The Different Types of Genetic Tests
Some say that the human genome is the only data set in healthcare that does not change, and that helps explain the recent popularity of genetic testing with more than 77,000 genetic tests currently in use and an average of 10 new tests entering the market daily.
“The human genome was mapped only fifteen years ago, but since then the adoption of genetic testing has skyrocketed. Much of this trend is explained by the critical role genetic testing plays in precision medicine (an approach to disease treatment and prevention that seeks to maximize effectiveness by taking into account individual variability in genes, environment, and lifestyle) as well as by advances in next-generation sequencing methods and concurrent reductions in sequencing costs,” wrote four healthcare professionals in a Health Affairs research article.
Discoveries in Genetics have Revolutionized Medical Science
The American Journal of Managed Care says that from a broader perspective, Genetic testing can potentially lead to improvements in population health and health equity and cut down on healthcare costs. The theory is that when specific populations are identified as being at greater risk of disease, targeted treatments will lead to better response rates and less healthcare waste.
“Discoveries in genetics have revolutionized medical science. With each new advance, the excitement around genetics and genomics testing continues to build,” wrote clinical service physician Ian Z. Chuang.
Once known for its use in paternity tests, prenatal testing, forensic evidence and genealogy usage, genetic testing is now used in everything from breast and ovarian cancer to age-related macular degeneration (AMD) to bipolar disorder to Parkinson’s disease to celiac disease to psoriasis.
“If the developments in genetics and computer technologies continue to progress at their current speed, history has shown us we can look forward to some amazing developments in human life in the very near future,” a research paper published in BioMed Research International concluded.
The History and Definition of Genetic Testing
DNAfit says that genetic science can trace its roots back to the 1860s and plant hereditary traits experiments by Gregor Mendel with the term “genetics” first used in 1905 by English biologist William Bateson.
Even though genetics as a theory was widely accepted by 1925, it wasn’t until the 1950s that the field opened after the discovery and study of DNA. In the last decade, genetic testing has gone mainstream, first with at-home ancestry DNA testing and then with the role molecular tests played during the COVID-19 pandemic.
The National Library of Medicine (NLM) says that genetic testing is a type of medical test that identifies changes in genes, chromosomes, or proteins.
“The results of a genetic test can confirm or rule out a suspected genetic condition or help determine a person’s chance of developing or passing on a genetic disorder,” explained the NLM.
The NLM says genetic testing specifically involves looking for changes in:
- Genes: Gene tests study DNA sequences to identify variations (mutations) in genes that can cause or increase the risk of a genetic disorder. Gene tests can be narrow or large in scope, analyzing an individual DNA building block (nucleotide), one or more genes, or all a person’s DNA (which is known as their genome).
- Chromosomes: Chromosomal genetic tests analyze whole chromosomes or long lengths of DNA to see if there are large genetic changes, such as an extra copy of a chromosome, that cause a genetic condition.
- Proteins: Biochemical genetic tests study the amount or activity level of proteins or enzymes; abnormalities in either can indicate changes to the DNA that result in a genetic disorder.
There are Many Types of Genetic Tests Now Available
“Many types of genetic tests are available to analyze changes in genes, chromosomes, or proteins. A health care provider will consider several factors when selecting the appropriate test, including what condition or conditions are suspected and the genetic variations typically associated with those conditions,” says the NLM.
The NLM breaks down the types of genetic tests available under four categories:
- Molecular tests: Look for changes in one or more genes. These types of tests determine the order of DNA building blocks (nucleotides) in an individual's genetic code, a process called DNA sequencing. These tests can vary in scope:
o Targeted single variant: Single variant tests look for a specific variant in one gene. The selected variant is known to cause a disorder (for example, the specific variant in the HBB gene that causes sickle cell disease). This type of test is often used to test family members of someone who is known to have a particular variant, to determine whether they have a familial condition. Also, direct-to-consumer genetic testing companies typically analyze a number of specific variants in particular genes (rather than finding all the variants in those genes) when providing health or disease risk information.
o Single gene: Single gene tests look for any genetic changes in one gene. These tests are typically used to confirm (or rule out) a specific diagnosis, particularly when there are many variants in the gene that can cause the suspected condition.
o Gene panel: Panel tests look for variants in more than one gene. This type of test is often used to pinpoint a diagnosis when a person has symptoms that may fit a wide array of conditions, or when the suspected condition can be caused by variants in many genes. (For example, there are hundreds of genetic causes of epilepsy.)
o Whole exome sequencing/whole genome sequencing: These tests analyze the bulk of an individual’s DNA to find genetic variations. Whole exome or whole genome sequencing is typically used when a single gene or panel testing has not provided a diagnosis, or when the suspected condition or genetic cause is unclear. Whole exome or whole genome sequencing is often more cost- and time-effective than performing multiple single gene or panel tests.
- Chromosomal tests: Analyze whole chromosomes or long lengths of DNA to identify large-scale changes. Changes that can be found include an extra or missing copy of a chromosome (trisomy or monosomy, respectively), a large piece of a chromosome that is added (duplicated) or missing (deleted), or rearrangements (translocations) of segments of chromosomes. Certain genetic conditions are associated with specific chromosomal changes, and a chromosomal test can be used when one of these conditions is suspected. (For example, Williams syndrome is caused by a deletion of a section of chromosome 7.)
- Gene expression tests: Look at which genes are turned on or off (expressed) in different types of cells. When a gene is turned on (active), the cell produces a molecule called mRNA from the instructions in the genes, and the mRNA molecule is used as a blueprint to make proteins. Gene expression tests study the mRNA in cells to determine which genes are active. Too much activity (overexpression) or too little activity (underexpression) of certain genes can be suggestive of particular genetic disorders, such as many types of cancer.
- Biochemical tests: Do not directly analyze DNA, but they study the amount or activity level of proteins or enzymes that are produced from genes. Abnormalities in these substances can indicate that there are changes in the DNA that underlie a genetic disorder. (For example, low levels of biotinidase enzyme activity are suggestive of biotinidase deficiency, which is caused by BTD gene variants.)
7 Common Uses of Genetic Testing
Another way to look at the different types of genetic testing is their common usage, which includes these seven:
- Diagnostic Testing: This type of genetic test is used to confirm or rule out a suspected genetic condition. It is typically ordered by a doctor or genetic counselor when a person has symptoms of a genetic disorder.
- Carrier Testing: This type of genetic test is used to determine if a person carries a gene for a genetic disorder. It is typically recommended for people who have a family history of a genetic condition or who are part of an ethnic group with a higher risk for certain genetic diseases.
- Prenatal Testing: This type of genetic test is performed during pregnancy to detect certain genetic disorders in the developing fetus. It can include amniocentesis or chorionic villus sampling (CVS) and non-invasive prenatal testing (NIPT).
- Newborn Screening: This is a type of genetic test performed on newborns to screen for certain genetic disorders that may not be apparent at birth. It is typically done using a blood sample taken from the baby's heel.
- Predictive and Presymptomatic Testing: These types of genetic tests are used to determine a person's risk of developing a genetic disorder before symptoms appear. They are typically done on people with a family history of a genetic condition.
- Pharmacogenomic Testing: This type of genetic test analyzes how a person's DNA might affect their response to certain medications. It can help doctors determine the most effective medication and dosage for an individual.
- Forensic Testing: This type of genetic test is used to identify individuals for legal or criminal purposes, such as identifying the perpetrator of a crime or establishing paternity.
Genetic testing has already revolutionized healthcare, and it is likely to play an increasingly important role in the future.
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