An accurate gene test will tell you if you do or do not have a disease-related gene mutation. If you do, a variety of factors can influence the gene's penetrance and the chances that you will actually develop disease. Nearly everyone with the familial adenomatous polyposis genes will - unless he or she takes effective preventive measures - someday develop colon cancer. On the other hand, women who carry the BRCA1 breast cancer susceptibility gene have an 80-percent chance of developing breast cancer by the age of 65; their risk is high but not absolute.
Of course, even family members who escape the inherited susceptibility gene are not exempt from risk. Like anyone else, they could develop mutations in that same gene during their lifetimes. Or, they could have inherited a different, unknown susceptibility gene.