New Kiwi crime tool unravels mixed DNA

A Kiwi backyard invention is changing crime scene investigations around the world. For the first time, police can take samples that contain more than one person's DNA and unravel the strands.

Among the crimes it has been used to solve was a multiple-homicide in Australia, where a spot of blood found at the killer's house contained blood from several of the victims.

In Nelson, it was used to solve an arson case where an unexploded incendiary device was found at the scene of a forest fire. The handle of a shopping bag found inside the device yielded a mixture of DNA.

Another case, in the United States, proved a soldier rolled a jeep despite his claim it had been stolen from him. A shoe found at the scene contained a mixture of his DNA and another person's.

In the past all this evidence would have been discarded. Mixed DNA was useless DNA. There was no way to separate out the strands of genetic code. But now, with a tool developed in the "backyard" of some Kiwi scientists, the problem that has long stymied international forensic scientists has been solved.

The software, called STRmix, has already been sold to the US Army, and the FBI is in talks to acquire the rights.

Institute of Environmental Science and Research scientists John Buckleton and Jo-Anne Bright, in collaboration with Duncan Taylor from Forensic Science South Australia, developed STRmix using standard mathematics and "Monte Carlo processes" - sampling thousands of different possibilities and using chance "many, many times".

The program was put into case work in August 2012 and has since generated "enormous international interest", Buckleton said.

In many cases of sexual assault, particularly those involving more than one offender, any DNA recovered was often mixed. "We had no tool that could interpret mixed, low-level strains. Uninterpretable DNA was put aside - the report goes out as inconclusive."

The tool works in seconds for simple cases or up to days for complex cases. It currently can separate four strands.

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New Kiwi crime tool unravels mixed DNA