Some cancers show a genetic predisposition - for example BRCA Mutation a form of breast CA.

The BRCA genes are involved in initiating DNA repair and cell cycle check points, they as as regulators of other tumour supressors genes such as p53 and help the cell repair and damage which may lead to cancer occurring in the cell.

In mutant versions of the BRCA genes they cannot perform their roles and damaged DNA may cause the cell to replicate uncontrollably as check point to prevent this has been removed. Genome stabiliy is affected and cancer can develop

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Yes, but the BRCA1 and BRCA2 tests can also be negative.

BRCA1 and BRCA2 are genes that can mutate into cancer.