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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 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.
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.
Some genetic disorders are caused when all or part of a chromosome is missing or when an extra chromosome or chromosome fragment is present.
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.
There 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 offer.
Examples of tests used for newborn screening include:
Other tests, such as newborn hearing tests, can tell whether a baby may need future hearing services or genetic testing. Approximately 50% of 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.
Genetic 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 victims. DNA fingerprinting is more accurate for this purpose than dental records, blood type, traditional fingerprinting, or ID tags.
The information from genetic testing can have a big impact on your life. 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:
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.
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.
- 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.
- 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.
|Primary Medical Reviewer||Sarah Marshall, MD - Family Medicine|
|Specialist Medical Reviewer||Siobhan M. Dolan, MD, MPH - Reproductive Genetics|
|Last Revised||November 3, 2011|
Last Revised: November 3, 2011
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