When Gregor Mendel published his studies of inherited characteristics of pea plants in 1866, he probably didn't know he was starting a sequence of events that would end in the 1987 conviction of someone in the United States based on DNA evidence. This article discusses the history and current status of the use of DNA evidence in the United States.



How DNA Evidence is Gathered and Used

DNA (deoxyribonucleic acid) is a nucleic acid consisting of two chains of nucleotides bonded together in a double helix, and is responsible for determining the inherited characteristics of each person. Historically, DNA could only be extracted reliably from clean specimens of blood or other body fluids. Due to recent scientific developments, DNA evidence can be extracted and amplified from a variety of samples, including licked stamps, dental floss, used razors, hair, and even sweaty t-shirts.

The DNA evidence is taken back to the laboratory where the sample is cleaned and prepared. The DNA is cut into small, manageable pieces using enzymes, and then it is categorized by size using a process known as “gel electrophoresis.” We all share some 99.9% of our DNA, but there are specific regions in our DNA that differ. In certain areas, given sequences of the bases adenine, thymine, cytosine, and guanine repeat themselves. The sequences, known as variable number tandem repeats, or VNTRs, create a unique personal blueprint that can be used as DNA evidence.

The VNTRs are marked with a radioactive compound that aids in being able to make an x-ray image of the DNA sequence. These images, which are the DNA evidence ultimately presented in courts, can then be compared to the DNA sample collected from a suspect.

The DNA sample from the crime scene and the suspect are compared at a number of different VNTRs, exponentially increasing the probability that a match between the two specimens is not an error. Statistically an innocent person would be more likely to win the lottery than to be inaccurately convicted using DNA evidence, assuming that the proper number of sequences is analyzed.

Where DNA Evidence Stands Now

The first conviction made using DNA evidence occurred in Portland, Oregon in 1987. Juries seemed hesitant at first to accept DNA evidence as conclusive, perhaps because of the complicated process - which has been extensively simplified for this article - that lawyers and specialists had to explain to the jurors. The process in its infancy left much room for defense attorneys to insert doubt into the cases against their clients. However, as science continued to develop, DNA evidence and technology gained a foothold in the United States' courts.

DNA evidence and associated technologies were thrust into the limelight when a man by the name of O.J. Simpson was accused of killing his ex-wife and her associate in 1995. DNA evidence also played a large role in the case of the disappearance of child beauty queen JonBenet Ramsey.

Just as DNA evidence has been used to convict people of crimes, innocent people wrongly accused have also been freed based on DNA evidence analyzed after the fact. Ten people have been freed from death row in the United States when DNA technology was finally made available to analyze their cases.

At the time of this writing, several states, prisons, and communities are developing programs to create DNA databases, especially from those considered dangerous felons or high risk criminals. The future of DNA evidence in the United States lies in the hands of legislatures, courts, and responsible DNA labs.