September 10 2026
DNA profiling is widely held to be the gold standard of forensic science and without doubt has revolutionised the investigation of crime. But does that mean DNA profile evidence, and its stablemate ‘forensic biology’, should be accepted without challenge? We think not.
Over the next few weeks we’ll be taking a look at some real world situations where DNA profile/forensic biology evidence hasn’t quite lived up to its ‘gold standard’ reputation. This week we’ll be focusing on the subject of identification; who DNA originated from, which is the fundamental question DNA profiling seeks to address. In future weeks we’ll explore wider issues of ‘What has been detected?’, ‘When did it get there?’ and ‘How did it get there?’. By the end of the series you’ll hopefully have gained a better insight into the potential pitfalls of DNA profile/forensic biology evidence, and an appreciation of why it should be challenged.
Part 1: Identification — A Question of “Who?”
Each year, thousands of people are identified as suspects in criminal matters as a result of a ‘hit’ on the National DNA Database. Historically, such matches were verified by the collection and analysis of a fresh reference sample, but that safeguard was removed over a decade ago. Whilst this decision can be justified on the grounds that DNA profiling and National DNA Database management are highly controlled, it does mean that there is now no automatic alert if there is a problem with the initial database search.
To illustrate why this should be a concern, in one of our recent cases a DNA profile held on the database was shown to originate from the brother of the person the database identified it to. This only came to light because the brother was arrested at the same time as the suspect and provided a fresh reference sample under his correct identity. Whilst this makes for an interesting anecdote, what would the outcome have been if the wronged brother had matched to a DNA deposit in a case where there was no other evidence or witnesses? The only way to be certain that a DNA profile match is robust is to go back to the beginning and make sure that the reference DNA profile for an accused person is reliable. As the police won’t do this as a matter of course, the defence team should if DNA evidence is disputed.
Even where the reference profile is correct, identification may not be as conclusive as it first seems. The commonly cited “one billion times more likely” figure sounds compelling, but it applies only where the alternative DNA source is unrelated to the person of interest – and is diminished entirely if the alternative source is an identical twin.
Further complexity also arises in mixed DNA profiles. Statistical evaluations often rely on assumptions regarding the number of contributors and their unrelatedness in order to produce a likelihood ratio. Challenging those assumptions can materially affect the probative value of the findings.
By challenging assumptions in statistical evaluations the probative value of a DNA profile match, and the extent to which findings address the question of ‘Who?’, may turn out not to be as clear cut as first perceived. This brings us back to the core message of this series – Do Not Assume when dealing with DNA evidence, even if at first glance it looks compelling.
Next week we’ll move on from identification and start to take a look at wider issues that can affect the significance of DNA profile/forensic biology evidence. In the meantime, if you have a case where DNA profile/forensic biology evidence is disputed, be sure to drop us a line.
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.
