CAN DNA DEMAND A VERDICT?
Since 1987, forensic DNA analysis has made appearances in U.S. courtrooms. Originally known as "DNA fingerprinting," this type of analysis is now called "DNA profiling" or "DNA typing" to distinguish it from traditional skin fingerprinting.
Although used in less than one percent of all criminal cases, DNA profiling has helped to acquit or convict suspects in many of the most violent crimes, including rape and murder.
How can DNA be used to identify an individual?
Almost every cell in our bodies contains DNA, the genetic material that programs how cells work. 99.9 percent of human DNA is the same in everyone, meaning that only 0.1 percent of our DNA is unique!
Each human cell contains three billion DNA base pairs. Our unique DNA, 0.1 percent of 3 billion, amounts to 3 million base pairs. This is more than enough to provide profiles that accurately identify a person. The only exception is identical twins, who share 100 percent identical DNA.
At a crime scene, DNA is everywhere. It is present in all kinds of evidence collected at the scene, including blood, hair, skin, saliva and semen. Scientists can analyze the DNA in evidence samples to see if it matches a suspect's DNA.
How is forensic DNA analyzed?
On the right, you can see how DNA evidence is collected and analyzed. In the past, DNA analysis required an evidence sample at least the size of a dime. Today's techniques can multiply the DNA, producing millions of copies from tiny amounts of evidence, such as the saliva from a cigarette butt. This approach is also helpful for analyzing poor-quality DNA in evidence samples collected from dirty crime scenes.
If people are blood relatives, is their DNA similar?
Though all people except identical twins have three million bases of unique DNA, blood relatives share more similar DNA than do unrelated individuals. If the DNA profiles from the evidence and a suspect are similar but not identical, blood relatives of the suspect may be investigated further.
What about human error?
Forensic investigators take many precautions to prevent mistakes, but human error can never be reduced to zero. The National Research Council (NRC) recommends that evidence samples be divided into several quantities soon after collection, so that if a mix-up were to occur, there would be backup samples to analyze.
To detect possible contamination of DNA samples during collection or handling, evidence DNA profiles are often compared with those from detectives at the crime scene, the victim, a randomly chosen person or a DNA profile from a database.
The NRC recommends that forensic DNA analysis be conducted by an unbiased outside laboratory that maintains a high level of quality control and a low error rate.
Is DNA evidence alone enough to acquit or convict?
It is easier to exclude a suspect than to convict someone based on a DNA match. The FBI estimates that one-third of initial rape suspects are excluded because DNA samples failed to match.
Forensic DNA is just one of many types of evidence. It is important to examine other clues such as motive, weapon, or additional evidence linking a suspect to the crime scene. The more evidence collected, the less likely it is that samples from a particular suspect were planted, either on purpose or by accident, at the crime scene.
Can DNA evidence exonerate wrongfully convicted prisoners?
The Innocence Project at New York's Benjamin N. Cardozo School of Law aims to exonerate prisoners wrongfully convicted of crimes. The project uses DNA profiling evidence to support the re-evaluation of criminal cases. But DNA evidence alone is not enough to get a person out of jail: the case must be re-examined by a judge, along with lawyers representing both sides of the case. Since 1992, the Innocence Project has exonerated over 100 prisoners, including eight who were on death row - one of whom was only five days from execution.
Thirty-three states restrict the time for post-trial submission of DNA evidence to six months or less. New York and Illinois, on the other hand, will reconsider cases with compelling DNA evidence regardless of when the trial ended. Unfortunately, the evidence from some cases has been lost or destroyed, making DNA analysis impossible.
Is this technology used appropriately for justice?
DNA profiling can be a powerful tool in criminal investigations. Its success in the courtroom depends upon many factors, including:
When used correctly, DNA profiling is a powerful forensic tool. It can be used to quickly eliminate a suspect, saving time in searches for perpetrators. And it can provide compelling evidence to support a conviction and, most importantly, reduce the chances of a wrongful conviction.
Funding for this feature was provided by the Educational Resources Development Council, University of Utah.
Author: Jacqui Wittmeyer
Need to know some DNA basics?
Find out more
DNA Forensics, from Human Genome Project Information, provided by the U.S. Department of Energy
DNA Untwisted, by Norah Rubin, Ph.D.
The Evaluation of Forensic DNA Evidence, from the National Academy Press
DNA as a forensic instrument, by Mark Fischetti.
Forensic DNA Articles, from the Denver, Colorado District Attorney's Office.