It is thought that up to 5% of all breast cancers are inherited owing to the presence of abnormal genes. These genes are passed on from mother to daughter, so having a first-degree relative (such as a sister or mother) with breast cancer will increase your chance of developing the disease. This is particularly true if the relative develops breast cancer in both her breasts, or before she reaches the age of 45 tears. A family history of breast cancer in a first-degree male relative is also associated with a significant increase in breast cancer risk. Breast cancer in a distant relative has little effect on your breast cancer risk.
Your chance of developing breast cancer doubles if one first-degree relative developed the disease before the age of 45 years. If two first-degree relatives developed the disease before the age of 45 years, then your chance of developing breast cancer is four times greater than normal.
Scientists have identified several genes responsible for transmitting breast cancer. Three of the most important of these breast cancer genes are called BRCA-1, BRCA-2 and P53. This inherited form of breast cancer usually develops before the age of 50 years. If a woman has not developed breast cancer by the age of 50 years, despite having a first-degree relative with breast cancer, it is unlikely that she carries the abnormal gene(s).
The BRCA-1 gene is also associated with ovarian cancer. So, the presence of other types of cancer, such cancer of the womb or ovary, in addition to breast cancer, also suggests the possibility that there is a cancer-causing gene in the family. Testing for breast cancer genes is now possible, but may require the presence of living relatives who have had breast cancer, and it may also take a long time to identify the gene.
A significant family history of prostate cancer (early age of onset before the age of 55, more than one relative) is also associated with an increased risk of breast cancer.