Hello Hannah
I agree with what Skye has said.
Last night I went to a public seminar given by the Alzheimer's Research Trust and this question was discussed. I wouldn't attempt to even try and summarise it here as it was all very scientific, but they did mention things like APoE.
One of the things discussed was that it tends to be early onset dementia that is linked to genetic/hereditary AD. However, they said it was sometimes difficult to monitor because, if someone died in their fifties (say in a car accident) and then their relative went on to develop it. They wouldn't actually know whether the one in the car crash would have got it. Does that make sense? Particularly true where relatives are older and where it might not be possible to establish what they died of. They did mention three generations as being a possible measure. However, it all seems a bit woolly.
One thing that did come over was that lifestyle is an absolute factor. Some of the stats they showed indicated obesity as an indicator for developing some types of dementia in some cases. The message seemed to be, eat healthily, take plenty of exercise, keep blood pressure under control and alcohol intake to a minimum!
Of course, it's all "some" and ifs and buts and maybes. Having spent some time on TP, there seems to be so many things that can be thrown into the mix.
Having read this it's not much help! I've just thought of something else, as well. There are ethical issues around genetic testing. For instance how it would affect people psychologically if they knew that 2 years down the line they would develop the illness?
Best wishes and take care.