I have previously written at some length about the perils of failing to properly scrutinise expert evidence. There is a particular danger that the strength of the prosecution case will be exaggerated where a suspect’s DNA is found at the crime scene with no other supporting evidence to link them to the offence. “DNA was found at the scene. It’s a 1:1 billion match. The forensics have got them bang to rights, surely?”
Others have written about how much reliance we can really place on match probability ratios depending on how they purport to have been calculated, but let us assume for a moment that there is little dispute that the profile does match the Defendant’s DNA.
Some prosecutors have relied on authorities such as R v FNC  EWCA Crim 1732 and R v Tsekiri  EWCA Crim 40 to assert that, provided a DNA profile matching the Defendant’s is found at the crime scene or on an article associated with the offence, there is a case to answer. That would be all well and good – except that is not what those authorities say.
In FNC, the victim was sexually assaulted on the Tube in 2003 when a man ejaculated onto her dress. In March 2014 the Defendant was arrested for an unrelated offence and his DNA was found to match the semen deposited on the victim’s dress years earlier.
The Court of Appeal held that “where DNA is directly deposited in the course of the commission of a crime by the offender, a very high DNA match with the Defendant is sufficient to raise a case for the Defendant to answer”.
This makes sense – in FNC the depositing of the semen was part and parcel of the offence. If DNA in that semen is found to have a high match probability ratio with the DNA of the Defendant, then (subject to questions about how that match probability is calculated) it stands to reason that the Defendant should answer for that.
This can be distinguished from, for example, a case in which someone’s blood is found on an article left at the scene of a burglary but there is no evidence that any of the actual burglars were bleeding or injured at the time the offence was committed (as was the case in Ogden  EWCA Crim 1294). In such a case the connection between the Defendant’s DNA and the commission of the offence is not so irresistible that it calls for an answer.
In Tsekiri, the victim was robbed just after getting into her car. The offender had opened the driver’s door and had taken a gold necklace from her. A mixed DNA profile was obtained from the driver’s door handle. The major DNA contribution to that mixed profile was said to match the Defendant’s. Again, it was argued that there had been no case to answer.
In his judgment dismissing the appeal and upholding the conviction, Davis J in the Court of Appeal set out a number of questions for Courts to consider when deciding whether or not there is a case to answer solely based on the presence of the Defendant’s DNA:
Davis J went on to state that this “is not an exhaustive list and each case will depend on its own facts”, and that there is “no evidential or legal principle which prevents a case solely dependent on the presence of the defendant’s DNA profile on an article left at the scene of a crime being considered by a jury”.
I would submit that by the same token, there is no evidential or legal principle which prevents an argument that there is no case to answer, either – after all, each case depends on its own facts.
Where the Prosecution’s case rests entirely or largely on DNA found at the scene, I would emphasise that the first step should be to examine how the DNA match probability ratio has been calculated. If there are clear problems with DNA analysis, then challenging that with the assistance of an expert may be the first port of call – if the match probability is undermined to the extent that it is no longer 1:1 billion, then any Prosecution arguments based on Tsekiri should fail.
Even in cases where a single DNA profile is obtained and there is no real scope to dispute that the DNA belongs to the Defendant, it is still not enough for the Prosecution to show that the DNA was found at the crime scene before there is a case to answer. Arguments should be made in line with the considerations set out in Tsekiri above.
Unless there is some evidence that the DNA was deposited in some way connected to the actual commission of the offence, or there is reason to believe that the person who committed the offence deposited their DNA in that way whilst present, then there are good grounds to argue that the case should not pass half-time. If a half-time submission fails, the same arguments can be used in closing to persuade the Court that, on the facts of the case, the Prosecution has not proved the connection between the Defendant’s DNA and the offence beyond reasonable doubt – and if that is the case, the Defendant should be acquitted.
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