DNA In The Courtroom


Written  August 2015 by:
Lisa Wilson - Furnival Chambers - Called 2006

Lisa enjoys a thriving defence practice, both as a led junior and junior alone. Renowned for her fearlessness in court and friendliness outside the court room, she is highly valued by lay and professional clients alike. Lisa receives instructions in cases covering all aspects of Fraud, Confiscation, Cash Seizure and Criminal Law including Manslaughter, Armed Robbery, GBH, Child Cruelty, Serious Sexual Offences and multi-handed Drugs Conspiracies.


What Is DNA?

  • Deoxyribonucleic acid – or DNA as it is known to you and me - contains all the genetic information about a person. It is a ‘blueprint’ for the human body and is unique to each individual (except for identical twins, who have exactly the same DNA). A person has the same DNA in every cell throughout their entire body.
  • By testing the DNA found in a person's cell, such as a cheek swab, scientists can produce a DNA profile for that individual. This is a series of 20 numbers plus an indicator that shows the gender of the person. In the mid-1980’s, English scientist Dr Alec Jeffreys, discovered that certain areas of the DNA strand contain patterns that repeat many times. The number of these repetitions varies between individuals. ‘DNA profiling’ involves the testing of regions of a person’s DNA that contain the short tandem repeats (STRs). This allows scientists to identify individuals through their DNA. The chances of one individual’s DNA profile matching another persons are extremely small – approximately one in a billion. STR profiling is a very sensitive technique and only a tiny quantity of DNA, such as a microscopic flake of skin, is needed.

  • The body easily sheds material, and as such we deposit our DNA throughout the day. For instance, when we have a drink, we will leave our saliva behind on the cup; when we sneeze, the tissue will contain a piece of us; when we shake hands with someone or touch something, we may leave a trace of our skin cells behind; and if we cut ourselves, our blood will leave a trail. 

  • DNA profiles can be used as evidence in criminal investigations when matched to samples of material taken from crime scenes. The use of DNA evidence in criminal investigations has grown in recent years. DNA testing has helped police identify criminals and solve difficult crimes. It has also helped prove that many suspected or convicted people are in fact innocent. The UK has the world’s first national DNA database. It contains over 5 million people’s DNA profiles – mostly those who have been suspects in investigations or convicted of crimes.

  • Providing that crime scene investigators properly collect the biological material and the police properly handle the exhibits, and assuming that the forensic scientists correctly employ established methods to analyse it, DNA evidence is extremely accurate. 

DNA at Crime Scenes

  • DNA linking the person charged with an offence (let’s take burglary as an example) to the scene of the crime (the burgled house) can be used by the prosecution to show guilt. This usually comes in two forms:

    • The first is evidence which is attached to an immovable part of the scene, such as blood on the smashed bedroom window.

    • The second type is evidence attached to a movable object found at the scene. This may include a DNA saliva sample from a drinking bottle found at the scene.

  • Obviously DNA evidence found on a static part of the scene implies that the defendant has been at the scene of the burglary. However, evidence on a movable object does not necessarily mean that the defendant has been involved in the burglary.  

Challenging the Evidence

  • DNA evidence is not unassailable. Perhaps most importantly, it cannot be dated. So whilst the existence of the blood may imply presence at the address, that alone cannot be used to tell a jury when the defendant was present. 

  • There is also a real danger of the evidence itself being compromised. DNA profiling is not a fool-proof science, particularly where very small crime samples are available for testing. In such circumstances the reliability of the DNA will depend upon the accuracy of the testing, the measurement involved and the profile matches.

  • When challenging DNA evidence, defence solicitors and barristers should focus on the behaviour of the investigators and forensic analysts. DNA evidence can be easily contaminated during collection and storage. Contamination can occur when DNA evidence mixes with the DNA of another person. This could occur as easily as the scientist handling an exhibit without gloves, or two exhibits touching each other, or even by the exhibit rubbing against the bag it is stored in (paper bags or envelopes are safer than plastic ones because plastic will retain moisture that may damage the biological material). This should be explored through instructing a defence expert and during cross examination. It is important to know what questions to ask.  

