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DESCRIPTION
Genetics is the study of inheritance and how the characteristics of one generation are passed on to another. Prior to pregnancy, or once pregnancy is diagnosed, a medical and family history of both parents will be taken by your midwife or health care provider to look for any conditions that could affect, or be inherited by, the fetus. Most couples are at little risk for transmitting a genetic problem. If there appears to be increased risk, certain screening tests will be recommended and, possibly, a referral to a genetic specialist for further testing and counseling.
Genetic counseling is used to obtain and provide expectant parents with as much objective information as possible to help them arrive at a decision that is based on their own desires, values and ethics. Counseling is not used to coerce a couple to take or not take any particular tests or to make any reproductive decisions that are not theirs alone.
Though some defects are genetically transmitted, other defects are the result of environmental effects as the fetus develops in the womb and many of the defects that occur are spontaneous without traceable cause.
GENES AND CHROMOSOMES OVERVIEW
Genes are the blueprints of heredity. Genes are made of hundreds to thousands of DNA bases. Each gene directs cells to produce one or more specific proteins, including enzymes and structural proteins. The human genome is the complete set of genes that every person inherits from his or her parents. It is present in virtually every cell of the body. The human genome consists of tens of thousands of pairs of genes. Each person inherits one copy of each gene from each parent. Genes are organized along string-like structures called chromosomes. Each individual inherits two sets of twenty-three chromosomes, one from each parent: two sets of twenty-two autosomes and one set of sex chromosomes (X, X or X, Y).
GENETIC VARIATIONS, MUTATIONS, & HUMAN DISEASE
The DNA base sequence of human genes is about 99.9 percent identical among individuals. About 1 of every 1,000 DNA bases varies among individuals, accounting for inherited differences in traits and disease susceptibility. Changes in a DNA base sequence, called mutations, account for inherited gene variations. Mutations may be harmful if they prevent a gene from making a normal copy of its specific protein. These mutations can cause, or increase susceptibility to, specific diseases.
Single-gene diseases are relatively rare diseases that result when a person inherits one gene with a harmful mutation or a pair of genes in which each has a harmful mutation. Inheritance of these mutated genes generally results in a 100 percent chance of developing a specific disease. Single-gene diseases include autosomal dominant diseases (e.g., Huntington disease), autosomal recessive diseases (e.g., sickle cell disease), and X-linked diseases (e.g., Duchenne muscular dystrophy).
Most diseases result from a complex set of both genetic and environmental causes. Inheritance of some harmful gene mutations increases the chance, although it does not ensure, that a person will develop a specific disease. These mutations are called inherited susceptibility mutations.
NY Department of Health: Genetic Testing & Screening in the Age of Genomic Medicine
TYPES OF GENETIC TESTING
Genetic testing is the analysis of human DNA, RNA, chromosomes, proteins, and certain metabolites in order to detect heritable disease-related genotypes, mutations, phenotypes, or karyotypes for clinical purposes. It can provide information about a person's genes and chromosomes throughout life. Available types of testing include:
- Newborn screening: Newborn screening is used just after birth to identify genetic disorders that can be treated early in life. The routine testing of infants for certain disorders is the most widespread use of genetic testing. Millions of babies are tested each year in the United States. All states currently test infants for phenylketonuria (a genetic disorder that causes mental retardation if left untreated) and congenital hypothyroidism (a disorder of the thyroid gland).
- Diagnostic testing: Diagnostic testing is used to diagnose or rule out a specific genetic or chromosomal condition. In many cases, genetic testing is used to confirm a diagnosis when a particular condition is suspected based on physical mutations and symptoms. Diagnostic testing can be performed at any time during a person's life, but is not available for all genes or all genetic conditions. The results of a diagnostic test can influence a person's choices about health care and the management of the symptoms of a disorder.
- Carrier testing: Carrier testing is used to identify people who carry one copy of a gene mutation that, when present in two copies, causes a genetic disorder. This type of testing is offered to individuals who have a family history of a genetic disorder and to people in ethnic groups with an increased risk of specific genetic conditions. If both parents are tested, the test can provide information about a couple's risk of having a child with a genetic condition.
- Prenatal testing: Prenatal testing is used to detect changes in a fetus's genes or chromosomes before birth. This type of testing is offered to couples with an increased risk of having a baby with a genetic or chromosomal disorder. In some cases, prenatal testing can lessen a couple's uncertainty or help them decide whether to abort the pregnancy. However, it cannot identify all possible inherited disorders and birth defects.
