What Is DNA Testing and Why Is It Useful?

Genetic testing can tell you about your genes and your family history

DNA structure, Illustration
What is DNA testing and what types of genetic tests are available?. Ktsdesign/Science Photo Library / Getty Images

Your body is made up of trillions of cells. Inside every one of those cells is a structure called the nucleus, which holds 23 pairs of chromosomes that carry your genetic information. Each chromosome is made up of a long string-like structure called DNA, which is made up of a chemical called deoxyribonucleic acid (DNA).

Half of your chromosomes are inherited from your mother and the other half are inherited from your father, giving you two copies of every gene.

Humans have around 25,000 genes in total. Your DNA is a sequence of these pairs of genes.

Your genes carry information about what makes you, you. All of those pairs add up to make a unique combination, like a code, which tells your body whether you'll have brown hair, olive skin, or your great aunt's piercing blue eyes. Your DNA also tells your cells how to behave, grow, and when to die.

Think of your DNA as if it were a ladder. The steps are made up of nucleotide bases. The order of the bases is like the letters of an alphabet. These letters are, in turn, arranged in ways that spell "words." These words are then transcribed (copied) and translated (interpreted) into the proteins (enzymes) that control everything in our body. If a gene is mutated, you can think of it as mixing up the letters of the alphabet. The word comes out wrong and the whole sentence is wrong. If that gene or area of DNA is responsible for an important function, problems result.

If it codes for the proteins that control cell division, cancer may occur. Sometimes excess DNA, like three copies of a chromosome instead of two, can lead to genetic conditions such as Down syndrome.

Getting Your DNA Tested

When you have your DNA tested, your DNA is unraveled and read. Since each cell has more than six feet of DNA inside each and every cell (the DNA is tightly coiled up to make up the chromosome, so that it fits inside the nucleus), each cell can hold a lot of genetic information completely unique from anyone else.

The only exception is identical twins, who share identical DNA. 

To do a DNA test, you'll need to take a sample of your cells. This sample can be from your blood, saliva, skin cells or even the inside of your cheek. If you have cancer, your DNA will be taken from a sample of your tumor. Usually, DNA test samples are taken by a doctor or clinic and sent away to a specialized genetic testing lab where your DNA is separated from your cells.

For a blood test, a needle and syringe are required to draw a sample of blood. To give a saliva sample, you spit into a tube. For the cheek swab, scraping the inside of your cheek with a large cotton swab should do the job.

It will take a week or more for results to come back from the lab, as it takes lab technicians time to unravel and read the genetic sequencing along the DNA strand.

Testing for Genetic Disorders

If your doctor suspects you or your child has a genetic disorder, they will order further tests to diagnose the problem. Scientists may look for a change in a particular gene associated with a particular genetic disorder. DNA testing might also be able to identify any family members who have or are at risk for, developing the genetic disorder.

One common test is the test for phenylketonuria, which is done on all newborn babies. If your child, due to heredity or mutation, lacks the gene which codes for the enzyme phenylalanine hydroxylase, any phenylalanine they eat can lead to brain damage. If, however, the abnormal gene is discovered at birth, children can lead relatively normal lives by following a restrictive diet.​

Another test your doctor might order is a karyotype. Karyotypes look at the chromosomes in DNA. Extra copies of DNA or missing pieces can indicate genetic abnormalities. If your child has 47 chromosomes instead of the usual 46 and has three copies of chromosome 21 instead of two, they have Down's syndrome.

Down syndrome is the most common chromosomal abnormalities. Many pregnancies with an abnormal number of chromosomes end in miscarriage. If you have a history of recurrent miscarriages, your doctor may recommend parental karyotype tests.

In some cases, you may not have a genetic condition, but if you carry one abnormal copy in your genes, you risk passing the condition down to your children. An example of this is cystic fibrosis. Cystic fibrosis is an autosomal recessive condition, meaning you have to inherit an abnormal copy of the CFTR gene from both of your parents in order to have the disease. Parents may choose to have genetic testing for cystic fibrosis to see if they are carriers.

If you carry a certain genetic condition, your doctor may advise preimplantation genetic testing, which is used to see if an embryo conceived by in vitro fertilization (IVF) has a genetic disorder as well as select embryos for implantation that is free of the disorder.

Another medical use of genetic testing is looking at tumors. Many tumors have genetic mutations which "drive" their growth. Genetic testing can tell whether or not a particular cancer has a genetic mutation treatable with targeted drug therapy. Most genetic changes in cancer cells occur after birth and are not hereditary with the exception of BRCA1 and BRCA2, which are associated with breast cancer.

Other Types of Genetic Testing

Paternity testing confirms that your child is genetically related to their father. Paternity tests are considered highly reliable with one rare exception, chimerism. If you are a chimera, your cells may have different DNA. This occurs most frequently in twins, who share some DNA with the other twin; in mothers who have given birth and may have picked up some DNA from their baby, and those who have had transfusions or organ transplants. Mosaic Down syndrome is another form of chimera.

Genetic testing is also used at crime scenes. Forensic investigation teams use special tests to increase the amount of DNA available for examination. DNA at a crime scene can come from blood cells, sperm cells, or other body cells such as hair or saliva. Mitochondrial DNA is rarely used in forensics.

A growing use of genetic testing is ancestry testing. DNA from two individuals can be compared to see if they are similar in certain important ways that would mean they are related. Mitochondrial DNA, in contrast to nuclear DNA, is not found in the nucleus of each cell, but rather is found in the mitochondria—organelles in the cytoplasm of the cell that act as the power plant of the cell and is comprised of only one chromosome wrapped in a circle. Mitochondrial DNA is much shorter than nuclear DNA and carries the blueprint (only 37 genes) for only a few enzymes involved in metabolism.

Mitochondrial DNA is passed on from the mother alone. A mitochondrial DNA test does not give any information about your father but could help to identify maternal lineage—even many years in the past. Mitochondrial DNA, in contrast to nuclear DNA is much easier to come by and can be used to make an identity (at least maternal) even when there is no nuclear DNA present. 

How Much Does Genetic Testing Cost?

The cost of a DNA test depends on what type of testing needs to be done. Some simple tests, like paternity tests or those used for easy-to-detect genetic mutations, might cost a few hundred dollars or less. More complicated testing, such as for rare genetic disorders, can cost thousands of dollars. Fortunately, thanks to genetic startups and companies that offer ancestry testing, more common tests have decreased in cost. 

Ethics of Genetic Testing

There is little controversy over the routine use of some antenatal tests. If certain conditions are discovered at birth, such as phenylketonuria, your child may have a healthier future. However, not all genetic conditions can be treated and some do not appear until adulthood. Huntington's disease, a progressive neurological disorder, develops later in life and has no treatment or cure. In cases like these, some people forego testing.

In other cases, you may be predisposed to "cancer genes," like BRCA1 and BRCA2. Knowing you have these genes can help you make important decisions about your health. However, it is your choice, and if you are better off with or without the information. 

Sources:

Chard, R., and M. Norton. Genetic Counseling for Patients Considering Screening and Diagnosis for Chromosomal Abnormalities. Clinical Laboratory Testing. 2016. 36(2):227-36.

Fonda Allen, J., Stoll, K., and B. Bernhardt. Pre- and Post-Test Genetic Counseling for Chromosomal and Mendelian Disorders. Seminars in Perinatology. 2016. 49(1):44-55.

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