How DNA Evidence Works
Technology has changed many aspects of the criminal justice system, and the use of DNA evidence represents one of the best examples of how technology has altered the criminal justice landscape.
DNA, or deoxyribonucleic acid, is the basic building block of life. The information encoded in an organism's DNA acts as a blueprint for the organism's biological development and functioning. DNA exists in the cells of all living organisms, and by testing the DNA found in a person's cell, scientists can come up with a DNA profile for that individual.
DNA profiling of individuals didnt even exist, however, until the mid-1980s, when an English scientist, Dr. Alec Jeffreys, discovered that certain areas of the DNA strand contain patterns that repeat many times. The number of these repetitions varies between individuals (except for identical twins, who have the exact same DNA), and Dr. Jeffreys developed a test to measure the variation in length of these repetitions. Using this test, Dr. Jeffreys found that he was able to identify individuals by comparing samples of their DNA. This test that Dr. Jeffreys developed became known as restriction fragment length polymorphism (RFLP).
RFLP is an accurate and reliable test, but it requires a relatively large amount of DNA to work. Laboratories now use tests based on the polymerase chain reaction (PCR) method, which allows for testing on very small amounts of DNA from biological samples.
Investigators can collect DNA evidence from a number of different sources. Almost any biological evidence can contain DNA, although not every sample contains sufficient amounts of DNA to enable DNA profiling.
Forensic investigators will analyze the biological samples to get a DNA profile of the individual(s) that the samples came from. If investigators already have suspect(s) in mind, they can collect samples to compare to the evidence collected at the scene. There are also databases of DNA profiles that investigators can use to identify suspects by comparing the database information to the DNA profile obtained from the biological evidence.
Assuming that investigators properly collect and handle biological evidence and that the forensic scientists employ accepted methods and conduct the analysis correctly, DNA evidence is extremely accurate. The chances of one individuals DNA profile matching another persons are extremely small about one in a billion.
Compared to fingerprinting or eyewitness testimony, which both have inherent flaws and inaccuracies, DNA evidence is a highly effective way to match a suspect to biological samples collected during a criminal investigation.
Because of its accuracy, criminal lawyers increasingly rely on DNA evidence to prove a defendants guilt or innocence. DNA evidence has also exonerated people through postconviction analysis of biological samples. Since DNA analysis didnt exist until recently, a reexamination of evidence collected during older investigations can reveal that the DNA profile of the person convicted of the crime does not match the DNA profile from biological samples collected at crime scenes.
DNA evidence is not unassailable, however. Errors in the collection and/or handling of the biological samples used for the DNA analysis can result in the exclusion of DNA evidence at trial. Similarly, if a lab contaminates the biological sample or is found to use unreliable methods, a judge may reject the DNA evidence at trial.
When challenging DNA evidence, defense attorneys will usually focus on the behavior of the investigators and forensic analysts in an attempt to cast doubt on the results of DNA profiles, rather than attack the reliability of DNA profiling as a whole. A well-known example of this is the defense strategy used in the O.J. Simpson trial.
Additionally, each state has difference rules regarding evidence, and any failure to comply with the particulars of each states requirements can result in a refusal of the court to examine DNA evidence.
The rise of DNA analysis has enabled a level of accuracy in criminal identification not possible before the development of certain technologies, and shows how technology is shaping, and will continue to shape, how the criminal justice system operates.
See DNA.gov for more information.