Genetics

Carrier Identification

Every person carries two copies of most genes (one copy from the mother and one from the father). A carrier is a person who has a change in one copy of a gene. The carrier does not have the genetic disease related to the abnormal gene. A carrier can pass this abnormal gene to a child. Carrier testing is a type of genetic testing that can determine whether people who have a family history of a specific disease, or who are in a group that has a greater chance of having a disease, are likely to pass that disease to their children. Information from this type of testing can guide a couple's decision about having children.

For many genetic disorders, carrier testing can help determine how likely it is that a child will have the disease:

• If both parents carry a change in the same gene, there is a 1-in-4 (25%) chance that their child will have the disease and a 2-in-4 (50%) chance that their child will be a carrier of the disease (but will not have it). There is also a 1-in-4 (25%) chance that the child will not get the changed gene and so will not have the disease nor be a carrier.
• If only one parent carries a change in that gene, the child has a 1-in-2 (50%) chance of being a carrier but almost no chance that he or she will have the disease.

Examples of screening tests to identify carriers for specific genetic disorders include:

• Cystic fibrosis carrier screening. This test identifies the most common changes (mutations) in the cystic fibrosis transmembrane regulator (CFTR) gene. Many couples planning to become pregnant have this type of screening to determine whether either or both of them carry a changed CFTR gene.
• Tay-Sachs test. This test is used to identify Tay-Sachs carriers. People of Ashkenazi Jewish or French-Canadian descent or those who have a family history of Tay-Sachs disease may choose to be tested to see if they are a Tay-Sachs carrier.

Prenatal Screening and Testing

There are two types of prenatal genetic tests: screening and diagnostic.

• Screening tests show the chance that a developing baby (fetus) has a certain genetic condition. It can't tell for sure that your baby has a problem. If the test result is "positive," it means that your baby is more likely to have that condition. So your doctor may want you to have a diagnostic test to make sure. If the screening test result is "negative," it means that your baby probably doesn't have that genetic condition. But it doesn't guarantee that you will have a normal pregnancy or baby.
• Diagnostic tests show if the developing baby has a certain genetic condition.

Examples of tests used for prenatal screening include:

• First-trimester screening (first part of integrated screening).
• Triple or quad screening (second part of integrated screening).

If prenatal screening shows an increased risk of problems, other tests may be done. These include:

• A diagnostic ultrasound.
• Chorionic villus sampling.
• Amniocentesis.

Newborn Screening

Shortly after birth, a blood sample is taken from a newborn to screen for diseases such as phenylketonuria (PKU) and congenital hypothyroidism. Sensors are placed on the newborn's hand and foot to check the amount of oxygen in the blood. This type of testing is important, because treatment is available to improve the health of the child. Newborn screening is required in the United States, but states vary on which tests they offer.

Examples of tests used for newborn screening include:

• Phenylketonuria (PKU) screen, which measures the amount of phenylalanine in a baby's blood. Babies found to have PKU should be put on a special low-protein diet to prevent intellectual disability.
• Cystic fibrosis screening test, which measures levels of immunoreactive trypsinogen (IRT), a digestive enzyme, in a newborn's blood sample. Abnormally high levels of IRT suggest cystic fibrosis, although further testing is needed to confirm the diagnosis.
• Hemoglobin test, which tests for types of hemoglobin in a baby's blood that may point to sickle cell trait or sickle cell disease. Babies who have sickle cell disease need special medical care throughout their lives.
• Congenital heart defect screening test, which checks for low oxygen levels in the blood. Low oxygen levels may be caused by a heart problem.

Other tests, such as newborn hearing tests, can tell whether a baby may need future hearing services or genetic testing. Approximately 50 out of 100 cases of newborn hearing loss are caused by genetic factors.^{footnote 1}

Late-Onset Diseases

This type of testing is done to determine whether you have a greater chance of having diseases that usually show up later in life. If you have a relative who has the disease, information from these tests can help you make decisions about preventing or slowing the progress of the disease.

Genetic testing is used to identify the risk of diseases such as:

• Breast cancer and ovarian cancer (BRCA-1 and BRCA-2 testing).
• Familial adenomatous polyposis (FAP), which causes abnormal growths inside the colon and increases your risk of colorectal cancer.
• Huntington disease, a rare disease that causes premature, severe deterioration of the brain.

Identification

Genetic testing used to determine the biological parent of a child is called forensic testing. It is also often used to help solve crimes by determining whether crime scene DNA evidence could be the same as the suspect's DNA.

Forensic testing has been used to identify unknown people, such as military personnel killed in action or crime victims.

Genetic Counseling

The information from genetic testing can have a big impact on your life. Medical geneticists and genetic counselors are trained to help you understand your risk of getting a disease related to genetics or of having a child with an inherited (genetic) disease, such as sickle cell disease, cystic fibrosis, or hemophilia. A genetic counselor can help you make well-informed decisions. Ask to have genetic counseling before making a decision about testing. Genetic counseling may involve:

• Discussing what problems an inherited disease may cause.
• Teaching you and your partner about how a specific disease is inherited or passed from you to your child.
• Discussing whether and how to test for an inherited disease before you become pregnant or before your child is born.
• Discussing the likelihood that you and your partner will have a child with an inherited disease, based on test results.

Genetic counseling can help you and your family:

• Understand medical facts, including what causes diseases, how a diagnosis is made, and what you may be able to do to help yourself manage a disease.
• Understand how your family history contributes to the development of a disease.
• Understand what you can do to help prevent a disease.
• Learn about caring for a family member who has a genetic disease, including getting referrals to specialists or joining support groups.

Medical geneticists and genetic counselors are trained to help you and your family make informed decisions that are right for you. They are sensitive to physical and emotional aspects of these decisions. Your privacy and confidentiality are carefully protected.

What to Think About

Before making a decision about testing, you should clearly understand how the results of the test may affect your life. Consider how the test results may influence your decisions. If testing will not change any of your decisions, you may feel the test is not worth doing.

• Fetal genetic testing may detect a serious disease or disorder, such as Down syndrome, that will greatly impact your child's life and the lives of caregivers. A pregnant woman who is considering genetic testing may want to consider her ethical, social, and religious beliefs to help her determine the actions she would take if test results show a genetic disease.
• A genetic test result is sensitive information. Your confidentiality should be maintained, and the release of information should be limited to those who are authorized to receive it.
• Genetic testing can sometimes reveal unintended information, such as whether a child's father is actually the biological father.