A 38-year-old woman presents with a palpable mass in the right breast discovered during self-examination. Mammography reveals a 2.5 cm irregular lesion with microcalcifications. Core needle biopsy confirms invasive ductal carcinoma, grade 2. On detailed family history, her mother was diagnosed with breast cancer at age 42, and her maternal grandmother had ovarian cancer at age 55. Genetic testing reveals a pathogenic mutation in the BRCA1 gene. Which of the following best explains the increased cancer susceptibility in this family?