CSI UIndy: Study shows secondary DNA can wind up at crime scenes
If a person’s DNA is found at a crime scene, that person had to have been in that room, jurors might assume. But that assumption might well be false, a recent University of Indianapolis study that started as a class project indicates.
With forensic science kits growing increasingly sensitive, samples may not only contain DNA from individuals who were actually at the scene or, say, personally handled a weapon. They also may include genetic samples from people with whom those individuals had contact, the study found.
This result suggests that investigators and jurors need to proceed with caution with "touch" DNA, small samples of material such as skin cells collected from a crime scene, the study’s authors say.In the past, scientists needed to have hair or bodily fluids to derive DNA. But the advent of touch DNA allows them to build a profile from just a few cells.
“There’s no nevers and no always in all of science. ... Forensic DNA is a very, very important tool, but it is subject to interpretation just like the other types of evidence that are presented,” said Krista Latham, a University of Indianapolis associate professor and director of the school’s molecular anthropology laboratory, in whose class the work originated. “This solidifies that this is good science, but it also emphasizes that there is interpretation that comes with this science.”
Because forensic science kits have only recently developed the capability to detect such minuscule amount of DNA, Latham and other experts said that they did not think that this study would open the door to overturning past convictions.
But the study could have implications for how forensic scientists present their evidence in the future, and it points to future avenues of research, experts uninvolved with the study agree.
“This certainly raises the question that we need to start thinking about it,” said Barbara Llewellyn, a forensic DNA consultant based in Atlanta. “This is just one study. We need to do more studies and talk as a community about it.”
When Cynthia Cale, who is studying for her master’s degree in biology, proposed doing the study as a project for Latham’s class, “forensic and historic DNA analysis” she did not think that it would necessarily reveal any problems with touch DNA.
Other researchers had dismissed concerns about touch DNA, saying that they did not think inadvertent transfer of DNA would have an impact on test results.
“Based on all of the prior research, I really thought it wasn’t going to go anywhere,” said Cale, lead forensic DNA analyst at Strand Diagnostics.
For the study, two people shook each other’s hands for two minutes. One person then handled a knife. In all but 15 percent of the time, enough DNA from the person who never touched the knife was found on the weapon to identify that person.
In five of the 20 samples, the person who was identified as the main or only contributor of DNA to the potential weapon had never even touched it. That could be because some people shed more DNA than others do, Cale said.
The results lead the researchers to conclude that the term “touch DNA” may mislead jurors and others into believing that means that the person had direct contact with the object or spot from which the DNA was gathered. The term “transfer DNA” would be more accurate, Latham said.
“That profile can’t tell you how it got there, when it got there, or even if it’s significant in terms of that crime,” Latham said. “Those are all questions that are based on interpretation.”
The findings will be published in the January issue of the Journal of Forensic Science.
Some experts said the study is too preliminary to generate much concern about the validity of DNA evidence gleaned from a crime scene.
Before the researchers asked their study subjects to shake hands, they had them wash their hands and then wear latex gloves for an hour and a half to maximize their own sweat and oils on their hands. Then the researchers had them vigorously shake hands.
“One thing that’s important to understand, these were very artificial conditions,” said David Foran, director of the forensic biology laboratory at Michigan State University. “Those aren’t real-world conditions.”
The University of Indianapolis researchers acknowledge that their results are preliminary. Cale plans to continue exploring the role of secondary DNA in touch DNA samples for her master’s thesis.
Future studies will minimize the amount of contact between the two individuals to see if that makes a difference. She also will explore how long the second person’s DNA persists on the hands of the first. This study asked participants to immediately touch the knife after shaking hands.
Such research could help jurors know how to assess DNA presented in court, said Fran Watson, a clinical professor of law and director of the wrongful conviction clinic at Indiana University Robert McKinney School of Lawn in Indianapolis.
A 2009 report from the National Academy of Sciences warned forensic scientists not to overstate their conclusions, but the lay public may be all too quick to accept what a scientist says without questioning.
“They call that the 'CSI' effect or the aura of infallibility. ... Science can sound so convincing,” Watson said. The whole point is we don’t know, even when we go into court and say this is certain and this is certain, we only know that it is a matter of degrees.”
Call Star reporter Shari Rudavsky at (317) 444-6354. Follow her on Twitter:@srudavsky.