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What Is DNA?
Deoxyribonucleic acid (DNA) is the genetic code found in all living cells. This code is a thread of bases, which are sequences of four chemicals in different arrangements. In humans, DNA is made up of about three billion bases.
Chromosomes are the structures within cells where the DNA is contained. Normal human DNA is held within 23 pairs of chromosomes, equaling 46 total chromosomes.
DNA contains the instructions that allow cells to function and enable all bodily processes. Those instructions are provided by genes, which are collections of bases within DNA. It is believed that humans have 20,000 to 25,000 genes, each of which is made up of between a few hundred and two million bases.
Most of your DNA and genes come from your biological parents. Your full set of inherited DNA is known as your genome. More than 99% of the human genome is identical in every person.
Hereditary DNA is found in all the cells in your body. During your life, individual cells can experience changes, known as somatic mutations, that can alter DNA. However, these mutations only affect the DNA in those specific cells. They do not change your hereditary DNA.
Because DNA determines how the cells in your body work, it is fundamental in understanding both health and disease. Variants (mutations) in genes can heighten the risk of health problems and affect how your body responds to medications.
In addition, since DNA is unique to every individual — except for identical twins, who share the same DNA — it can be used for identification, and since it is hereditary, it can help demonstrate ancestry and family relationships.
The Role of DNA Tests
DNA influences every aspect of health, and as a result, genetic testing, which analyzes DNA, has widespread applications.
In the context of health and medicine, some of the existing and developing uses of DNA testing include:
- Diagnosis of a genetic disorder: Certain medical disorders are characterized by a specific variant or alteration in genes that can be identified by DNA testing.
- Carrier testing for hereditary disorders: A carrier is a person who has an abnormal component of their DNA that can be passed down to the next generation, potentially causing or increasing the risk of a specific inherited medical disorder in their children. Genetic testing can be used to determine whether a person is a carrier.
- Predictive testing: As with carrier testing, predictive or presymptomatic testing attempts to identify people who have genetic variants that can cause a heightened risk of certain kinds of medical problems.
- Prenatal screening: DNA tests can determine if a fetus has genetic abnormalities that mean the fetus has a serious disease or developmental disorder.
- Pharmacogenetics: Genes are one of several factors that may explain why not everyone responds to medications in the same way. Pharmacogenetics uses DNA testing to try to help select the right drug and/or reduce their side effects when given to a specific person.
- Transplantation testing: A challenge in organ and stem cell transplants is the possibility that the recipient’s immune system will reject the transplant. Testing for a specific set of genes in both the donor and recipient can determine if there is a genetic match, which improves the chances of successful transplantation.
- Lifestyle and genetic traits: This category of DNA testing attempts to analyze genes specific to certain bodily processes like your senses, metabolism, or circadian rhythm in order to allow you to optimize your lifestyle habits.
Not all of these applications of genetic testing are equally validated or utilized. While some gene variants are well-studied, others are still not fully understood. DNA testing can provide information about the risk of certain health conditions, but in some cases it cannot definitively determine whether you will or will not develop a disease.
Many types of health-related DNA testing focus on genes that exist in your inherited genome, but genetic testing can also be done on specific tissues that have undergone somatic mutations during your lifetime. For example, genetic testing of a cancerous tumor may show certain DNA mutations or characteristics of gene expression that can help determine the most effective type of treatment.
With these diverse uses, genetic testing can be involved in diagnosing health conditions, screening for problems in people without symptoms, and informing options for medical treatment. As researchers continue to learn more about human genetics, the number of genetic tests and their applications in health and medicine will continue to grow.
DNA testing is also used outside of the health context. Examples of non-medical applications of DNA testing include:
- Forensic identity testing: Because DNA has a signature specific to an individual, DNA testing can prove a person’s identity. In criminal investigations, this type of forensic testing can determine the identity of both victims and perpetrators.
- Ancestry testing: People who want to trace their family lineage often turn to DNA ancestry testing. Learning about the presence of certain genes and DNA inheritance patterns may offer insight into your family’s origins.
- Kinship testing: Parents and their children share specific parts of their DNA, which means that DNA can be used in paternity testing or otherwise proving whether people are biologically related.
Who should get testing?
The benefits and downsides of genetic testing depend significantly on the situation in which it is used. In most cases, it is recommended to consult with a physician or genetic counselor before testing. A genetic counselor’s job is to describe the kind of information that genetic testing provides and how it may affect your health.
A genetic counselor can also describe the possible downsides of DNA testing. Genetic tests may reveal unexpected or worrisome results, including gene alterations with unknown health significance.
Even for non-medical genetic tests, it is often beneficial to first consult with a doctor or genetic counselor to make sure that you are informed about the details of the test and the results that it will provide.
Getting test results
When a genetic test is prescribed by a doctor or genetic counselor, you can expect to get your test results from them. At that time, you will typically have an opportunity to ask questions and make sure that you understand what your results mean and whether any follow-up care is necessary.
For at-home DNA testing, results may be available by mail or through a website or smartphone app. Some home testing companies provide consultations with genetic counselors to review your test results, but others offer only limited explanations of the findings of your test.
Types of DNA Testing
DNA testing can be done in several different ways. Tests may look at only one small piece of the genome to see if there is a DNA variant. Panel tests can be used to examine multiple genes at the same time. Large-scale testing can analyze huge sections of DNA or even the entire genome.
Distinct genetic testing techniques are available depending on the purpose of the test and the amount of DNA being analyzed. The cost of testing and the interpretation of results can vary based on the selected testing method.
