A great number of diseases, such as diabetes, congenital heart defects, spina bifida, and cancer, are now known to have a hereditary component. These are called "multifactorial" diseases because they are caused not by a single gene mutation, but by a combination of genetic and environmental factors working together in ways that aren't yet fully understood.
Genetic screening for mutations associated with multifactorial diseases can only tell you about your risk. It cannot predict whether you will develop the disease. For instance, a woman who inherits an alteration in the BRCA2 gene is more likely than other women to develop breast cancer, but she may also remain disease-free. The altered gene is only one risk factor among many. Lifestyle, environment, and other other biological factors also play a part.
If you are concerned about a health problem that seems to run in your family, and want to know whether your genes put you or your children at increased risk, you may want to talk to a genetic counselor about what tests are available.