I think this is not as straight forward as the medics will have us think. My farther was diagnosed first in our family followed by my mother, then 6 years ago I was diagnosed followed by my older sister one year later and my younger brother last year (2017). I have one younger brother left who is 57, that has shown no signs yet? Interestingly no on either of my parents side have been diagnosed nor did their paternal parents, that we are aware of.