Family History, Inheritance, and Breast Cancer Risk

I have been doing some research and came across this in a study by Cornell University:

What are breast cancer families?

Breast cancer families are families in which breast cancer is inherited and family members are at greater than average risk of breast cancer. Simply finding several cases of breast cancer within one family does not mean a pattern of genetic inheritance. Determination of family inheritance requires a detailed examination of breast cancer in present and earlier generations by someone trained in genetic analysis. Breast cancer is common enough (affecting one in eight or 12% of all American women over their lifetimes) that several cases could occur within a family merely by chance alone. For example, if a woman with breast cancer has ten female relatives who have lived to 80 years old, there is a 50% chance that one of them will also have developed breast cancer.

Breast cancer families are frequently described as families with three or more close relatives with breast cancer. Members of these families are at high risk for developing breast cancer at a young age that may affect both breasts. The existence of breast cancer families has been noted since the mid-1800s.

Several types of family patterns exist. There are families with breast cancer alone, families with both breast and ovarian cancer, and families with several types of cancer including breast cancer. Breast cancer families also differ in the number of family members affected with cancer. Some families have many members with breast cancer and very high risk and other families have fewer family members affected and lower, but still, higher than average breast cancer risk.

http://envirocancer.cornell.edu/factsheet/general/fs48.inheritance.cfm