An inherited metabolic disorder in which there are abnormally high blood sugar levels. In advanced stages, this often results in blindness from cataracts, nerve damage, gangrene in the feet and legs leading to amputation, heart disease, and kidney failure. Type 1 diabetes melitis (juvenile onset diabetes) is due to decreased production of insulin by the pancreas. Type 2 diabetes melitis is due to increased resistance of cells in the body to insulin. The gene or genes for diabetes are incompletely penetrant.