Meet an expert. I can't do justice to your question in 400 characters. Check out these great links on breast cancer. http://1.usa.gov/nmutde / http://1.Usa.Gov/nmv740 / http://1.Usa.Gov/nmvhzl that should get you started.
Exception not rule. Most breast cancer diagnoses are not "hereditary". The currently identified mutations (brca 1/2) account for only ~ 10-15% of newly diagnosed breast cancer. The primary risk factors for developing breast cancer are being female and having breast tissue. This is the basis for general population screening including regular self exams, regular clinical exams, and screening imaging (mammography).