I just had a terrible daydream where my dad gets diagnosed with early-onset alzheimer's. The scenario I pictured was horrible, I'm a bit shaken, I don't ever want that to happen. My great-grandmother on his side of the family lived to be over 100. I spoke with her a year or so before she died, her brain had clearly deteriorated somewhat. I don't know if she had dementia or something, but she acted as though she did, hardly speaking, people babying her through the conversation, she would occasionally say "what a nice family!". His mother, my grandmother, is still around and sharp as ever. His father died soon after I was born, lung cancer or something. My dad is aged 50 and has a drinking problem, he also has sleep apnea. What are the odds of something happening to his brain?