In normal patients, BRCA1 encodes a tumor suppressor protein (i.e., a protein that prevent tumors by checking DNA for errors and either repairing the damage or killing the cell before it turns into cancer). In patients with a BRCA1 mutation, this check system is broken.
In patients with a BRCA1 (or BRCA2, another similar gene) mutation, the risk of breast cancer is about 60% by 90 years old. BRCA1/2 are also linked to ovarian cancer with a risk of 55% for BRCA1 mutations and 25% for BRCA2. If there is a family history of breast AND ovarian cancer, it would be worth your family members with cancer to be screened for BRCA1/2 mutations as this could effect the other women in the family. This tumor suppression gene mutations are very prominent in a family - nearly every woman in the family (or one side of a family) will have breast or ovarian cancer. If you have a family member that has had breast AND ovarian cancer, this would be highly suspicious. Additionally, BRCA gene mutations have a higher incidence in families of Ashkenazi Jewish ethnicity.
Finally, there are many other genes (some that we know and others that we don't) that may be associated with breast cancer. Testing for many of these is not worth the cost or the benefit. As such, it is important that you talk with your primary care physician
about your concern and family history to ensure that you have appropriate cancer screening
) at the appropriate intervals.