How DNA Evidence Works

DNA evidence is powerful, but it does have limitations. One limitation is related to misconceptions about what a DNA match really means. Matching DNA from a crime scene to DNA taken from a suspect is not an absolute guarantee of the suspect's guilt. Instead, forensic experts prefer to talk about probability. For example, they might make a statement like this: The chance is 1/7,000 that an unrelated person would by chance have the same DNA profile as that obtained from the evidence. Combine that statistical analysis with other evidence, and you can see how prosecutors can make strong cases against a suspect.

A contributing factor to public misconception is how DNA analysis is portrayed in movies and television. Some lawyers and judges complain that a so-called "CSI effect" is influencing criminal justice. The CSI effect manifests itself when jurors demand DNA tests in cases where they are unnecessary or rely too heavily on DNA evidence to the exclusion of other physical evidence taken at a crime scene.

Even more troubling are cases of DNA fraud -- instances where criminals plant fake DNA samples at a crime scene. In 1992, Canadian physician John Schneeberger planted fake DNA evidence in his own body to avoid suspicion in a rape case. Planting fake DNA obtained from someone else is only part of the problem. Scientists at Nucleix, an Israeli company, recently reported that they could, with access to profiles stored in one of the DNA databases, manufacture a sample of DNA without obtaining any tissue from that person.

Nucleix has developed a test to distinguish real DNA samples from fake ones, with the goal of selling the test to forensic laboratories. But taking these extra precautions to ensure the validity of its results will only slow down busy labs even more. In fact, forensic casework backlogs are becoming a serious problem. A study conducted by the Bureau of Justice Statistics found that more than half a million cases were backlogged in forensic labs, which means felons and other violent offenders could be walking the streets while their DNA evidence sits in a queue, waiting to be tested [source: Houck].

As advances in DNA testing are made, some of these challenges may become less severe. But other, unforeseen challenged will likely emerge. Up next, we'll examine some of these advances and their implications.