Scientists believe that both copies of either BRCA1 or BRCA2 must be mutated in order for cancer to occur. This should make the event exceedingly rare since the chance of a random mutation striking both copies of the same gene is minuscule. Inheritance of both mutated genes from two parents who are carriers should also be rare. Twenty five percent of their offspring, or 1 out of every 10,000 Ashkenazic Jewish offspring would inherit both genes. The problem is that inheriting two mutant copies from two different parents is not the only way to get two mutant copies of a BRCA gene .
Mutations can occur in our cells at any time.
They are rare events but they do occur, and they happen randomly. In addition, our cells have repair enzymes which can detect and fix many of those mutations. Even if an occasional adult cell dies because of having two faulty copies of an important gene, it is likely to go unnoticed in our adult bodies where over a trillion cells perform the various functions, and there are many backup cells to compensate for malfunctioning ones. However, the BRCA genes in their normal forms are such important genes since they control cell growth and suppress cancer. When BRCA genes are not functioning properly, the cells begin to grow out of control, and cancer develops.