Would you want to go through your life constantly thinking “my hands are a little trembly today, is this it? Am I doomed?”
Parkinson’s, if it is caused via genetics, is unpreventable (parkinson’s is also caused by environmental factors). Really, most diseases which someone is predetermined to have via genetics are usually not preventable (other than maybe breast cancer). You just have to hope you don’t get it. Knowing you’re susceptible to it doesn’t really help you much other than cause you to worry the rest of your life about every little ache and pain. And through the placebo effect, you might make things worse in some way.