As researchers continue to map the estimated 100,000 human genes, they have learned that as many as 4,000 diseases have a genetic, or inherited, component. In some, such as cystic fibrosis and sickle-cell anemia, genes are directly responsible for the disease. In others, such as breast, colorectal, and ovarian cancers, genes impart a susceptibility to develop the disease.

Along with genes identified as links to diseases, the number of genetic tests available to determine whether a person will develop a certain disease or whether a person is at increased risk is growing. These tests can be both a blessing and a curse.

On the bright side, genetic testing may help further understanding of and, ultimately, treatments for a variety of illnesses. A negative result may take a load off your mind and save you or a family member from needless screening tests (although the lack of a genetic flaw does not necessarily mean that you are immune to a disease). A positive result may give you an opportunity to take steps to reduce your risk of developing a disease, if possible, or to prepare for the possibility that you will develop it.

On the negative side, although genetic tests can accurately detect genetic flaws, they may not be able to tell you whether, or when, those flaws will actually lead to disease. So a positive test result does not tell you the whole story. Likewise, because many diseases have multiple causes, a negative test result does not necessarily preclude disease development. In addition, people who test positively for a genetic flaw may worry about developing the disease or become upset or depressed. And some employers and insurers discriminate against people who are genetically at risk for various diseases.

If you believe you may be at risk for a genetically inherited disease and would like to know what your risk is, check with your doctor to see if a test is available. But before you undergo such a test, make sure you know exactly what the test can and cannot tell you. Make sure you have weighed the pros and cons. And make sure that qualified genetic counseling will be available to you.