Top of the page Genetics Topic OverviewWhat 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 TestingEvery 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 ScreeningShortly 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 1Predictive and Presymptomatic TestingThis 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 TestingGenetic 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 CounselingThe 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 AboutBefore 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. Other Places To Get HelpOrganization National Library of Medicine: Genetics Home Reference (U.S.) www.ghr.nlm.nih.gov Related InformationAlpha-Fetoprotein (AFP) in Blood Amniocentesis Ashkenazi Jewish Genetic Panel (AJGP) Birth Defects Testing Breast Cancer (BRCA) Gene Test Chorionic Villus Sampling (CVS) Cystic Fibrosis Down Syndrome Human Chorionic Gonadotropin (HCG) Huntington Disease Genetic Test Karyotype Test Phenylketonuria (PKU) Phenylketonuria (PKU) Test Sickle Cell Disease Sickle Cell Test Tay-Sachs Disease Tay-Sachs Enzyme TestReferencesCitations 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.
CreditsByHealthwise StaffPrimary Medical Reviewer Kathleen Romito, MD – Family Medicine John Pope, MD – Pediatrics Martin J. Gabica, MD – Family Medicine Elizabeth T. Russo, MD – Internal Medicine Adam Husney, MD – Family Medicine Specialist Medical Reviewer Siobhan M. Dolan, MD, MPH – Reproductive Genetics Current as ofFebruary 23, 2018 Top of PageNext Section:
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February 23, 2018Author:
Healthwise Staff Medical Review:
Kathleen Romito, MD – Family Medicine & John Pope, MD – Pediatrics & Martin J. Gabica, MD – Family Medicine & Elizabeth T. Russo, MD – Internal Medicine & Adam Husney, MD – Family Medicine & Siobhan M. Dolan, MD, MPH – Reproductive GeneticsGenetic 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.