It's not irrational at all. If there's not yet any pragmatic benefit to knowing that you have a significant risk of getting a serious disease in the future, it's probably better not to know.
You can select which tests to make available to yourself, though, and avoid finding out about things like Parkinson's.
Even though there's no current treatment for early-onset Alzheimer's, I'd certainly want to know if I was at risk. I'd live my life much differently if I thought my brain would rot at age 50.
Not knowing if you'll get an untreatable disease doesn't stop you from getting an untreatable disease! It just means it'll sneak up on you and force you to make tough decisions with less preparation.
"I'd live my life much differently if I thought my brain would rot at age 50."
Not to get all Carpe Diem on you, but I'd suggest if this is true, you should probably re-evaluate your life. Whether your brain starts to rot at age 50, or you get hit by a bus in two weeks, you don't get to control how much time you have. You can only control what you do with that time.
If you live every day like it's your last then that precludes sensible things like retirement planning. On the other hand, if you know you're not going to be mentally capable of enjoying your retirement anyway, then now is the time to blow that cash.
It would change my career and retirement planning quite a bit. I'd retire early. I would forget having kids, since I wouldn't be around to support them after their mid-teens.
Think about it from the flip side: If you knew you were going to live to age 200 in good health, wouldn't you retire later? Maybe you'd spend more time gallivanting around the world like a recent college grad. Maybe you would wait longer to settle down and have children. Any deviation from the current typical life-span should definitely change your behavior.
You can select which tests to make available to yourself, though, and avoid finding out about things like Parkinson's.