Through research on the human genome, we now know that many common diseases usually caused by genetic alterations in the genes of an individual's cells — such as breast cancer and colon cancer — also have rare hereditary forms. In these cases, gene variants that cause or strongly predispose a person to these cancers run in a family and significantly increase each member's risk of developing the disease.

Geneticists group genetic disorders into three categories:

• Single gene disorders are caused by a mutation in a single gene. The mutation may be present on one or both chromosomes (one chromosome inherited from each parent). Sickle cell disease, cystic fibrosis and Tay-Sachs disease are examples of single gene disorders.

• Chromosome disorders are caused by an excess or deficiency of the genes that are located on chromosomes, or structural changes within chromosomes. Down syndrome, for example, is caused by an extra copy of chromosome 21, but no individual gene on the chromosome is abnormal.

• Multifactorial inheritance disorders are caused by a combination of small variations in genes, often in concert with environmental factors. Heart disease and most cancers are examples of these disorders. Behaviors are multifactorial, complex traits involving multiple genes that are affected by a variety of other factors. Researchers are learning more about the genetic contribution to behavioral disorders such as alcoholism, obesity, mental illness and Alzheimer disease. There is currently no recommended genetic testing for behavioral disorders such as alcoholism or obesity.

Genetic tests look for alterations in a person's genes or changes in the level of key proteins coded for by specific genes. Abnormal results on these tests could mean that someone has an inherited disorder.

Types of genetic tests include:

• Gene tests (when you test DNA)
• Chromosomal tests (called karyotype and FISH)
• Biochemical tests (when you test enzyme levels)

Gene tests look for signs of a disease or disorder in DNA taken from a person's blood, body fluids or tissues. The tests can look for large changes, such as a gene that has a section missing or added, or small changes, such as a missing, added or altered chemical base within the DNA strand. Other important changes can be genes with too many copies, genes that are too active, genes that are turned off or those that are lost entirely.

Gene tests examine a person's DNA in a variety of ways. Some tests use probes. These are short strings of DNA with base sequences complementary to those of an altered gene. Probes look for their complements within a person's genome. If the altered is found, the probe binds to it, identifying the alteration.

Another type of gene test directly compares the sequence of DNA bases in a patient's gene to a normal version of the gene.

What Is a Chromosomal Test?

Chromosomes are structures in the nucleus of a cell that contain DNA. Chromosomal tests look at features of a person's chromosomes, including their structure, number and arrangement. These tests look for changes, such as pieces of chromosomes being switched or being in a different location.

Types of chromosomal tests include:

• Karyotype — This test gives a picture of all of a person's chromosomes from the largest to the smallest.

• FISH analysis (fluorescent in situ hybridization) — This test identifies certain regions on chromosomes using fluorescent DNA probes. FISH analysis can find small pieces of chromosomes that are missing or have extra copies. These small changes can be missed by the karyotype test.

Biochemical tests look at the level of key proteins. This level signals genes that are not working normally. These types of tests are used for newborn screening. For example, this screening can detect infants who have metabolic conditions such as phenylketonuria (PKU).

What Information Can Genetic Testing Give?

Genetic testing can:

• Give a diagnosis if someone has symptoms.

• Show whether a person is a carrier for a genetic disease. Carriers have an altered gene, but will not get the disease. However, they can pass the altered gene on to their children.

• Help expectant parents know whether an unborn child will have a genetic condition. This is called prenatal testing.

• Screen newborn infants for abnormal or missing proteins that can cause disease. This is called newborn screening.

• Show whether a person has an inherited disposition to a certain disease before symptoms start.

• Determine the type or dose of a medicine that is best for a certain person. This is called pharmacogenetics.

A genetic test result that shows a person has an alteration causing disease also can give benefits. Such a test result might lead a person to take steps to lower their chance of disease. For example, someone could screen for the disease or could make changes to their health habits like diet and exercise. A genetic test result showing that a person has an alteration causing disease can lower their uncertainty. This information also can help people to make informed choices about their future.

What Are Reasons to Get Different Types of Genetic Tests?

Diagnostic testing is to confirm a diagnosis when a person has signs or symptoms of a genetic disease. The genetic test used depends on the disease for which a person is tested. For example, if a patient has physical features that suggest Down syndrome, a chromosomal test is used. To test for Duchenne muscular dystrophy, a gene test is done to look for missing sections in the dystrophin gene.

