What is the pattern of inheritance in families with a faulty BRCA1 or BRCA2 gene?

Two factors influence the pattern of inheritance of BRCA1 or BRCA2 gene(s) in families.

1. The BRCA1 and BRCA2 genes are located on chromosomes 17 and 13 respectively.
2. The effects of changes in the BRCA1 and BRCA2 genes are ‘dominant’ over the information in the working copy of the genes on the partner chromosomes, meaning a person only needs to inherit one faulty copy to have the increased risk of cancer.

The pattern of inheritance in families of the faulty genes causing predisposition to breast and/or ovarian cancer is therefore described as autosomal dominant inheritance.

In Figure the autosomal dominant faulty gene causing predisposition to breast and/or ovarian cancer is represented by ‘D’; the working copy by ‘d’.
The faulty gene is represented by ‘D’; the working copy by ‘d’.

Where one of the parents is a carrier of a faulty BRCA1 or BRCA2 gene, in every pregnancy, each of their children has:

• 1 chance in 2 (50% chance) of inheriting the faulty gene from the affected parent 
• 1 chance in 2 (50% chance) of not inheriting the faulty gene

Some important things to note:

• Cancer will not develop in a woman who is a carrier of a faulty BRCA1 or BRCA2 gene unless further mutations occur in additional other ‘cancer protection’ genes in the cells during her life 
• Women who have not inherited a faulty gene are not at increased risk of developing breast and/or ovarian cancer over their lifetime and cannot pass the faulty gene on to their own children. However, they still have the same risk for developing breast and/or ovarian cancer as the average woman in the Australian population.

While the Figure shows the father as the parent carrying the faulty BRCA1 or BRCA2 gene, the same situation would arise if it was the mother.

• A faulty BRCA1 or BRCA2 gene can be inherited from either the mother or the father
• Environmental factors causing mutations in the BRCA1 or BRCA2 gene(s) are still largely unknown. The identification of these factors and preventing their action paves the way for the prevention of many cancers. This is the subject of intense research 
• The identification of the environmental factors causing mutations in other ‘cancer protection’ genes over the woman’s lifetime that eventually lead to breast and ovarian cancer are also unknown 
• Women of Ashkenazi Jewish ancestry have a higher chance of being a carrier of a faulty BRCA1 or BRCA2 gene