It is a fairly long medical journey, because so many things have to be ruled out. Eventually, in consultation with the family doc, the diagnosis is determined. But it is pretty bleak, currently. Nothing is really out there that can address AD in its various forms. Alzheimer's is pretty non-discriminating.
Different families are more prone to it.
That is in fact how the genes were first isolated. That clue from Down's actually gave researchers their first hint as to where to go looking for genes that are responsible for the disease. They started with chromosome 21 and hit research paydirt. But that's the only specialized population I can think of. That has been shown by a number of groups to be quite false however.