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What is DNA Evidence?

One of the most reliable forms of evidence in many criminal cases is in our genes, encoded in DNA (deoxyribonucleic acid). DNA evidence can be collected from blood, hair, skin cells, and other bodily substances. It can even be used to solve old crimes that occurred prior to the development of DNA-testing technology. Similar to fingerprints, each individual has a unique DNA profile (except for identical twins, who share the same genetic code). But unlike fingerprints, only a minuscule amount of genetic material is needed to identify a suspect.

This article provides a basic overview of DNA as a source of evidence in criminal cases. For more detailed information, see How DNA Evidence Works; DNA: Greater Accuracy; and Post-Conviction DNA Analysis.

DNA Evidence: A Brief History

The science of DNA testing was developed in 1985 by British scientist Alec Jeffreys. Genetic evidence was first tested using his method one year later to solve a double homicide in England and to link the suspect to other previously unsolved rapes and murders in the area. In 1987, a Florida rapist became the first criminal defendant in the United States to be convicted through DNA. Genetic material collected at crime scenes and preserved in evidence lockers also has become an important factor in exonerating those who were wrongly convicted of violent crimes.

As DNA became the gold standard for identifying criminal suspects, the FBI and police departments throughout the U.S. started assembling databases (see The National DNA Database System). Additionally, sex offenders in all states are now required to submit DNA samples to their local police department. Unfortunately, many crime labs are overwhelmed with backlogs of genetic samples and may be unable to process them in a timely fashion.

How Does DNA Testing Work?

A sufficient amount of DNA may be found in virtually any type of biological evidence. For violent crimes, such evidence typically comes from blood or other bodily fluids. Hair and skin cells left at the crime scene also may provide investigators with enough DNA for testing purposes.

DNA evidence is analyzed using the polymerase chain reaction (PCR) method, which allows for very small samples to be tested and identified. Once the sample is tested, it may be cross-referenced with DNA profiles already in a database or with genetic data provided by a suspect.

While DNA testing is not completely foolproof, it is more than 99 percent accurate (in fact, there is only a one in one billion chance that the DNA of two individuals will match). Typically, errors in testing are the result of mix-ups in the lab or the contamination of samples. Additionally, each state has specific rules for DNA sample collection and handling. Courts might not allow the use of genetic evidence in court if these requirements are not met.

Other Uses of Genetic Evidence

In addition to criminal investigations and trials, DNA can also be used to exonerate wrongly accused individuals (see DNA as an Exoneration Tool). This is particularly important for those convicted of serious crimes solely on the basis of eyewitness testimony, which is not always reliable. More than 250 people have been exonerated through post-conviction DNA tests, according to the Innocence Project.

Also, DNA can be used to determine paternity in child support cases; to identify the remains of crime and accident victims; and to conduct genealogical research.

If you have additional questions about DNA evidence or related matters, see FindLaw’s Criminal Evidence section.

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