Genetic tests are done by analyzing small samples of blood or body tissues. They determine whether you, your partner, or your baby carry genes for certain inherited disorders.

Genetic testing has developed enough so that doctors can often pinpoint missing or defective genes. The type of genetic test needed to make a specific diagnosis depends on the particular illness that a doctor suspects.

Many different types of body fluids and tissues can be used in genetic testing. For deoxyribonucleic acid (DNA) screening, only a very tiny bit of blood, skin, bone, or other tissue is needed.

For genetic testing before birth, pregnant women may decide toundergo amniocentesis or chorionic villus sampling. There is also a blood test available to women to screen for some disorders. If this screening test finds a possible problem, amniocentesis or chorionic villus sampling may be recommended.

Amniocentesis is a test usually performed between weeks 15 and 20of a woman's pregnancy. The doctor inserts a hollow needle into the woman's abdomen to remove a small amount of amniotic fluid from around the developing fetus. This fluid can be tested to check for genetic problems and to determine the sex of the child. When there's risk of premature birth, amniocentesis may be done to see how far the baby's lungs have matured. Amniocentesis carries a slight risk of inducing a miscarriage.

Chorionic villus sampling (CVS) is usually performed between the 10th and 12th weeks of pregnancy. The doctor removes a small piece of the placenta to check for genetic problems in the fetus. Because chorionic villus sampling is an invasive test, there's a small risk that it can induce a miscarriage.

A doctor may recommend genetic counseling or testing for any of the following reasons:

Although advances in genetic testing have improved doctors' ability to diagnose and treat certain illnesses, there are still some limits. Genetic tests can identify a particular problem gene, but can't always predict how severely that gene will affect the person who carries it. In cystic fibrosis, for example, finding a problem gene on chromosome number 7 can't necessarily predict whether a child will have serious lung problems or milder respiratory symptoms.

Also, simply having problem genes is only half the story because many illnesses develop from a mix of high-risk genes and environmental factors. Knowing that you carry high-risk genes may actually be an advantage if it gives you the chance to modify your lifestyle to avoid becoming sick.

As research continues, genes are being identified that put people at risk for illnesses like cancer, heart disease, psychiatric disorders, and many other medical problems. The hope is that someday it will be possible to develop specific types of gene therapy to totally prevent some diseases and illnesses.

Gene therapy is already being studied as a possible way to treat conditions like cystic fibrosis, cancer, and ADA deficiency (an immune deficiency), sickle cell disease, hemophilia, and thalassemia. However, severe complications have occurred in some patients receiving gene therapy, so current research with gene therapy is very carefully controlled.

Although genetic treatments for some conditions may be a long way off, there is still great hope that many more genetic cures will be found. The Human Genome Project, which was completed in 2003, identified and mapped out all of the genes (about 25,000) carried in our human chromosomes. The map is just the start, but it's a very hopeful beginning.

Date reviewed: April 2014

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Genetic Testing - kidshealth.org

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