Only about 5 percent of those patients who develop breast cancer, have will have one or more identifiable genetic risk factor for breast cancer. This means majority of the breast cancers are unrelated to genetic risk factors. However if a woman inherits a genetic mutation like BRCA1 and BRCA2 the lifetime risk of development of breast cancer in that woman may be as high as 85 percent.
You may read more on BRCA1 and BRCA2 in the section dealing with this topic.
Apart form BRCA1 and BRCA2 there are rare genetic forms of breast cancer may be associated with mutation of the PTEN gene, ATM gene or p53 gene giving rises to Cowden's syndrome, or ataxea-telangiextasia, or Li-Fraumeni syndrome respectively.