Prosecutors in Missouri and around the country tend to be fairly confident when forensic scientists can link a suspect to the crime they allegedly committed using deoxyribonucleic acid analysis, but DNA can also be used by defense attorneys to exonerate individuals who have been wrongfully convicted. The cells of every living organism contain DNA, and analyzing it can link tissue samples with the organism they came from with incredible accuracy.
DNA analysis was first performed in the 1980s using the restriction fragment length polymorphism technique, but the polymerase chain reaction method is more commonly used today as much smaller sample sizes can be used. When DNA tests are performed correctly, the likelihood of a mistaken identification is extremely small. While there is still some academic debate on the issue, most scientists believe that the chances of two individuals who are not identical twins having the same DNA is about one in a billion.
However, there are situations where evidence based on DNA analysis may not be reliable. While the testing techniques are sound, they may produce misleading results if the samples used were not gathered and handled correctly. There have also been cases where forensic scientists mixed up DNA samples or used unauthorized techniques to conduct tests.
While DNA analysis may be a reliable way to place people at a particular place, it cannot reveal when they were there or what they were doing. When DNA evidence is likely to be an important part of a criminal trial, experienced defense attorneys may accept the underlying science but raise questions about how samples were collected and handled or the conclusions drawn by expert witnesses. If a clear chain of custody cannot be established for DNA samples and prosecutors have little else in the way of evidence, attorneys may seek to have the charges against their clients dismissed.