Well my mum and Dad both had it so i checked it out and i think we have to agree to disagree but no it's not, my mums parents lived long lives and neither had dementia, my Mum got it through having General Anaesthetic at the age of 80 having a breast removal, my mum was also the oldest of 9 children and none of her brothers or sisters have or had the disease either, my cousin is a Sister in a local hospital too and she also told me it is not Hereditary....even things like a bang on the head when you are younger can bring this on....as we see in our country now with ex professional footballers who are getting this disease because of heading heavy footballs all their careers.
http://www.alzheimer-europe.org/Dementia/Alzheimer-s-disease/Is-Alzheimer-s-disease-hereditary
https://www.alzheimers.org.uk/info/20010/risk_factors_and_prevention/117/genetics_of_dementia/8