Amanda Knox and Forensic DNA Evidence

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The inquiry into the murder of British student Meredith Kercher in Perugia, Italy, has played out in the media for the past six years.  The 21 year old was found stabbed and sexually assaulted in her bedroom in an apartment she shared with three other female students.  One of these, the US citizen Amanda Knox, was charged with the murder together with Raffaele Sollecito, the former Italian boyfriend of Ms Knox.  The Ivory Coast born Rudy Guede is currently serving a 16 year sentence for the sexual assault and murder; his conviction was apparently uncontroversial.  Knox and Sollecito were convicted in 2009 but after spending four years in prison an appellate court in 2011 acquitted them.  This year the Italian Supreme Court overturned the acquittal and directed that a new appellate trial begin in Florence in September (1).

The key evidence in the case was DNA obtained from a knife found at Sollecito’s flat in the kitchen drawer, and from bra clasps from the deceased located at the crime scene.  At the appeal the DNA evidence was challenged by two Italian scientists (2, 3).  The knife allegedly had traces of DNA from Amanda Knox on the handle and of Meredith Kercher on the blade.  The DNA alleged to have come from Knox was not disputed (she regularly visited her boyfriend’s flat) but the DNA profile alleged to have come from Kercher was low level.  The appeal found that the necessary precautions regarding the testing of low level DNA samples were not followed in the analysis of the DNA from the blade.  Furthermore, this DNA could not be related to blood or any specific biological matter.  It was not obvious why the knife was believed to be evidential and questions were raised about handling and packaging.

The bra clasps were recovered from the scene 46 days after the crime in a context highly suggestive of environmental contamination.  Y chromosome DNA analysis indicated the presence of at least three males on the clasps but there was debate about the ‘routine’ nuclear DNA analysis.  It was agreed by all parties that Sollecito may have contributed to the DNA but the question still remained as to how it was deposited.

This case highlights again the caution required in evaluating the relevance of any DNA evidence and the probative value of a DNA profile.  In particular, inferring an activity such as stabbing cannot be scientifically supported by analysis of a DNA profile alone.  Transfer of DNA is also a consideration in every case, as mentioned in this writer’s blog of 10 September 2013.

The Florence appellate court directed further DNA testing of material from the knife, a DNA trace previously untested.  An expert witness said that this DNA showed “considerable affinity” with Knox’s own DNA, and did not match Kercher (4).  A verdict is expected in January next year.

  1. D. Longhini, 16 October 2013, Amanda Knox update: DNA test result is a win for American accused of murder in Italy, available at CBS News online
  2. J.M. Taupin, 2013, An introduction to forensic DNA evidence for criminal justice professionals, CRC Press, Boca Raton, Florida
  3. Conti-Vecchiotti report 2011, translated at
  4. C. Barry, 7 November 2013, Knox’s knife DNA casts doubt on murder weapon,

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