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Genetic Testing

What Is Genetic Testing?

Genetic testing can help doctors look for missing or defective genes. This information helps them know if a person, their partner, or their baby is likely to have certain medical conditions.

Genetic tests are when small samples of blood or body tissues are analyzed. Many different types of body fluids and tissues can be used. The type of genetic test needed to make a diagnosis depends on which condition a doctor checks for.

What Is Genetic Testing During Pregnancy?

For genetic testing before birth, a blood test can screen pregnant women for some disorders. To check for others, or if the screening blood test finds a possible problem, doctors may recommend amniocentesis or chorionic villus sampling:

  • Amniocentesis is a test usually done between weeks 15 and 20 of 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. The fluid is checked for genetic problems and can show the sex of the child. When there's risk of premature birth, amniocentesis can show how far the baby's lungs have matured. Amniocentesis carries a slight risk of inducing a miscarriage.
  • Chorionic villus sampling (CVS) usually is done between weeks 10 and 12 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.

Why Do Doctors Recommend Genetic Testing?

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

  • The pregnant woman is over age 34. A pregnant woman's chances of having a child with a chromosomal problem (such as trisomy) increase if she is older than 34. Children of older fathers are at risk for new dominant genetic mutations — those caused by a single genetic defect that hasn't run in the family before.
  • A standard prenatal screening test had an abnormal result. Doctors may recommend genetic testing if a screening test showed a possible genetic problem.
  • A couple plans to start a family and one of them or a close relative has an inherited illness. Some people are carriers of genes for genetic illnesses, even though they don't show signs of the illness themselves. This happens because some genetic illnesses are recessive. This means they cause symptoms only if a person inherits two copies of the problem gene, one from each parent. Children who inherit one problem gene from one parent but a normal gene from the other parent won't have symptoms of a recessive illness. But they will have a 50% chance of passing the problem gene to their children.
  • A parent already has one child with a serious birth defect. Not all children who have birth defects have genetic problems. Sometimes, exposure to a toxin (poison), infection, or physical trauma before birth causes a birth defect. Often, the cause isn't known. Even if a child has a genetic problem, it might not have been inherited. Some happen because of a spontaneous error in the child's cells, not the parents' cells.
  • A woman has had two or more miscarriages. Severe chromosome problems in the fetus can sometimes lead to a spontaneous miscarriage. Several miscarriages may point to a genetic problem.
  • A woman has delivered a stillborn child with physical signs of a genetic illness. Many serious genetic illnesses cause specific and distinctive physical problems.
  • A child has medical problems that might be genetic. When a child has medical problems involving more than one body system, genetic testing might help doctors find the cause and make a diagnosis.
  • A child has medical problems known to be part of a genetic syndrome. Genetic testing can confirm the diagnosis. In some cases, it also might help find the type or severity of a genetic illness. This can help doctors find the best treatment.

Looking Ahead

Progress in genetic testing has improved how doctors diagnose and treat some illnesses. But it has limits. Genetic tests can identify a particular problem gene. But they can't always determine how that gene will affect the person who carries it. In cystic fibrosis, for example, finding a problem gene on chromosome number 7 can't predict whether a child will have serious lung problems or milder respiratory symptoms.

Also, having problem genes is only part of the story. Many illnesses develop from a mix of high-risk genes and environmental things, some of which a person can control. Someone who knows they carry high-risk genes might be able to make lifestyle changes to avoid becoming sick.

Research has identified genes that put people at risk for cancer, heart disease, psychiatric disorders, and many other medical problems. The hope is to someday develop specific types of gene therapy to prevent some diseases and illnesses.

Gene therapy is 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. But some patients have had severe complications while receiving gene therapy. So the research is carefully controlled.

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

Reviewed by: Larissa Hirsch, MD
Date Reviewed: 19-08-2019

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