The Types of Genetic Testing
Although most genetic diseases can be linked to both environmental and genetic factors, knowing whether you are predisposed to a specific disease often proves to be an important first step in combating its effects. Genetic testing is the analysis of DNA, most commonly obtained from a blood sample, and is primarily used to screen for genetic diseases--even though the testing receives much more media attention for its highly publicized role in court, where it is used as an identification tool in paternity and criminal cases. People who have a family history of a genetic disease, show signs of a genetic disease or have concerns of passing on a genetic disease to their offspring are candidates for genetic testing.
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Carrier Identification
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Carrier identification is genetic testing undertaken by couples who have a history of recessive genetic disease in their family and who are considering having children. Most people have two copies of their genes, one maternally received and one paternally received. A carrier is a person who has an abnormal change in one copy of a gene. Although the carrier does not suffer from any effects of the abnormality, that abnormal gene can be passed to a birth child.
Carrier identification tests provide couples some guidance in their decision on whether to have children. If both parents are carriers of the abnormality, there is a 1-in-4 chance that their child will have the disease, a 2-in-4 chance that the child will be a carrier and a 1-in-4 chance that the child will neither have the disease nor carry the abnormal gene. If only one parent is a carrier, chances are 1-in-2 that the child will also be a carrier but almost certainly will not suffer from the disease. The most common carrier identification testing includes screen for cystic fibrosis, sickle cell and Tay-Sachs.
Prenatal Diagnosis
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Prenatal diagnosis is genetic testing of a fetus. This testing is undertaken if there is a risk of mental retardation or physical deterioration in the unborn child. Because the chances of a woman giving birth to a baby with abnormalities increase dramatically after the age of 35, prenatal diagnosis is a common procedure among pregnant women in this age group. Prenatal testing is usually performed between the weeks 16 and 18 of pregnancy by amniocentesis, a procedure that removes a sample of fluid from the amniotic sac.
Prenatal diagnosis is most commonly used to test for Down syndrome and Spina bifida, which are both classified as neural tube defects. The alpha feto protein (AFP) blood test is used to screen for these defects: An elevated AFP may indicate defect to the brain and spinal cord development and suggest Spina bifida, and a low AFP level may indicate the possibility of Down syndrome. If abnormal levels are detected, more tests (using ultrasound and additional amniocentesis) will be performed to further evaluate the fetus for chromosomal abnormalities and birth defects. Although not routinely performed, the cells of the amniotic fluid can also be examined for specific inherited genetic abnormalities such as Tay-Sachs disease, cystic fibrosis and sickle cell anemia. -
Newborn Screening
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Newborn screening is the most common form of genetic testing because it is mandated in all 50 U.S. states. Although each state sets its own guidelines, all states screen for both phenylketonuria and congenital hypothyroidism. Approximately 3,000 to 5,000 infants are diagnosed annually with a genetic disorder as a result of state-mandated newborn screening. Screening is usually done from blood drawn from the newborn's heel. If screened levels are abnormal, the attending physician is likely to order additional tests and screen for other conditions that are not state mandated. This early diagnosis allows for immediate intervention and proper treatment to begin as quickly as possible.
Late Onset Disorders
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Late onset disorders are diseases that often affect people later in life, such as cancer, Huntington's disease and familial adenomatous polyposis (a condition that causes abnormal growths inside the colon and increases risk of colorectal cancer).The goal of testing is to determine whether you have a greater chance of contracting a specific disease. Testing is sometimes performed on people who have a particular family history of disease. For example, a breast cancer (BRCA) gene test is used to detect mutated genes, referred to as BRCA1 and BRCA2, that control normal cell growth. Locating these genes can indicate the likelihood of developing breast or ovarian cancer. This type of testing is also referred to as presymptomatic genetic testing. In certain cases, genetic testing can also be used after symptoms have appeared to confirm a suspected diagnosis for such diseases as adult polycystic kidney disease, hemochromatosis (iron overload) and Charcot-Marie-Tooth disease.
Identification
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Identification genetic testing is used to make a positive identification of a specific individual. Because each person has a unique DNA "fingerprint," a genetic test provides a profile of one or more genetic markers that positively link an individual to a collected DNA sample. Identification testing can be used to determine paternity by comparing DNA samples between the suspected father and the child. DNA fingerprinting has also received significant attention in the criminal courts, where DNA samples taken from crime scenes can link suspects to the crime. In addition, genetic testing is sometimes used to identify the victims of crime, a major accident or disaster and war.
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