BRCA1 is a gene involved in repair of double-stranded DNA breaks. Mutant forms of this gene are ...

(a) BRCA1 is a tumor suppressor gene. Tumor suppressor genes generally conform to Knudson's two-hit mutational model of cancer. For a cell to become cancerous as a result of BRCA1 mutation both copies of the gene need to have become mutant and non-functional. If a woman is born with one defective copy her odds of ending up with cells with two defective copies are greatly increased over that of a person whose cells started out with two good copies, but the chance is still less than 100.
(b) In Mendelian terms breast cancer caused by a hereditary mutation in the BRCA1 gene would appear to be dominant but with partial penetrance.