DNA testing is a powerful identification tool. It has been used both to convict and to exonerate people accused of a crime, establish paternity with very close to 100 percent accuracy, and identify victims of accidents who could not be identified by any other means.
Q: How does DNA testing work?
A: Most of your DNA (in fact, more than 99 percent of it) is exactly the same as every other person in the world. However, there is a small group of sections, spread throughout the total genome, that makes you the unique person that you are. No one else has the same sequence of bases in all of these sections of DNA unless you have an identical twin.
DNA analysis looks at some of these specific sections, called markers. A small sample of DNA is taken from cells found in body fluids, skin, hair follicles, or the inside of the cheek. A DNA test using a blood sample is no more accurate than a test using cells from a cheek swab. The swab does not collect saliva for the tests. It actually picks up loose cells that were shed by the tissue of the inside of the mouth. Because all cells contain the same DNA, the swab will provide the same DNA as a blood sample. If a hair sample is used, it must be pulled, not cut, because hair is not made of living cells. When a hair is pulled, however, it carries with it some cells from the living follicle.
After the DNA is isolated from the cells, millions of copies are made using an enzyme that speeds DNA reproduction. Other reactions then break the DNA molecules apart at specific locations to isolate particular markers. These markers are compared to the unknown sample.
Any one specific marker is shared by many people, but the chance of two people having two identical markers is much smaller. In an ideal situation, we would look at the whole DNA sample, comparing all the possible markers to make identification. With today’s technology, though, this is not possible. DNA analysts look at a number of different markers to create a “DNA fingerprint.” The more markers identified, the better the chances that the match between two samples is accurate.
Without looking at the whole of a person’s DNA, it is not possible to absolutely identify a particular person as the source of a DNA sample. In the case of identical twins, it is never possible. However, by using many markers, the odds of a correct identification are extremely favorable. Normally, between 6 and 13 markers are compared. At the higher end of this range, it is extremely unlikely that an incorrect identification will occur, but not absolutely impossible.
On the other hand, it is easy to identify a mismatch in DNA analysis. If any of the markers is different, then the DNA does not match. In a criminal investigation, therefore, a suspect can be eliminated as the source of a sample based on the absence of a single marker. All DNA from a particular person matches from one end of each molecule to the other.
Because samples can remain useful for a very long time, DNA analysis has been used to reopen cases that were once considered solved. The results have far greater reliability than other forms of identification, including eyewitness testimony. A number of people have been cleared of crimes of which they had been convicted, sometimes decades earlier, by analysis of evidence held in storage.
From The Complete Idiot’s Guide to the Science of Everything by Steve Miller