All of our genes are in pairs. This means that we have inherited two BRCA1 genes; one from our mother and one from our father. We have also inherited two BRCA2 genes; one from our mother and one from our father.
A harmful BRCA1 or BRCA2 mutation can be inherited from a person's mother or father. Each child of a parent who carries a mutation in one of these genes has a 50% chance of inheriting the mutation.
BRCA mutations are inherited in an autosomal dominant pattern, which means that a single copy of the mutated BRCA1 or BRCA2 gene in each cell is enough to increase your chance of developing certain cancers.
When a person with a BRCA mutation has children, they may pass along a copy of their normal gene or a copy of the gene with the mutation. Therefore, each child has a 1 in 2 (or 50%) chance of inheriting the BRCA mutation and a 1 in 2 (or 50%) chance of inheriting the working BRCA gene copy, as is shown below:
However, sometimes an individual may carry a harmful BRCA1 or BRCA2 mutation that was not inherited from either parent, but which arose spontaneously within their own DNA. Mutations are occurring all the time within our DNA and, therefore, these BRCA mutations may arise on their own accord, without having been inherited from a parent. These mutations are known as somatic mutations.