Weak, noisy and incomplete: it's easy to see why the
genetic profiles constructed from tiny traces of crime scene DNA can
fail to meet the high standards needed for criminal courts. But these
low-template DNA (LTDNA) profiles could prove useful, thanks to new
software.
Forensic scientists can now construct a partial DNA profile from just a few cells, says David Balding
at University College London. However, natural contamination from DNA
in the environment, together with the fact that it is impossible to
build a complete genetic profile from so few cells, means that
interpreting LTDNA evidence is challenging. Consequently, court cases
where it plays a central role – such as the trial of Amanda Knox for the
murder of Meredith Kercher – often become controversial.
That largely reflects a lack of proper
statistical techniques to handle the data, Balding says. His software
is a step towards improving that.
It compares a full DNA profile of a
suspect with an incomplete DNA profile found at a crime scene. By
incorporating factors such as the natural decay of a DNA sample, or the
presence of DNA from another person entirely, the software can provide a
probability score that a suspect was at the crime scene. Using the
software on data from Knox's trial suggests that it was very unlikely
that her DNA was at the crime scene. Italy's supreme court ruled in June
that Knox and her then-boyfriend, Raffaele Sollecito, acquitted in 2011, should be retried for the murder.
Peter Gill,
a forensic geneticist at the University of Oslo in Norway, says the new
software is based on theories he developed a decade ago, which he
regularly uses to help train police. But he adds that implementation of
those theories in courts, particularly in the UK, is long overdue.
"Current methods of LTDNA interpretation are ineffective," he says.
Several groups are working on software
to improve the handling of LTDNA, Gill says, but the exact methods used
by most are protected for commercial reasons. Balding's software is
open source. "In order to gain court acceptance I argue that
transparency of software is required and therefore the way forward is
open source," says Gill. "Balding's software satisfies this criterion."
The next step is to present the software's results in an intuitive manner that courts can understand, says Balding.
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