I was quite shocked to find that a friend's mum and dad (both in their eighties) shared the assumption that her daughters (in their thirties) were bound to develop dementia. This was based on the fact that both the friend and her mother developed dementia but ignored the fact that each of them had an entirely different kind of dementia.
I mentioned this to my step-daughter. As S's mum had vascular dementia, we had discussed it before and I had found reliable information online suggesting that the number of people who will almost certainly develop dementia as a result of their genes is fairly small. After our latest conversation she sent me a PDF document from Alzheimer's Research UK entitled 'Genes and Dementia' which I feel is pretty reassuring.
You can see the document by following this link: