The Basics About Genetic Testing One of the most powerful medical technologies we have today is genetic testing. There are a few different ways doctors can use genetic tests: To find out if a person is a carrier for a certain disease. To learn if a person has an inherited predisposition to a certain disease, like breast or ovarian cancer (also known as susceptibility testing). To help expecting parents know whether their unborn child will have a genetic disease or disorder (prenatal testing). To confirm diagnosis of certain diseases or disorders (for example, Alzheimer's disease). Today there are tests for about 450 genetic diseases. However, the majority of these tests remain available only to research laboratories and are offered only to family members of someone who has been diagnosed with a genetic condition. About 50 genetic tests are available for clinical use. The Basics Genetic testing starts with that fundamental building block, the gene. As any student of genetics knows, we inherit two copies of each gene, one from our mother and one from our father -- so you have two copies of each gene, governing everything from color blindness to whether you can roll or "flip" your tongue to whether you have a predisposition to a specific genetic disease. A few genetic diseases are inherited in a dominant manner. A dominant condition is one that is "expressed," or that determines a specific trait. Huntington disease is an example of a dominant gene disorder. This means that if you have the gene for Huntington disease, you will get the disease. Each child of a parent with an abnormal copy of the gene for Huntington disease gene (or any other dominant gene) has a 50 percent chance of inheriting that gene copy. A genetic test can show whether you have an abnormal copy of the Huntington gene and will get the disease. The model currently used for testing for the Huntington gene includes extensive pre- and post-test genetic counseling to help patients and their families deal with the psychological and emotional issues that are involved with learning the test results. Most genetic diseases we know about today are recessive disorders. In a recessive condition, there must be two abnormal copies of a gene for it to be expressed. Cystic fibrosis is an example of a recessive disorder. A person who has cystic fibrosis has two abnormal copies of the cystic fibrosis gene. That means that one copy of the cystic fibrosis gene was inherited from the person's mother, and one copy was inherited from the person's father. A person who has only one abnormal copy of a gene for a recessive condition is known as a carrier. A carrier of a genetic condition won't get the disease, but could pass the gene for that disease along to children. If both parents are carriers for a specific genetic condition, like cystic fibrosis, their children will have a 1 in 4 chance of getting that condition, and a 1 in 2 chance of carrying one abnormal gene copy. Susceptibility Testing Doctors now can test for genetic susceptibility for diseases like breast, ovarian or colon cancer, heart disease or Alzheimer's disease. (These tests are most often offered to members of families at high risk for genetic conditions or to people who participate in research studies.) Being predisposed to a disease does not necessarily mean you will get the disease -- it means you will have a certain increased risk of developing the disease. For example, a person who tests positive for a genetic mutation in BRCA1 would have a 55 to 85 percent chance of developing breast cancer by age 70, and a 40 to 60 percent chance of developing ovarian cancer. But those diseases are also affected by environmental factors or "risk modifiers." Genetic counselors and some doctors can help you understand what genetic predisposition to a disease will mean in your individual case. Prenatal Testing A prenatal genetic test is an optional medical test available to people who are considered to be at increased risk for having children with a chromosomal abnormality or an inherited genetic condition. Prenatal testing is often used to look for conditions such as Down syndrome, spina bifida, cystic fibrosis, Tay-Sachs disease and others that would show up in early childhood. Two procedures are used in prenatal testing. Amniocentesis, which involves taking a sample of amniotic fluid from the womb for analysis, can be done during the 16th through 20th weeks of pregnancy. Another procedure, chorionic villus sampling (CVS), can be done earlier, at nine to 12 weeks. But these tests do carry a risk of miscarriage: depending on the mother's age and other factors, amniocentesis causes miscarriage in between 1 in 200 and 1 in 400 cases, and CVS has a risk of 1 in 100. Prenatal testing is offered to pregnant women over age 35, because their babies are at greater risk for having abnormal chromosomes. For example, a 35-year-old woman has about a 1 in 200 chance of having a baby with a chromosomal abnormality like Down syndrome. A 40-year-old woman has closer to 1 in 50 chance. But prenatal testing is increasingly being offered to pregnant women of all ages. Current advances in genetic research make it possible to determine more and more conditions through prenatal testing, but we still have not entered an era in which doctors can medically treat most conditions that are discovered prenatally. Families who learn that their developing baby is affected with a certain condition must choose between terminating the pregnancy or having the baby and learning as much as they can about how to manage the condition. To learn more about genetic testing, read our Interview with a Genetic Counselor.