There are at least two categories of relaionship - inherited and familial. e.g. Huntington'disease (previously known as Huntington's Chorea) is proven to be hereditary. Cancer is not hereditary but is know to "run in families" which is why doctors always ask for family histories. However it is just suspected that if one has relatives who have had cancer, wherever, the risk is higher. Glaucoma is another disease which is familial but no hereditary link has been proven. As far as diagnosis for your 2x gt.grandfather is concerned, medical knowledge was so primitive at that time there is absolutely no way you could say it was definitely such-and-such. These are really only symptoms and, as you say, could have had a dozen causes.