Genetics

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:

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:

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.

Other Places To Get Help

Organization

National Library of Medicine: Genetics Home Reference (U.S.)
www.ghr.nlm.nih.gov

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

ByHealthwise 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

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