My last column was on medical genealogy. This is a follow-up on genetic testing.

Many of the causes of our illnesses are inherited from our ancestors. Almost a third of known diseases have genetic links. These include colon cancer, heart disease, alcoholism and high blood pressure.A medical genealogy or medical family tree can reveal patterns. If you have prepared a medical genealogy and found that a disease seems to run in your family, you might want to consider genetic testing. This form of testing can help you plan.DNA, or deoxyribonucleic acid, is in the genes you get from your parents. DNA guides the cells in your body. If your DNA contains a mutation, you could develop a medical condition.A test can reveal mutations that raise the risk of developing a disease. Positive results for certain diseases can induce people to take preventive action, such as surgical removal of endangered organs.About 900 genetic tests are now offered by diagnostic laboratories. The tests cost from less than $100 to a few thousand dollars. Your health insurance may not cover testing.Testing usually requires a blood sample, but may require hair, skin or other tissue samples, such as cells from the inside of your cheek.Genetic testing should be viewed as a fallible tool. A positive result for a mutation doesn't mean you'll get a disease. And a negative result doesn't mean you are immune.Multiple mutations can cause a disease. Multiple genes can be responsible for a single disease. There are gene changes that develop without any link to your ancestors; they happen because you smoke or get too much sun or sometimes for no known reason.If you decide to try genetic testing, remember that what you learn about yourself could be reassuring, but it could also be upsetting.Genetic testing is a subject to discuss first with your personal physician. You may be referred to a medical-genetics specialist, who is trained to interpret the results of tests.To find genetics professionals in your area, contact the National Society of Genetic Counselors at

www.nsgc.org; GeneTests at

www.genetests.org; or the American Society of Human Genetics at

www.ashg.org. To learn more about the medical conditions present in your family and about support groups, contact the Genetic Alliance at

www.geneticalliance.org.You may also want to consult a lawyer to protect your interests. Results of genetic tests are usually kept in your medical records. The Genetic Information Nondiscrimination Act is a Federal law that prohibits discrimination in health coverage and employment based on genetic information.The views of the author do not necessarily state or reflect those of the Times News.