Arlo, Nancy and Janice each have a 50/50 chance of developing a devastating nerve disorder. A laboratory test can tell them if in fact they will fall victim. In their shoes, would you take the test? Thousands of others face a similar choice: to know, or not know, if they will carry the genetic time bomb of Huntington's disease. NOVA looks at this incurable disease which affects 20,000 people in... see more