Below are examples of specific genetic tests that can diagnose genetic disorders and indicate who is at risk for developing a condition or passing it on to their children:
|Tests Related to Genetic Disorders, Health Risk, and/or Carrier Status|
|Cystic fibrosis||Cystic fibrosis transmembrane conductance regulator (CFTR) gene variants|
|Sickle cell anemia||Hemoglobin gene variants|
|Tay-Sachs disease||HEXA gene variants|
|Huntington disease||CAG mutation in HTT/HD gene|
|Hereditary hemochromatosis||HFE gene variants|
|Hereditary breast and ovarian cancer syndrome||BRCA1 and BRCA2 gene variants|
|Lynch syndrome||Single or multi-gene variant testing|
DNA tests may be performed on a fetus to screen for genetic disorders:
|Tests Related to Prenatal Screening|
|Down syndrome (trisomy 21)||Extra copy of chromosome 21|
|Edwards syndrome (trisomy 18)||Extra copy of chromosome 18|
|Patau syndrome (trisomy 13)||Extra copy of chromosome 13|
Pharmacogenomic tests, such as the following, can help predict how a person will respond to certain drugs:
|Tests Related to Pharmacogenomics|
|Warfarin||Excess blood clotting||VKORC1 and CYP2C9 gene variants|
|Thiopurines||Immune system disorders and some types of cancer||TPMT gene variants|
|Clopidogrel||Excess blood clotting||CYP2C19 gene variant|
|Irinotecan||Colorectal cancer||UGT1A1 gene variant|
|Abacavir||HIV||HLA-B*5701 gene variant|
|Carbamazepine, phenytoin||Epilepsy and some psychiatric disorders||HLA-A*3101 or HLA-B*1502 gene variant|
In patients with cancer, tumor DNA testing can identify genetic changes in the cancer cells. Targeted treatments are available for some genetic variants, including those listed in the table below:
|Tests Related to Genetic Variants in Cancerous Tumors|
|Type of Cancer||Genetic Analysis|
|Breast cancer||Overexpression of HER2/neu protein|
|Chronic myelogenous leukemia (CML)||BCR-ABL gene fusion|
|Non-small cell lung cancer||EGFR, ALK, KRAS, and ROS1 gene variants; PDL1 protein overexpression|
|Myeloproliferative neoplasms||JAK2 gene variants|
In organ and stem cell transplantation, a type of genetic analysis called human leukocyte antigens (HLA) testing is commonly used. The HLA genes help manage the body’s immune reactions, and an HLA match between donors and recipients can improve transplant success.
DNA tests for lifestyle traits can focus on a variety of genes, but the validity of this type of testing continues to be studied. Other genetic tests related to forensic identity, ancestry, and kinship can involve different degrees of DNA analysis and testing methods based on the scope and purpose of the specific test.
Getting a DNA Test
Most genetic tests involve either a blood sample taken from a vein in your arm or a saliva sample. For health-related genetic testing, a doctor or genetic counselor can prescribe testing after reviewing your situation and discussing the potential benefits and downsides of specific genetic tests.
Working with a health professional can help ensure that you are informed about the test, that your DNA sample is properly collected, and that analysis is conducted by a certified laboratory. It is also more common for insurance to cover genetic testing when it is medically prescribed.
Some types of DNA testing, especially non-medical tests, do not require a doctor or genetic counselor to order them. However, even in these cases, it can be helpful to talk with a health professional beforehand to make sure you know the pros and cons and afterward to review your results and their significance.
Several types of at-home DNA tests are available. In most cases, these tests involve taking a sample of your saliva or blood and sending it to a laboratory for analysis.
Types of at-home genetic tests include ancestry tests, kinship and paternity tests, lifestyle analysis, and some carrier and predictive health risk testing.
Some companies sell at-home tests for just one aspect of DNA analysis while others offer a range of types of analysis that you can choose from. Certain at-home testing options provide just the raw data from your DNA that can be submitted to third-party companies for analysis.
It is important to be aware of potential concerns related to at-home DNA testing:
- Privacy protection: By nature, your DNA contains abundant personal information, so make sure to consider the privacy implications of submitting a DNA sample. Privacy policies can help clarify how your data will be used and protected, but these documents are often confusing or may be modified at a later date.
- Unproven test interpretation: For many health-related genetic tests, the significance of a DNA variant remains poorly understood. This means that some tests provide potentially confusing or misleading results. Relatedly, small studies have found diverging interpretations of disease risk for the same DNA sample between different at-home testing companies.
- Unexpected information: Sometimes genetic tests, especially paternity or ancestry tests, can reveal information about family relationships that may be unexpected and upsetting.
- Minimal regulation: Many types of at-home DNA testing are not closely regulated by the Food and Drug Administration, which can make it hard to know which tests are reputable.
Carefully considering the benefits and risks of genetic testing can help you determine if it is right for you and avoid unanticipated consequences. Talking with a doctor or genetic counselor before and after testing can also help improve your understanding of any genetic test and its potential significance.
Sources and Resources
These resources provide additional background information about genetics and genetic testing:
- National Library of Medicine: Help Me Understand Genetics
- University of Utah: Genetic Science Learning Center
- CDC: Genetic Counseling and Testing
ACMG Board of Directors. Direct-to-consumer genetic testing: a revised position statement of the American College of Medical Genetics and Genomics. Genet Med. 2016;18(2):207-208. doi:10.1038/gim.2015.190
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