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DNA evidence: Court primer highlights important questions

December 15 2017

Court Primer Highlights

The recently published ‘Forensic DNA analysis – a primer for Courts’ document serves as a timely reminder of the complexity of DNA profile evidence in criminal cases.  With advances in DNA profiling technology allowing profiles to be generated from just a few cells, just knowing whose DNA is present is simply not enough to assess the probative value of the DNA evidence.  The primer highlights two other important questions:

  • From what body fluid has it originated? and
  • How did it get there?

The forensic specialist you instruct needs to assess both the significance of the DNA and use their expertise to evaluate the findings in the case circumstances.  As highlighted in the primer, consideration of indirect DNA transfer, for example, is increasingly important.  In this respect, ‘information is king’ and subtle changes in accounts, or assumptions made about accounts in the absence of complete information, are critical. 

Take a look at example 4 in Appendix 4 of the primer.  A scenario is presented where DNA has been detected on the trigger of a gun.  The authors highlight the transfer requirements if the DNA were to have transferred directly or indirectly via a plastic bag upon which the person of interest’s DNA has been detected on the handles.  They opine that a scientist is likely to conclude that the findings are more likely if the trigger had been touched by the person of interest than if DNA were to have transferred from the bag handles.  This conclusion assumes, however, that there is no reason for the bag handles to have touched the trigger, or that any person who handled the bag by its handles has not subsequently touched the trigger of the gun.  Whilst the presented conclusion might be valid if DNA on the bag handles were low level, the handles were not in contact with the gun when it was first encountered and the officer who made the gun safe did not touch the handles prior to handling the gun, the interpretation would be quite different if any/all of these assumptions were reversed.  Consequently, whilst the primer’s example serves to illustrate how a forensic scientist might seek to formulate an opinion, it also demonstrates the inherent risks from making unsubstantiated assumptions.

The primer serves also as a cautionary guide to the dangers of Streamlined Forensic Stage 1 Reports that often communicate a DNA profile match with no assessment of what this means in the case context.  Such reports are inherently dangerous as they invite the reader to make gross assumptions about the significance of the scientific findings.  In our opinion, without a comprehensive assessment of the findings in the context of the complete case circumstances by an appropriately qualified person there is great potential for them to be misunderstood and overstated.   

If you would like to discuss any of the issues highlighted above, or have a case involving DNA profile evidence that you think would benefit from probative evaluation, please call Lee Fagan at our Durham office or Dr Helen Davey at our Huntingdon office.

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