Diabetes has a genetic component, but just carrying the gene is not a guarantee of getting the disease.  Type 1 is caused by autoimmune destruction of pancreatic beta cells, and doctors aren't sure why it happens (just like other autoimmune diseases).  Type 2 is caused by a progressive lack of sensitivity to insulin, followed by slow death of beta cells from overwork, and this is almost always set off by obesity or old age when all organs are declining in function anyways. As for both types, weight gain is from overeating.  I always hear diabetics bitching that the insulin is causing weight gain, which is complete and utter bullshit.  Yes, insulin is necessary for the body to retain excess weight, but that won't happen without eating too much food.