First off, it can't tell for certain - just that you have a high percentage chance of getting Parkinsons.

Now with that information, you might decide to live more of your life now... You might decide to have children earlier knowing that you will see the grow up before you enter the risky phase in your life when you could develop the condition.

Like Sergey Brin is now doing, you might pay close attention to the research of the condition - perhaps even donating more.