Although Type 2 Diabetes is generally considered a life-style disease, they have found 6 gene variations that can up your risk of developing the disease. The more variations you inherit, the more likely you are to develop the disease, unless you take measures (like eating low carb and exercising!) to prevent it.
Researchers find six more diabetes genes: study - Yahoo! News
This explains why some people eat a lousy diet, never exercise, gain a lot of weight, and never develop diabetes, and others get a full-blown, terrible case involving heart and kidney failure, losing limbs, and going blind.
Unfortunately, even if a person knew that he had all 6 gene variations, unless he did his own research on the best diet to follow, he'd probably be given the old "eat less, reduce fat, eat more carbs" mantra that doesn't work. So, just knowing that they possess the variations isn't going to help them. Getting the right information on controlling the disease will.