Medical Genetics: How Genetic Testing Is Used

Genetic testing can help find genetic diseases in many kinds of situations. For example, tests can find diseases that:

• Run in a family

• Don’t yet have symptoms

• May be in an embryo to be implanted for in vitro fertilization (IVF)

• May be found in a baby in the womb or a newborn

• May be found in a child or adult

Many kinds of tests are used for many reasons. The types of testing include:

• Diagnostic genetic testing

• Predictive genetic testing

• Presymptomatic genetic testing

• Carrier testing

• Prenatal diagnosis

• Preimplantation studies

• Newborn screening

Diagnostic genetic testing

Genetic testing is used to find or confirm the diagnosis of a genetic condition. This kind of testing may be done before birth, or any time during a person's life. Test results may help determine the course of a disease and the choice of treatment. Types of tests include chromosome studies, direct DNA studies (sequencing), and biochemical testing.

Predictive genetic testing

This kind of testing helps show how much of a chance a person with or without a family history of a disease may have of developing that disease. Predictive testing is available for health problems, such as some types of cancer, cardiovascular disease, and some single-gene disorders.

Presymptomatic genetic testing

This kind of testing is used to find out if a person with a family history of a disease who has no symptoms has the gene changes of the disease. This can help show if the person has the disease before symptoms start.

Carrier testing

A carrier is someone who has a nonworking copy and a normal copy of the same gene. They may not have a genetic disease. But the person may be able to pass their nonworking copy of the gene on to a child. Carrier testing is done to see if a person has 1 copy of a changed gene for a disease. The disease may be inherited in an autosomal recessive manner. This means that the disease occurs only if a person has 2 copies of the changed gene. Couples who both carry the same gene of an autosomal recessive condition have a 1 in 4 chance with each pregnancy to have a child with that disease.

A recessive disease may also be X-linked recessive. This means that the changed gene is on the X chromosome. Females have two X chromosomes, and males have one X and one Y chromosome. Because females have two X chromosomes, they can be carriers of a nonworking gene on one of their X chromosomes. But they are not affected by a disease if the other X has a normal copy of the gene. Males are often affected by a disease if they have the changed gene on their only copy of the X chromosome. This is because they don’t have the normal copy of the gene on the Y chromosome. Carrier testing for X-linked conditions is usually done in females.

Prenatal diagnosis

This kind of testing is used to diagnose a genetic disorder in a baby growing in the womb. These kinds of tests include:

• Blood tests for the mother (maternal serum screening)

• Ultrasound

• Amniocentesis

• Chorionic villus sampling (CVS)

• Percutaneous umbilical blood sampling (PUBS)

• Noninvasive prenatal screening

Preimplantation studies

These kinds of tests are done after IVF. They’re done to check for a genetic disorder in an embryo before it’s implanted into the mother's womb.

Newborn screening

Newborn screening is done on newborn babies. The tests are part of state public health programs. The tests look for certain genetic diseases that can be diagnosed early and treated. Millions of babies are tested each year in the U.S. for diseases, such as phenylketonuria and hypothyroidism. Phenylketonuria can cause intellectual disability if not treated. Hypothyroidism means the thyroid gland does not make enough thyroid hormone.