Why BRCA is related to cancer risk
This is why BRCA-positive people have a higher risk for cancers. It also explains why they get them earlier in life. They were born already having one mutated BRCA allele ready to fuel cancer cells.
Normally, people who have two normal BRCA alleles would need both copies of their BRCA to spontaneously mutate before getting cancer. BRCA-negative people spend most of their lives cancer free, and if they end up with breast cancer or ovarian cancer they get these later on in life after much time as passed, allowing for spontaneous mutations.
Basically, BRCA-positive families only need to “win” this unlucky lottery once while others need to win it twice before getting cancer.