DNA Evidence

Here’s how it works.

Until the 1990s, the only sure-fire way to establish the identity of an individual was to examine his or her fingerprints. That is because each individual’s fingerprints have a unique pattern. Now, DNA is rapidly becoming the method of choice when it comes to linking individuals with crime scenes and criminal assaults.

Most people know that our inherited characteristics come from chromosomes and genes. Since the 1950s, science has also known that the chromosomes consist of self-replicating molecules known as deoxyribonucleic acid, or DNA, and that our genes consist of subsets of these very-large molecules. Now, as the end of the century rapidly approaches, we are on the verge of learning which genes result in which characteristics. While this knowledge promises untold rewards in the treatment of disease, agricultural production and an understanding of life itself, it is also being used as a means to uniquely identify individuals and to link individuals to criminal activities.

The universally accepted theory underlying DNA analysis is that every person (except an identical twin) has certain elements of his or her DNA 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. And yet 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.

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. A number of prisoners who were sentenced to death have been cleared through this technology.

The conclusion that a DNA match proves the defendant’s guilt is based primarily on the assumption that the probability against one individual matching another 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 — or even to no odds at all — 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.