Genetics

Topic Overview

What are genes?

Genes are the part of a body cell that contain the biological information that parents pass to their children. Genes control the growth and development of cells. Genes are made of DNA (deoxyribonucleic acid), a substance inside the center (nucleus) of cells that contains instructions for the development of the cell.

You inherit half of your genetic information from your mother and the other half from your father. Genes, alone or in combination, determine what features (genetic traits) a person inherits from his or her parents, such as blood type, hair color, eye color, and other characteristics, including risks of developing certain diseases. Certain changes in genes or chromosomes may cause problems in various body processes or functions.

What are chromosomes?

Many genes together make up larger structures within the cell called chromosomes. Each cell normally contains 23 pairs of chromosomes.

A human has 46 chromosomes (23 pairs). One chromosome from each pair comes from the mother, and one chromosome from each pair comes from the father. One of the 23 pairs determines your sex. These sex chromosomes are called X and Y.

• For a child to be female, she must inherit an X chromosome from each parent (XX).
• For a child to be male, he must inherit an X chromosome from his mother and a Y chromosome from his father (XY).

Some genetic disorders are caused when all or part of a chromosome is missing or when an extra chromosome or chromosome fragment is present.

What is genetic testing?

Genetic testing examines a DNA sample for gene changes, or it may analyze the number, arrangement, and characteristics of the chromosomes. Testing may be performed on samples of blood, semen, urine, saliva, stool, body tissues, bone, or hair.

Should I have genetic testing?

You may choose to have genetic testing if you are concerned that you have an increased risk for having or getting a disease that has a genetic cause. The information you obtain from the tests may help you make decisions about your life. For example:

• If tests show that you have an increased risk for passing on a disease to your child, you may choose to have more genetic testing while you are pregnant (prenatal testing). Or you may decide to adopt a child.
• If tests show that you have an increased risk for developing a disease such as breast cancer, you may make decisions that help lower your risk of breast cancer.
• You may feel reassured if the tests are normal.

You may decide to have a genetic test during pregnancy to find out whether your fetus has a disorder, such as Down syndrome. Information obtained from the test can help you decide how to manage your pregnancy.

Genetic testing can be used to find out the identity of a child's father (paternity). It can also be used in crime scene investigation.

What are the main types of genetic testing?

There are six main types of genetic testing:

• Carrier testing determines whether people who have a family history of a specific disease or who are in a group that has an increased chance for that disease are likely to pass on that disease to their children. Information obtained from this type of testing can help guide a couple as they make decisions about pregnancy.
• Prenatal testing determines whether a fetus has a disorder, such as Down syndrome. Information gained from this type of testing can help guide decisions about how to manage a pregnancy, including the decision about whether to end the pregnancy.
• Newborn screening checks for various metabolic diseases, such as phenylketonuria (PKU). Information obtained from newborn screening can help guide medical treatment to ensure the best possible outcome for the baby.
• Predictive and presymptomatic testing determines whether you carry a genetic change that increases your risk for developing a disease, such as breast cancer or Huntington disease, later in life. This might be of interest if you have a relative who has the disease. Information from this type of testing can help you make decisions about preventing or managing the disease.
• Forensic testing is used for legal purposes and not for finding genetic changes associated with diseases. It can be used to determine paternity, help solve crimes, and identify a body.
• Diagnostic testing may be used to confirm a specific genetic condition when a person has symptoms of a disease.

What are the risks of genetic testing?

Information from genetic testing can affect your life and the lives of your family members. The issues involved include:

• Psychological. The emotions you may experience if you learn that you have a greater chance of having or passing on a serious disease can cause you to feel anxious or depressed. This may also affect your relationship with your partner or other family members. Genetic counseling is recommended prior to genetic testing.
• Medical. A person who tests positive for a disease-specific gene change may decide to use preventive or treatment options to reduce the impact or severity of the disease. Although many treatment options have been shown to be effective, others may be potentially dangerous or of unproven value.
• Privacy. Because genetic testing is expensive, few people are able to afford it without help from their insurance companies. Many people worry that genetic information released to insurance companies may affect future employment options or insurance availability. The discovery of a genetic disease that is not causing symptoms now (such as breast cancer or Huntington disease) should not affect your future ability to gain employment or health insurance coverage. A law in the United States, called the Genetic Information Nondiscrimination Act of 2008 (GINA), helps protect people who have DNA differences that may affect their health. But it has some limits. For example, this law doesn't apply to life insurance, disability insurance, or long-term care insurance. And it doesn't protect people who work for companies with fewer than 15 employees.

Carrier Testing

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 test results show an increased risk of problems, further diagnostic genetic testing (karyotype) can be used to examine the size, shape, and number of chromosomes. A karyotype can be done on cells taken from the placenta (chorionic villus sampling) in the late first trimester or from the amniotic fluid (amniocentesis) in the second trimester. Extra, missing, or abnormal positions of chromosome pieces can cause problems with growth, development, and body functions.

Newborn Screening

Shortly after birth, a blood sample is taken from a newborn to screen for diseases such as phenylketonuria (PKU) and congenital hypothyroidism. 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.

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}

Predictive and Presymptomatic Testing

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.

Forensic Testing

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.