- Predictive and presymptomatic testing: Predictive and presymptomatic types of testing are used to detect gene mutations associated with disorders that appear after birth, often later in life. These tests can be helpful to people who have a family member with a genetic disorder, but who have no features of the disorder themselves at the time of testing. Predictive testing can identify mutations that increase a person's chances of developing disorders with a genetic basis, such as certain types of cancer. For example, an individual with a mutation in BRCA1 has a 65% cumulative risk of breast cancer. Presymptomatic testing can determine whether a person will develop a genetic disorder, such as hemochromatosis (an iron overload disorder), before any signs or symptoms appear. The results of predictive and presymptomatic testing can provide information about a person's risk of developing a specific disorder and help with making decisions about medical care.
- Forensic testing: Forensic testing uses DNA sequences to identify an individual for legal purposes. Unlike the tests described above, forensic testing is not used to detect gene mutations associated with disease. This type of testing can identify crime or catastrophe victims, rule out or implicate a crime suspect, or establish biological relationships between people (for example, paternity).
- Research testing: Research testing include finding unknown genes, learning how genes work and advancing our understanding of genetic conditions. The results of testing done as part of a research study are usually not available to patients or their healthcare providers.
POSSIBLE INDICATIONS FOR GENETIC TESTING
Advanced maternal age (35 or older).
Parental chromosome abnormality.
Couples who have already had a child with a genetic abnormality.
Couples whose blood tests indicate they are carriers of a disorder.
Closely related couples such as first cousins.
A pregnant woman whose routine prenatal tests detected a fetal defect.
One of the partners has a congenital defect such as congenital heart disease.
Couples who have a family history of a hereditary disorder or mental retardation of unknown origin.
A woman with a history of multiple miscarriages (generally 3 or more) or stillbirth.
Either parent belongs to a race or ethnic group at high risk for a specific genetic disease (e.g., sickle-cell disease in African-Americans, Tay-Sachs disease in Jewish persons of Ashkenazic background, and beta-thalassemia in people of southern European ancestry).
Either parent has been exposed to high doses of radiation, drugs, occupational exposure to gases or chemicals, or other environmental agents which, could result in congenital abnormalities.
MEDICAL PROCEDURE FOR GENETIC TESTING
Genetic testing is often done as part of a genetic consultation. Once a person decides to proceed with genetic testing, a medical geneticist, genetic counselor, primary care health care provider, or specialist can order the test after obtaining informed consent.
Genetic tests are performed on a sample of blood, hair, skin, amniotic fluid (the fluid that surrounds a fetus during pregnancy), or other tissue. For example, a medical procedure called a buccal smear uses a small brush or cotton swab to collect a sample of cells from the inside surface of the cheek. The sample is sent to a laboratory where technicians look for specific changes in chromosomes, DNA, or proteins, depending on the suspected disorder. The laboratory reports the test results in writing to a person's health care provider or genetic counselor.
Routine newborn screening tests are done on a small blood sample, using a 'heel stick', by pricking the babies heel and blotting the blood on a special paper. Unlike other types of genetic testing, a parent will usually only receive the result if it is positive.
INTERPRETING GENETIC TEST RESULTS
The results of genetic tests are not always straightforward, which often makes them challenging to interpret and explain. When interpreting test results, healthcare professionals consider a person's medical history, family history, and the type of genetic test that was done.
A positive test result means that the laboratory found a change in a particular gene, chromosome, or protein of interest. Depending on the purpose of the test, this result may confirm a diagnosis, indicate that a person is a carrier of a particular genetic mutation, identify an increased risk of developing a disease (such as cancer) in the future, or suggest a need for further testing. Because family members have some genetic material in common, a positive test result may also have implications for certain blood relatives of the person undergoing testing. It is important to note that a positive result of a predictive or presymptomatic genetic test usually cannot establish the exact risk of developing a disorder. Also, health professionals typically cannot use a positive test result to predict the course or severity of a condition.
A negative test result means that the laboratory did not find a dangerous copy of the gene, chromosome, or protein under consideration. This result can indicate that a person is not affected by a particular disorder, is not a carrier of a specific genetic mutation, or does not have an increased risk of developing a certain disease. It is possible, however, that the test missed a disease-causing genetic alteration because many tests cannot detect all genetic changes that can cause a particular disorder. Further testing may be required to confirm a negative result.
In some cases, a negative result might not give any useful information. This type of result is called uninformative, indeterminate, inconclusive, or ambiguous. Uninformative test results sometimes occur because everyone has common, natural variations in their DNA, called polymorphisms, that do not affect health. If a genetic test finds a change in DNA that has not been associated with a disorder in other people, it can be difficult to tell whether it is a natural polymorphism or a disease-causing mutation. An uninformative result cannot confirm or rule out a specific diagnosis, and it cannot indicate whether a person has an increased risk of developing a disorder. In some cases, testing other affected and unaffected family members can help clarify this type of result.
