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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
DNA (deoxyribonucleic acid), a substance inside the
center (nucleus) of cells that contains instructions for the development of the
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.
Many genes together make up
larger structures within the cell called chromosomes. Each cell normally
contains 23 pairs of chromosomes.
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.
genetic disorders are caused when all or part of a
chromosome is missing or when an extra chromosome or chromosome fragment is
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.
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:
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.
are six main types of genetic testing:
Information from genetic testing can affect your life and the lives of
your family members. The issues involved include:
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:
Examples of screening tests to identify carriers for
specific genetic disorders include:
There are two types of prenatal genetic tests: screening and diagnostic.
Examples of tests used for
prenatal screening include:
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.
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
Examples of tests used for newborn screening
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
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.
testing is used to identify the risk of late-onset diseases such as:
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
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:
Genetic counseling can help you and your family:
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
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
The Genetics Home Reference provides information on hundreds of genetic conditions. The website has many tools for learning about human genetics and the way genetic changes can cause
disease. It also has links to additional resources for people who
have genetic conditions and for their families.
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).
Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.
Tsai A C-H, et al. (2012). Genetics and dysmorphology. In WW Hay Jr et al., eds., Current Diagnosis and Treatment: Pediatrics, 21st ed., pp.
1088–1122. New York: McGraw-Hill Medical.
October 31, 2013
Kathleen Romito, MD - Family Medicine & Siobhan M. Dolan, MD, MPH - Reproductive Genetics
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