BRCA1 is a gene involved in repair of double-stranded DNA breaks. Mutant forms of this gene are linked to a substantial proportion of familial breast cancers

Question

A woman who inherits a certain allele of the gene has about a 60-80% chance of getting breast cancer, as well as an elevated chance of getting ovarian cancer, in her lifetime. a. Why don't a higher percentage of women with the mutation get breast cancer? b. How would the breast cancer inheritance caused by BRCA1 be described in Mendelian terms?

Answer 1

(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.