A good family health history is important in predicting future medical needs, but it's hugely underused. But now the government will beging offering a free new service to help people compile one at home, e-mail it to relatives who can fill in the gaps, and even pop it straight into their doctors' computers. The goal is to create a family health tree that will help genetics specialists look for patterns of inherited illnesses that can provide a powerful window on someone's brewing health risks.