You may have to pretend a couple of things to think about this question.
Say you have a chronic, debilitating, possibly disabling illness, like MS. Or even some form of cancer or something. Whatever illness, it is something that couldn't be prevented. Say you also have a young child. I'll let you decide how young is "young" because your answer might be different if the child is 14 as opposed to 4.
If you could know now, while your child is young, whether your child will inherit the disease from you, would you want to know? You wouldn't know when the child will have the first symptoms just that eventually, someday...
My answer? I wouldn't want to know. Even if I found out my child will eventually have whatever disease, it could be years before she shows any signs or symptoms. She could be very old; I might even be dead before she was diagnosed. I would rather live happily in ignorance than wonder every time she complains of a headache or funny vision or pins and needles whether that was the time she would be diagnosed.
Does that make sense? It would be bad enough not knowing, but to know for sure but not to know when would really suck. I mean really suck: suck the joy out of stuff. You know? Because I would always worry.
Maybe that's selfish, but if I was always worried, wouldn't my kid always be worried too?