The risk for "passing on" autoimmune Type 1 diabetes (which sounds like the kind your father had-it usually presents suddenly, often with coma, and CANNOT be prevented) is 7% for a father, and 2-4% for the mother. If BOTH parents have T1, the risk is about 30%. Most people who inherit the genes for T1 never develop it because they are not exposed to the "trigger" whatever that may be.

If there are multiple other autoimmune diseases present in the parent or parent's family such as MS, Lupus, Autoimmune Thyroid Disease, Celiac Disease, etc. the risk may be much higher because it signifies a strong genetic component to developing autoimmune diseases such as Type 1 diabetes.

For Type 2 diabetes (the common lifestyle related kind), I think the risk is high, 50% chance if both parents have it. Of course, Type 2 can easily be prevented in most cases by living a healthy lifestyle and making the effort to prevent it. Lifestyle usually overrides genetics, but not always of course.

There are rare exceptions like MODY (Type 2 in young thin people), which have a very strong genetic component, but thankfully, MODY is fairly rare.