GENETIC TESTING COSTS
The cost of genetic testing can range from under $100 to more than $2,000, depending on the nature and complexity of the test. The cost increases if more than one test is necessary or if multiple family members must be tested to obtain a meaningful result. In countries with private health insurance, in many cases, health insurance plans will cover the costs of genetic testing when it is recommended by a person's health care provider.
From the date that a sample is taken, it may take a few weeks to several months to receive the test results. Results for prenatal testing are usually available more quickly because time is an important consideration in making decisions about a pregnancy and whether or not a pregnancy should be terminated.
PRELIMINARY SCREENING TESTS FOR GENETIC (CHROMOSOMAL) DEFECT
Maternal blood test-measures the serum level of alpha-fetoprotein and other hormones which can identify a fetus with high risk for neural tube defect (cranial and spine abnormalities) or chromosome abnormality such as Down's syndrome. There is a possibility of false-negative or false-positive results.
Ultrasound is test that utilizes high-frequency sound waves that produce images of the fetus and placenta. It can detect major defects in the heart, bones, brain, abdomen and spinal cord.
PRENATAL SCREENING TESTS CONCLUSIVE FOR GENETIC (CHROMOSOMAL) DEFECT
Amniocentesis is a test that utilizes a large needle to take a sample of amniotic fluid (liquid surrounding the baby in the uterus) that is then analyzed for many genetic disorders.
Chorionic Villus Sampling is a sampling of cells in the chorionic villus (part of the placenta) that is analyzed for the presence of various defects or diseases.
Fetoscopy (rarely used) is an optical instrument with a lighted tip is inserted into the uterus to observe the fetus. A blood sample can be taken at the same time for laboratory analysis.
Cordocentesis is a test in which a sample of fetal blood is obtained from the umbilical cord with a fine needle guided by ultrasound. The blood sample is analyzed for defects or disorders.
OUTCOME OF PRENATAL GENETIC TESTING
Tests may indicate no problems.
Tests may indicate a possible problem that requires further testing.
Testing may show a definite problem. The parents will need to make a decision about continuing with the pregnancy if the defect is serious. Though the decision is the parent's alone, the midwife and medical care team will provide counseling and important information to assist them in a nondirective, but understanding manner.
The determination of serious genetic defects in a couple's history may present problems about future pregnancies and lead to decisions about permanent sterilization and discussions about other methods of contraception such as donor insemination. Genetic counseling will assist in determining the best alternative options available.
BENEFITS OF GENETIC TESTING
Genetic testing has potential benefits whether the results are positive or negative for a gene mutation. Test results can provide a sense of relief from uncertainty and help people make informed decisions about managing their health care. For example, a negative result can eliminate the need for unnecessary checkups and screening tests in some cases. A positive result can direct a person toward available prevention, monitoring, and treatment options. Some test results can also help people make decisions about having children. Newborn screening can identify genetic disorders early in life so treatment can be started as early as possible.
RISKS & LIMITATIONS OF GENETIC TESTING
The physical risks associated with most genetic tests are very small, particularly for those tests that require only a blood sample or buccal smear (a procedure that samples cells from the inside surface of the cheek). The procedures used for prenatal testing carry a small but real risk of losing the pregnancy (miscarriage) because they require a sample of amniotic fluid or tissue from around the fetus.
Many of the risks associated with genetic testing involve the emotional, social, or financial consequences of the test results. People may feel angry, depressed, anxious, or guilty about their results. In some cases, genetic testing creates tension within a family because the results can reveal information about other family members in addition to the person who is tested. The possibility of genetic discrimination in employment or insurance is also a concern.
Genetic testing can provide only limited information about an inherited condition. The test often can't determine if a person will show symptoms of a disorder, how severe the symptoms will be, or whether the disorder will progress over time. Another major limitation is the lack of treatment strategies for many genetic disorders once they are diagnosed.
A genetics professional can explain in detail the benefits, risks, and limitations of a particular test. It is important that any person who is considering genetic testing understand and weigh these factors before making a decision.
Many people are also concerned about the privacy implications of genetic testimony. In the United States, federal law requires that this kind of medical information to be kept confidential.
NOTIFY YOUR MIDWIFE OR HEALTH CARE PROVIDER IF...
You or a family member has questions or concerns about genetic screening or counseling.
You or a family member are at-risk for a genetic disorder and need to be tested.
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