What Is Genetic Testing?
Doctors use genetic tests to obtain information about your health. The broadest definition of genetic testing includes all tests that are ordered to look for evidence of inherited traits or diseases, or diseases caused by a new genetic change that was not inherited. Some genetic tests analyze your chromosomes or DNA, the chemical material you inherited from your parents. Other tests examine the chromosomes or proteins. Genetic tests search for specific changes in DNA or the proteins made from the DNA. Some changes could increase your chance of developing a particular disease. Other changes might not affect you, but could put your children at risk. Your doctor will first ask you what diseases or disorders people in your family (living and dead) have had. The doctor will perform a physical exam and perhaps order some laboratory tests. Armed with this information, you and your doctor can discuss whether genetic testing might be useful or reassuring for you. (This information is especially valuable when planning a pregnancy.)
What Happens During A Genetic Test?
It's simple. Blood is drawn from your arm in the usual way and sent to a special lab. Sometimes, a sample of cells from inside your mouth can be used. In the lab, scientists isolate the DNA from the cells in your blood or study your chromosomes or the proteins they create.
What Do The Tests Look For?
Many genetic tests today involve sequencing the DNA or looking at the chromosomes for missing or extra pieces. Some genetic tests do not involve DNA or chromosome analysis. Commonly ordered tests, such as red blood cell counts and blood levels of liver enzymes, can provide the initial screening tests for some inherited conditions.
Different testing methods are used. Large changes can be seen under a microscope. These changes include an extra copy of a chromosome, as in Down syndrome, or a missing chromosome, as in Turner syndrome.
Other tests compare your DNA to known mutations. These mutations can confirm the diagnosis of an existing disease or can increase a person's chance of developing a disease. In general, the test will only look for "hot spots" on the DNA: particular areas of the chromosomes that research has shown to be involved in disease, such as specific genes. There are probably other mutations that scientists don't know about yet that lead to disease. These mutations would not be detected in this type of genetic test.
It is more common now for genetic tests to look at the whole gene. This increases the chances of finding a change that has never been seen before. This can make it difficult to know what the change means for you.
Finally, some genetic tests look for changes in proteins, which reflect changes in your DNA. Such tests look for the presence, absence or function of a protein. This information can tell doctors if the gene that makes the protein is working properly. For example, newborn babies are screened for a disease called phenylketonuria, a condition in which an important protein is missing due to a defective gene. Untreated, phenylketonuria can lead to severe mental retardation. The test does not check the gene directly. It looks for high levels of a blood chemical that builds up when the gene isn’t working.
Who Gets Genetic Tests?
There are a number of reasons why your doctor might suggest you have a genetic test.
- If a certain disease, such as breast or ovarian cancer, runs in your family, a genetic test might tell you if you have a higher-than-average chance of developing the disease. This type of testing is called "susceptibility testing."
- If a parent has a genetic problem such as Huntington's disease, "predictive testing" can tell you if you have inherited that parent's normal gene or mutated gene.
- If you already have some symptoms or signs of a genetic disease, a genetic test may confirm or deny the diagnosis. This is called "diagnostic testing." This is especially helpful when standard evaluation does not provide a definitive answer. One example is fragile X syndrome, a common cause of mental retardation. Without the diagnostic test, it's hard to differentiate a child with fragile X from any other child with mental retardation.
- If you are thinking about having children, a genetic test can tell you ahead of time if you have a mutation that could increase your child's chance of developing a genetic disease. This is referred to as "carrier testing." One example is cystic fibrosis, which affects one in four children born to parents who are both carriers. DNA testing can tell if you are a carrier for this disorder and can help you plan for healthy children. If your ancestors were from a part of the world where certain genetic mutations are known to be common, such as those which cause thalassemia (Mediterranean and Far East Asian areas) or sickle cell disease (Africa) or Tay-Sachs disease (Eastern European Ashkenazi Jewish), carrier testing may be useful.
- If you are pregnant, genetic testing procedures such as amniocentesis, chorionic villus sampling, ultrasound imaging, and DNA testing, as well as screening blood tests for the mother, can help tell if your fetus is developing normally or might be born with certain disorders, such as Down syndrome. This is referred to as "prenatal testing."
- In a newborn baby, genetic tests can reveal certain disorders, such as phenylketonuria, in time to treat and prevent abnormal mental and physical development. This is called "newborn screening."