DNA Evidence in Criminal Cases

DNA testing is now common in criminal trials and in proving innocence after wrongful convictions.

By , Attorney · UC Law San Francisco

DNA technology is rapidly becoming the method of choice when it comes to linking individuals with crime scenes and criminal assaults. DNA evidence is increasingly used in criminal trials, and has also become a powerful tool in proving the innocence of wrongly-convicted prisoners. (For more on DNA, see Can officers collect DNA samples from suspects?)

This is a change from the past -- until the 1990s, the only sure-fire way to establish the identity of an individual was to examine his or her fingerprints. Because each individual's fingerprints have a unique pattern, fingerprint evidence is readily admitted into court.

How DNA Evidence Works

The universally accepted theory underlying DNA analysis is that every person (except an identical twin) has certain elements of his or her DNA (deoxyribonucleic acid, which makes up chromosomes) that are unique.

Different methodologies allow experts to identify these distinguishing elements. The most common of these methodologies is "RFLP," or the "Restriction Fragment Length Polymorphism Technique." By comparing an individual's known DNA with a sample of DNA from a crime scene (for example, in a droplet of blood or a strand of hair), an expert can give an opinion concerning the likelihood that both samples came from the same individual.

This sort of technology is extremely complex; few people are able to understand it. Nevertheless, the major crime labs -- and, increasingly, local police agencies -- are becoming adept at comparing DNA samples left at the scene of a crime with DNA taken from a suspect and concluding on that basis that the suspect is the culprit.

The conclusion that a DNA match proves the defendant's guilt is based primarily on the assumption that the probability against one individual's DNA matching another's is in the hundreds of millions, or even billions, depending on who is crunching the numbers. However, as overwhelming as these figures may seem, it's still possible to whittle them down to far less overwhelming odds if it can be shown that the methods used by the laboratories doing the testing were flawed in some manner. It is this approach, among others, that the defense team in the O.J. Simpson case used to mount its defense against what appeared to be overwhelming DNA evidence implicating O.J. Simpson as the guilty party.

DNA As a Tool in Proving Innocence in Wrongly-Convicted Prisoners

DNA evidence is also proving to be a powerful tool in determining the innocence of prisoners who were tried before DNA testing in its current form was an option. If bodily samples, such as blood or semen, were collected from the crime scene or victim and that evidence is still available for DNA testing, a showing that the prisoner's DNA doesn't match the crime-scene DNA can be a powerful reason to conclude that the prisoner is innocent and should be released. DNA evidence has thus far resulted in the release of about 250 wrongly-convicted prisoners (many of whom had served many years in prison or were even facing the death penalty). Most instances of erroneous convictions are due to mistaken eyewitness identifications.

For More Information

To learn more about DNA evidence and for answers to other questions about every part of a criminal case, see The Criminal Law Handbook: Know Your Rights, Survive the System, by Paul Bergman and Sara Berman (Nolo). If you need a criminal defense attorney, check out Nolo's trusted Lawyer Directory for a lawyer in your area.

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