Download What is genetic testing?

Genetics
Table of Contents
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Topic Overview
Carrier Identification
Prenatal Screening and Testing
Newborn Screening
Late-Onset Diseases
Identification
Genetic Counseling
What to Think About
Other Places To Get Help
Related Information
References
Credits
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
contained in 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 identification 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.
• Late-onset disease testing determines whether you carry a genetic change that
increases your risk for developing a disease, such as breast cancer or Huntington's
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.
• Genetic identification (DNA fingerprinting) can be used to determine paternity, help
solve crimes, and identify a body.
• Compatibility testing looks at genetic markers to see if an organ for transplant is
suitable for a person who needs it.
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's 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), protects people who have DNA differences that may affect their health.
This law does not cover life insurance, disability insurance, or long-term care
insurance.
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 identification 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 the abnormal 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 abnormal gene and so will not have the disease nor be a
carrier.
• If only one parent carries the abnormal 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 defective 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.1
Late-Onset Diseases
This type of testing is done to determine whether you have a greater chance of having
diseases that show up later in life (late-onset diseases). 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 late-onset 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's 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 DNA
fingerprinting. 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.
DNA fingerprinting 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 the identity
of a child's father (paternity).
• The discovery of a genetic disease may affect your future ability to get work or
some types of private insurance coverage. The discovery of a genetic disease that
is not causing symptoms now (such as breast cancer or Huntington's 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), protects people who have DNA differences
that may affect their health. This law does not cover life insurance, disability
insurance, or long-term care insurance.
Other Places To Get Help
Organization
National Library of Medicine: Genetics Home Reference (U.S.)
www.ghr.nlm.nih.gov
Related Information
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Alpha-Fetoprotein (AFP) in Blood
Amniocentesis
Ashkenazi Jewish Genetic Panel (AJGP)
Autosomal Recessive Disease
Birth Defects Testing
Breast Cancer (BRCA) Gene Test
Chorionic Villus Sampling (CVS)
Cystic Fibrosis
DNA Fingerprinting
Human Chorionic Gonadotropin (HCG)
Huntington's Disease Genetic Test
Karyotype Test
Phenylketonuria (PKU)
Phenylketonuria (PKU) Test
Sickle Cell Disease
Sickle Cell Test
Tay-Sachs Disease
Tay-Sachs Test
References
Citations
1. Genetic Evaluation of Congenital Hearing Loss Expert Panel (2002;
reaffirmed 2005). Genetics evaluation guidelines for the etiologic diagnosis
of congenital hearing loss. Genetics in Medicine, 4(3): 162–171.
Other Works Consulted
• National Cancer Institute (2013). Cancer Genetics Overview (PDQ).
Available online: http://www.cancer.gov/cancertopics/pdq/genetics/
overview/healthprofessional.
• Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and
Laboratory Tests, 4th ed. St. Louis: Mosby.
• Saenz M et al. (2014). Genetics and dysmorphology. In WW Hay Jr, et al.,
eds., Current Diagnosis and Treatment: Pediatrics, 22nd ed., pp.
1134–1170. New York: McGraw-Hill.
Credits
By Healthwise Staff
Primary Medical Reviewer Kathleen Romito, MD - Family Medicine
John Pope, MD - Pediatrics
Specialist Medical Reviewer Siobhan M. Dolan, MD, MPH - Reproductive Genetics
Current as of October 13, 2016
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