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What Is Genetic Testing?

DNA, Genes and Chromosones

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Updated January 28, 2008

A karyotype (chromosome test) can be done using a blood sample.

A karyotype (chromosome test) can be done using a blood sample.

Photo © A.D.A.M.
Genetic testing looks for changes in a person's genes, chromosomes, or in the levels of certain important proteins.

Types of genetic tests include:

  • Gene tests (examining a person's DNA)
  • Chromosome tests (checking for abnormal chromosomes)
  • Biochemical tests (checking the levels of certain proteins)

Gene Tests

Gene tests look for changes (mutations) in a person's DNA. For the test, a person usually gives a blood sample and the DNA is taken out of some of the white blood cells. It is not too difficult to separate DNA from the other parts of the cells; in fact, schoolchildren can do it as a science project using peas (see Science in School: Discovering DNA).

Once the DNA is separated out, scientists hunt for the gene along the DNA strand to see if it looks abnormal.

Chromosome Tests

One type of chromosome test is called a karyotype. This test gives a picture of all of a person's chromosomes from the largest to the smallest. Scientists can see any major problems with the chromosomes. For example, in Down's syndrome (Trisomy 21), there are 3 copies of the 21st chromosome instead of the normal pair.

Another type of chromosome test, called FISH analysis (fluorescent in situ hybridization), can find small changes in the chromosomes that may be missed by the karyotype.

A newer type of chromosome test is called array CGH. It is a very sensitive test and can also find small changes in the chromosomes.

Biochemical Tests

Biochemical tests check the levels of certain proteins associated with disorders. For example, in alpha-1 antitrypsin deficiency the level of alpha-1 antitrypsin in the body will be abnormally low.

Where Is Genetic Testing Done?

Because most genetic disorders are rare, testing is done only by specialized laboratories. A physician, usually a genetics professional (geneticist), orders the test(s). The tests are often very expensive and may or may not be covered by health insurance.

You may find companies offering home genetics testing kits on the Internet. These do-it-yourself test kits have not been proven to be accurate, and they may not even be testing for what they claim to be. You should talk to a genetics professional before you purchase or use this type of kit.

Reasons for Genetic Testing

There are many reasons why a person might want or need to have a genetic test:
  • To confirm a diagnosis when someone has symptoms of a genetic disorder. Confirming a diagnosis may take several types of genetic testing.
  • To predict the risk of developing a certain disease. For example, if a woman inherits a gene associated with breast cancer she would be at higher risk for developing it.
  • Presymptomatic testing shows which family members are at risk for a certain genetic condition. This type of testing may be done when one family member is found to have a disease-causing genetic alteration. Those family members who also have the genetic alteration can then receive appropriate medical treatment.
  • Carrier testing shows whether a person has the genetic mutation for an inherited disorder. A person who is a carrier does not develop symptoms of the disorder but can pass the genetic mutation on to his or her children.
  • Prenatal testing can show whether an unborn child has inherited a genetic condition.
  • Newborn screening tests check the levels of certain important proteins in a baby's blood. Certain newborn screening tests are legally required of all babies born in the United States and many other countries.
  • Pharmacogenetic testing looks at a person's genes to see what types of medicines might be best for him or her. For example, a test given to people with chronic myelogenous leukemia can show who would benefit from being treated with a drug called Gleevec.

Sources:

"What Is Genetic Testing?" About Genetic Services. 19 Mar 2004. GeneTests. 21 Jan 2008

Burton, Jess, & Jon Turney. The Rough Guide to Genes & Cloning. London: Rough Guides Ltd., 2007.

"Frequently Asked Questions About Genetic Testing." Genetics and Genomics for Patients and the Public. 17 Dec 2007. National Human Genome Research Institute. 21 Jan 2008

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