Final answer:
Inheritance of the BRCA1 gene mutation significantly raises the risk of breast cancer, but it does not guarantee that an individual will develop the disease, as cancer development is influenced by a combination of genetic and environmental factors.
Step-by-step explanation:
Not everyone who inherits a mutation in the BRCA1 gene will eventually develop breast cancer; inheritance of the mutation merely increases the risk compared to the general population.
Inherited mutations in the BRCA1 or BRCA2 genes do significantly elevate the risk of developing breast and ovarian cancer, with statistics showing a 55 to 65% probability of women with a harmful BRCA1 mutation being diagnosed with breast cancer by age 70. However, this does not guarantee that cancer will occur. Various factors, including environmental, lifestyle, and other genetic influences, interact with inherited mutations to affect cancer risk. For example, not all individuals with a BRCA1 mutation will develop cancer, as evidenced by Rebecca's 72-year-old mother, pointing out the complexity of cancer development. Moreover, regular cancer screenings are often recommended for those with confirmed BRCA1 or BRCA2 mutations to improve early detection and treatment outcomes should cancer develop.