We're getting better at detecting trace amounts of DNA. How to analyze that DNA? That's where things get more complicated. Michigan is among the leaders in using new DNA analysis software in the courts. Taylor Dunivin reports that this software is increasingly being used by police and prosecutors around the state.
When you touch a countertop in a store, you could be leaving behind your DNA, and so could a robber. In the past, such a DNA mixture was too complicated to be useful to investigators, but new software is helping make the most of DNA collected at a crime scene.
STRmix is a new technology that Michigan courts say can be used to analyze DNA profiles.
Dr. David Foran is with the Forensic Biology Laboratory at Michigan State University. He explains that DNA analysis was simpler when DNA profiles came from just one individual. Think DNA from a pool of blood. Today, investigators can get DNA profiles from countertops, computers, and other surfaces we touch. These samples have trace amounts of DNA, and Foran says they often have DNA from multiple people. “STRmix is a very powerful tool," he says. “It takes something that is potentially very complicated, A mixture of several peoples DNAs and simplifies it and it simplifies it for the DNA analyst from the crime lab and it also simplifies it for the jury and judge.”
Forensic scientists can get a DNA profile, but traditional analysis methods are inconclusive. As techniques become more sensitive, the statistics get more complicated. It’s not math you can do by hand. That’s where STRmix comes in. Foran compares it to his car. He doesn’t know how it works, but he still drives it.
John Buckleton is principal scientist with the Institute of Environmental Science and Research in New Zealand, where he helped create STRmix. In a Skype conversation, Buckleton explains that STRmix allows forensic scientists to analyze samples that were once out of reach. “The STRmix software has improved the usability of DNA in court from 40% to 70%," Buckleton explains, "and therefore evidence is available in a much higher fraction of criminal cases than previously.”
To date, STRmix has been the subject of 28 admissability hearings in the United States, and seven of those occurred in Michigan. Ingham County Chief Assistant Prosecuting Attorney Mike Cheltenham states that "the courts are ruling that it is acceptable, it’s reliable, it’s based on solid science and you can move forward with it.”
Cheltenham recalls a homicide case from 2011. DNA was found on a car door handle, but it could not be analyzed. The sample was a mixture of DNA from three different people. With STRmix, the lab was able to analyze that sample five years later. It was the first case presenting STRmix in Ingham County. In an admissibility hearing, a judge allowed the evidence.
Cheltenham notes that prosecutors and the crime labs hoped for STRmix to be challenged in court so a precedent would be set. That happened in a case last October. While the state Court of Appeals has ruled that STRmix DNA analysis is admissible in Michigan, the state Supreme Court has not done so.
STRmix creator John Buckleton says that while STRmix can be used by prosecutors, it may also benefit defendants, adding “the main thing that we do is exonerate people. The most powerful part of STRmix is to exclude someone.”
Cheltenham agrees that STRmix analysis might, in some cases, lead to exonerations. “If there are people out there who have been convicted of prior DNA evidence that was not as solid as STRmix," Cheltenham conclundes, "then our hope is, even from the prosecution side, that those people are exonerated by this new technology.”
The Michigan State Police Department handles thousands of cases every year. Kristin Schelling, the biology technical leader for the State Police Forensic Science Division, says their lab is employing STRmix analysis with increasing frequency in cases where there’s a DNA mixture, stating that “almost every single case uses STRmix at this point.”
While STRmix can help prove that an individual was at the scene of some crimes, it has its limitations. It cannot produce usable analysis of DNA mixtures from more than five people. It doesn’t seem likely that STRmix or similar software will ever be able to reliably pinpoint one individual out of a DNA mixture beyond that.
Taylor Dunivin is a doctoral student in the department of Microbiology and Molecular Genetics at Michigan State University. Reporting on this story was supported in part by a grant from the Science and Society at State program at MSU.