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.
The next step is to present those results in an intuitive manner that courts can understand, says Balding.
Journal reference: PNAS , DOI: 10.1073/pnas.1219739110
This article will appear in print under the headline "How to tell if DNA will be useful as evidence"
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