What Is DNA Testing and Why Is It Useful?

What are the Different Types of Genetic Testing?

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

What is DNA testing? The answer to that question begs another question."What exactly are you referring to?" There are many different ways in which genetic tests may be used which range from screening tests for rare genetic disorders to paternity testing to forensics. In order to understand these answers let's back up first and talk about DNA.

Our DNA Contains Our Genes

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. That string-like structure is your DNA, as it is made 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, for this reason you have two copies of every gene. Humans have around 25,000 genes in total and your DNA is a sequence of these pairs of genes.

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

When a person undergoes a DNA test, those string-like structures are unraveled and read.

Since you have 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 (except for identical twins, who share identical DNA).

How Does Our DNA Dictate What Happens in Our Bodies?

It can be confusing to hear of genetic diseases, and you may have wondered why an abnormality in a single gene can cause so many problems. It helps to understand how genes work. As noted above, our DNA is made up of sections called genes. Some traits, such as eye color, are determined by the message in a single gene, whereas other traits, such as height are related to a combination of different genes.

DNA is like 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, in turn are transcribed (copied) and translated (interpreted) into the proteins (enzymes) that control all that happens 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 (acts as a blueprint) for the proteins that control cell division, a cancer may occur. If there is extra DNA such as with Down syndrome (which has three 21st chromosomes instead of two) a number of abnormalities can occur.

How DNA Testing Is Done

To do a DNA test, a sample of your cells is required. This sample can be from your blood, saliva, skin cells or even the inside of your cheek. (In a cancerous tumor, DNA is obtained from a tumor sample.) Usually, DNA test samples are taken by a doctor or clinic and sent away to a specialized genetic testing lab where the DNA is separated out from your cells.

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

It takes 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.

Why Would You Need a Test Done?

There are many different types of DNA tests which are done for a wide range of reasons. Some of these are discussed below.

Genetic Disorder Testing

Scientists may look for a change (mutation) in a particular gene that is 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. An example is a test for phenylketonuria done on all newborn babies. If babies (due to heredity or mutation) lack a normal gene which codes for the enzyme phenylalanine hydroxylase, phenylalanine that the babies get in their diets can lead to brain damage. If, however, the abnormal gene is discovered at birth, the children can lead relatively normal lives (yet with a restricted diet.) You may have noticed the PKU warning if you've ever looked closely at a can of diet Coke.

Carrier Testing

Carrier testing may be used to identify family members who have genes that indicate they are at risk for passing on a genetic disorder to their children; these individuals do not have symptoms of the genetic disorder themselves. An example of this is the test for cystic fibrosis. Cystic fibrosis is an autosomal recessive condition, meaning that abnormal copies of the CFTR gene from both parents are necessary for a child to have cystic fibrosis. Parents may choose to have genetic testing for cystic fibrosis to see if they are both carriers.

Karyotype Evaluation

If a baby is born with some abnormal findings, a karyotype test is often done. This genetic test looks at the chromosomes in DNA. If instead of 46 chromosomes there are 47, and there are three copies of chromosome 21 instead of two, this represents Down's syndrome. Many pregnancies with an abnormal number of chromosomes end in miscarriage. Parental karyotype tests may be recommended for those who have recurrent miscarriages.

Paternity Testing

Paternity testing confirms if a child has a similar genetic makeup to a possible father. There is a rare exception to the reliability of paternity testing that can be understood if you have a basic understanding of DNA mentioned earlier. There are rare people who are considered chimeras  This means that some of their cells have one type of DNA and other cells have different DNA. Most often this occurs in a child who was a twin and shared some DNA with the other twin, in mothers who have given birth and may have picked up some DNA from the baby, or in those who have had transfusions or organ transplants. Mosaic Down syndrome is a form of chimera.

Crime Scene Investigation (Forensics)

Forensic investigation teams use special tests to increase the amount of DNA available for examination. The DNA at a crime scene can come from blood cells, sperm cells, or other body cells such as hair or saliva. Mitochondrial DNA (see below) may rarely be used in forensics.

Ancestry and Relationships

DNA from two individuals can be compared to see if they are similar in certain important ways that would mean they are related. You may hear mention of mitochondrial DNA as well. 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 have been nicknamed the power plant of the cell. It is comprised of only one chromosome wrapped in a circle. It 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 would not give any information about a person's 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. It has recently begun to be used in forensics for this reason.

Preimplantation DNA Test (PGD)

Preimplantation genetic testing is used to see if an an embryo conceived by in vitro fertilization (IVF) has a genetic disorder. This allows the couple going through IVF to select embryos for implantation that are free of the disorder.

Molecular Testing of Cancerous Tumors

We are learning that many tumors have genetic mutations which "drive" the growth of the tumor. Genetic testing is done to learn whether or not a particular cancer has a genetic mutation which is treatable with one of the available targeted therapy drugs. It's important to note that with most genetic changes in cancer cells, the genetic change occurred after birth and is not hereditary. An exception is the BRCA1 and BRCA2 gene mutations associated with breast cancer.

How Much Does It 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, might cost thousands of dollars.

Pros and Cons of Genetic Testing

Depending on the purpose of the test, there can be some controversy. There is little controversy over the routine use of some antenatal tests. If certain conditions are discovered at birth, as above with phenylketonuria, the child can be given a special diet and spared disability. On the other hand, the mpact and ethics of a few tests is often debated. Huntington's chorea is a progressive neurological disorder. Genetic testing allows people to test and know if they may develop the disease. Since there is nothing that can be done to prevent the disease, many people do not want to know if they are predisposed. Another area of controversy occurs with testing for so-called "cancer genes." It's important to note that cancer gene testing does not test for all genetic causes of cancer, but only mutations in specific genes. In other words, you may still be genetically predisposed to a cancer if your test is negative. There can also be problems if a test is positive. In this case it's important for people to ask themselves the question before the test is ordered: "What would I do differently if I have the gene?" Learn more about the pros and cons of genetic testing.

The National Institutes of Health has an online laboratory directory that allows you to search for labs that offer DNA testing for specific diseases/genes.

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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