Predictive testing can show which people have a higher chance of getting a disease before symptoms appear. For example, one type of predictive test screens for inherited disposition to certain cancers, such as colon and breast cancer. Someone with an inherited disposition has an increased chance of getting a disease. This does not mean that the person will certainly get the disease.

Presymptomatic testing shows which family members are at risk for a certain genetic condition. This type of testing is done with people who do not have symptoms when the alteration causing disease in the family is known. For some diseases, this type of testing can lead to prevention or treatment options. For example, when a disease-causing alteration is found in a family, testing is recommended for all close blood relatives (such as parents and siblings). Those family members with the alteration can be offered removal of the thyroid. With other types of diseases, there are no prevention or treatment options. For example, there is no treatment for family members who have an alteration causing Huntington's disease and those who have this alteration are certain to get the disease.

Carrier testing can tell individuals if they are carriers of a gene alteration for a type of inherited disorder called an autosomal recessive disorder. A person who has only one altered copy of a gene for this type of disorder is called a carrier. Carriers will not get the disease, but can pass on the alteration to their children. If both parents are carriers, their children might inherit an alteration from each parent and get the disease. Examples of this type of disorder are cystic fibrosis and Tay-Sachs.

Prenatal testing is available to pregnant women:

• Who are age 35 or older. This is because they are at higher risk for having a child with a chromosomal abnormality.

• Who have a family history of an inherited condition such as Duchenne muscular dystrophy.

• When their ancestry or ethnic background means that they might have a higher chance of an inherited disorder such as sickle cell anemia, thalassemia or Tay-Sachs disease.

• To screen for common genetic disorders such as spina bifida and Down syndrome.

• Newborn screening is the most widespread type of genetic testing. It is an important public health program that can find disorders in newborns that have long-term health effects. Newborn screening tests infant blood samples for abnormal or missing gene products. For example, infants are often screened for Phenylketonuria (PKU). This is an enzyme deficiency that can lead to severe mental retardation if the child is not treated. Newborn screening now tests for more diseases. In the past, newborn screening focused on a few metabolic disorders that led to mental retardation. Newborn screening programs now test for disorders that can cause infectious disease, premature death, hearing disorders and heart problems. A new technology called tandem mass spectrometry allows screening of up to 30 other metabolic disorders. The number of disorders screened for varies from state to state. Some states offer newborn screening for up to 30 diseases, while others offer newborn screening for the most common disorders.

• Pharmacogenetic testing is a new type of genetic testing. This type of test examines a person's genes to look at how drugs would move through the body and be broken down. The goal of pharmacogenetic testing is to have drug treatments that are specific to each person. For example, a test used in patients who have chronic myelogenous leukemia can show which patients would benefit from a medicine called Gleevac. Another test looks at a liver enzyme called cytochrome P450, which breaks down certain types of drugs. Gene alterations can affect how well people's bodies break down certain drugs. People with a less active form of the enzyme might get too much of a drug. Pharmacogenetic testing can help make sure that people get the right amount of a medicine.

People have many different reasons for being tested or not being tested. For many, it is important to know whether a disease can be prevented if a gene alteration causing a disease is found. For example, those who have inherited forms of breast or colon cancer have options such as screening or early treatment. Pharmacogenetic testing can find the best medicine or dose of a medicine for a certain person.

In other cases, there is no treatment. There are no preventive steps or cures for Huntington's disease. But test results might help a person make life decisions, such as career choice, family planning or insurance coverage.

People can seek advice from a genetic counselor. Genetic counselors help individuals and families think about the scientific, emotional and ethical factors that affect the testing decision.

What About Direct-to-Consumer Genetic Tests Available on the Internet?

A number of companies offer self-administered genetic tests for many different reasons. Genetic testing offered via the Internet usually uses a cell sample scraped from inside the cheek, which is mailed to a test laboratory. One type of test offered, called nutrigenetic testing, says that it will match an individual's diet with their genetic make-up.

A recent report issued by the Government Accountability Office found that these tests can be misleading to consumers because they make predictions that cannot be scientifically proven. Furthermore, these tests may not provide meaningful information and could even provide harmful information to consumers.

Other types of self-administered genetic testing offered on the Internet include those for specific diseases (cancer, Alzheimer's disease) and genetic tests that can help trace family background and ancestry. These tests also may be misleading to consumers.

Before using genetic tests offered on the Internet, you should check with a genetic professional about the genetic test of interest.

Updated: January 2007

Source: National Human Genome Research Institute, National Institutes of Health