Final answer:
Less than 10% of all cancers are hereditary cancer syndromes, with a combination of genetic and environmental factors influencing most cancers. Genes like BRCA1 and BRCA2 greatly increase the risk but do not affect everyone who carries the mutations.
Step-by-step explanation:
The proportion of all cancers that are hereditary cancer syndromes is estimated to be less than 10%. Most cancers are a result of a combination of genetic and environmental factors, with only a smaller percentage being strongly influenced by inherited genetic mutations. Specific genes like BRCA1 and BRCA2 significantly increase the risk of certain cancers such as breast and ovarian cancer, but not all individuals with these mutations will develop cancer. Even among individuals with a family history of cancers, like in the provided case study, inherited mutations do increase risk but do not guarantee that cancer will develop.