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DNA Evidence


This practise note provides an explanation into what DNA identification evidence is, how it is obtained and used in line with Code D of the Police and Criminal Evidence Act 1984 (PACE). It further discusses how DNA evidence may be deployed and the ways in which it can be challenged.


Perpetrators of an offence are identified through evidence which identifies them as a particular person being in the place of a crime. Commonly, identification evidences that is used in the courts is evidence of footprint impressions, scientific evidence of DNA, ear impressions, voice identification, handwriting and facial recognition.[1] All identification evidence is circumstantial evidence, which is used to persuade a jury to come to the conclusion the perpetrator was present at the crime and of their guilt for the offence of which they are charged, beyond a reasonable doubt. For example, fingerprint evidence from a murder weapon combined with DNA samples of the defendant on the dead victim will result in a successful prosecution.


The identification evidence is very important in criminal cases because the prosecution will need to prove all elements of the offence, along with that the defendant was present at the scene and was the person who committed the offence. DNA evidence is vital as it places a person at the scene of the crime or in close proximity to the victim. This element is discussed in the case analysis practice note.


In a criminal case, proof of identity can be inferred through DNA, which will contaminate a document or item with the suspects DNA. As stated above, DNA evidence is similar to handwriting evidence, in that it is circumstantial. It is not direct evidence that the suspect has committed the crime such as CCTV.

Where DNA samples found at the crime scene matches the suspects DNA profile, then the significance of the match needs to be assessed. A statistical assessment is carried out into the likelihood of whether the DNA match profile has a common source. The way in which this is done, is through match "probability" or "random occurrence ratio". A forensic scientist is asked by the Court to provide an expert witness testimony to calculate the probability ratio based on the likelihood of the match occurring within general population or if it was possible with some evidence that a relative may have left the sample.[2]

Where the DNA evidence provides a random match probability in the order of one in a billion, it may permit the finding of the defendant's guilt even in the absence of any other supporting evidence depending on:

  • The circumstances of the charge; and

  • The relevance of the deposit of the DNA to that charge (See R v FNC [2015] EWCA Crim 1732); it is discussed in the case analysis practice note).

However, where DNA evidence arises in a criminal case where a sample has been retrieved from an article, rather than being left during the commission of a crime, then it is probable that there will be a need for further evidence which supports the prosecution before the Court deem the Defendant has a case to answer, as seen in Ogden [2013] EWCA Crim 1294.

The Court of Appeal, in R v Jones [2020] EWCA Crim 1021, held that, where there was no expert evidence which supported the argument that the secondary transfer of DNA was in this case improbable, then the trial judge was wrong not to allow an application of no case to answer (Linden JJ, Irwin LJ and Holgate LJ).


The Forensic Science Regulator, on the 11th September 2020 published guidance on how contamination of DNA evidence at a crime scene can be avoided. The guidance advises on how to avoid and control the introduction of disrupting factors to DNA exhibits, at the starting point of a forensic process especially during the examination of the scene of a crime.[3] Reducing the risk of contamination can be made in the following ways:

  • Lowering the contamination chance by staff using barrier clothing;

  • Access restricted to areas which contain exhibits;

  • Scene examination equipment cleaned along with surfaces both before and after use.

  • Rendering consumables free from detectable levels of DNA; and

  • Making sure that any equipment used at the scene is efficiently cleaned after each use based upon risk assessments.


[1] Koehler, Jonathan J, “Error and Exaggeration in the Presentation of DNA Evidence at Trial”, (1993) 34(1) jurimetrics 21-39 [2] Connors, Edward., Convicted by juries, exonerated by science: Case studies in the use of DNA evidence to establish innocence after trial, (DIANE Publishing, 1996) [3] Tully, G. (2020) Forensic Science Regulator Guidance: Expert Report Guidance, (Issue 4). (Birmingham: Forensic Science Regulator)


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