October 01 2026
This is the final part in our series of articles looking at some of the challenges associated with DNA profile/forensic biology evidence. Over the last three weeks we’ve focused on A question of 'Who?', A question of 'What?' and A question of 'When?'; this week we’re going to take a look at the ‘activity level’ consideration of A question of ‘How?’.
DNA can be deposited by direct contact, projection (e.g. coughing or sneezing), or through indirect (secondary or tertiary) transfer. The probative value of DNA evidence frequently depends on whether these mechanisms can be distinguished.
Indirect transfer is particularly significant in cases involving handled items. In a recent case, DNA attributed to a suspect was found on the handles of a carrier bag discarded at the scene of a burglary. The statistical evaluation was highly persuasive and at face value the findings looked compelling. However, examination of continuity material revealed photographs of a police officer handling the bag immediately after handling the suspect’s coat and footwear. Whilst the officer wore disposable gloves (which was reported as adherence to best practice), it has been demonstrated that DNA/biological material can transfer via gloves. Indirect transfer during police handling was therefore as plausible an explanation for the findings as direct handling by the suspect, and findings that had appeared compelling once again were anything but.
Examples of poor handling practices – drugs placed on carpets, firearms laid on beds for photographs – are regrettably common. Such practices can create transfer pathways that fundamentally alter the interpretation of forensic findings. The take home message from this is that even where a DNA match is reliable, it does not follow that it has probative value; how and when biological material was deposited are often decisive considerations.
This concludes our four-part series looking at some of the challenges associated with DNA profile/forensic biology evidence. We hope you have found it interesting and thought provoking, but most importantly we hope you’ll take away that whilst DNA profiling is a powerful investigative tool, it is not infallible. Identification, source, relevance and activity must each be addressed explicitly; where assumptions are made, conclusions may be flawed. No aspect of DNA profile or forensic biology evidence should be regarded as beyond challenge. Where such evidence is disputed, informed forensic review is essential; Do Not Assume, ask an expert!
Keith Borer Consultants is a leading provider of forensic science expert witness consultancy and has been providing services to the Criminal Justice System for more than four decades. If you have a case involving disputed DNA or forensic biology evidence, or any of the broad range of specialisms our numerous experts cover, please do not hesitate to email kbc@keithborer.co.uk or call us on 0191 332 4999.