Defendant Cannot Be Charged Solely On The Basis Of Moveable Object DNA Evidence 

  • It is established law that DNA on a moveable object is not sufficient evidence to found a case against a defendant, in the absence of other compelling evidence. This principle was helpfully set out by the Court of Appeal in Doheny and Adams

“The significance of the DNA evidence will depend critically upon what else is known about the suspect. If he has a convincing alibi at the other end of England at the time of the crime, it will appear highly improbable that he can have been responsible for the crime, despite his matching DNA profile. If, however, he was near the scene of the crime when it was committed, or has been identified as a suspect because of other evidence which suggests that he may have been responsible for the crime, the DNA evidence becomes very significant. The possibility that two of the only 26 men in the United Kingdom with the matching DNA should have been in the vicinity of the crime will seem almost incredible … ”

  • This statement of principle has been repeatedly confirmed by the Court of Appeal, such that it is clear that where a movable item is left at the crime scene with a DNA profile which matches the defendant's, that on its own is not sufficient to support a conviction. 

  • Lashley had been convicted of robbery of a sub post-office after being linked to the crime through the saliva on a half-smoked cigarette left behind at the scene. The Court of Appeal held that although DNA analysis of the cigarette revealed a profile which matched the appellant, the judge should have stopped the case at half time as there was no other evidence against him. His conviction was accordingly quashed.

  • In R v Ogden, a scarf had been left behind at the scene of a dwelling burglary. There was blood on the scarf. The DNA profile from the blood matched the DNA of the appellant. The Court of Appeal quashed the conviction, and reiterated the principle that where there is no evidence other than DNA on a moveable object, this is insufficient to found a conviction.

  • This principle has been confirmed in the very recent case of R v Bryon [2015] WLR (D) 180. In that case, however, the Court of Appeal upheld the burglary conviction because the appellant’s previous burglary conviction, which contained “most unusual” factors “carried out in exactly the same manner”, was sufficient to rebut coincidence.  

The Role of the Advocate 

  • There is increasing pressure on defence advocates to simply agree short ‘Streamlined Forensic Reports’ at the preliminary hearing, and the Judiciary often robustly press advocates to justify any derogation from this. For the sake of our clients, it is vital that we stand firm against this pressure.
  • SFR’s are usually silent on a number of important matters which the defence may wish to explore – both through cross examination and by instructing their own expert. For instance, 
    • Was incorrect data used by the scientist?
    • Is her suggested number of contributors a robust assertion, or may there be more?
    • Have her previous examinations been sufficiently audited or independently peer reviewed?
    • How many alleles were considered?
    • How much DNA exists in the sample?
    • Is there anything within the profile suggestive of stutter peaks/ contribution by another party?
    • Are the laboratory conditions up to the required standard?
    • Consideration of the scientists notes may assist in determining the reliability of her assessment of the components – e.g. has she placed brackets around any of the artefacts on the profile?
    • Contamination 
    • Transfer, including secondary transfer
    • Hidden perpetrator effect

  • It is crucial that we ensure that the jury understand the significance of DNA matches and mismatches in the profiles. In particular, cross examination and careful directions to the jury must be geared towards ensuring they do not confuse the 'match probability' (the probability that a person that is chosen at random has a matching DNA profile to the sample from the scene) with the probability that a person with matching DNA committed the crime.

Conclusion: DNA – Don’t Need to Agree

  • In many ways, DNA has revolutionised the criminal justice system. The rise of DNA analysis has enabled a level of accuracy in criminal identification not possible before the development of these technologies. Used to convict the guilty and to free those wrongly convicted, it is rightly heralded as a major breakthrough in the fight against crime. However, as with all things, the increasing use of it brings with it major risks. As courts and laboratories find themselves facing increased workloads and back-logs, the temptation to cut corners and simply accept results at face-value also rises. Defence teams must remain vigilant and stand firm, be prepared to say no to SFR’s and not to be intimidated by asking questions of the scientists. Science, after all, is never fool proof. 


Expert Criminal Solicitor: 
Mohammed Zeb: Director - Stuart Miller Solicitors

The expert Criminal Solicitors at Stuart Miller Solicitors are proficient with Criminal Law legislation, knowledgeable in creating the best strategy & able to analyse your case papers  to find the grain of detail required to achieve success for you. Having Specialist Criminal Lawyers on your side is vital to achieve the best result and ensure fairness in the proceedings. Challenging the admissibility of evidence, making applications to dismiss, fighting for your rights every step of the way requires motivation and enthusiasm in a Lawyer.  Our Criminal Lawyers have the vibrancy and energy needed to defend you. Our Specialist Criminal Solicitors are leading experts focused on maximising your chances of success.


This document is provided for information purposes only and does not constitute legal advice. Professional legal advice should be obtained before taking or refraining from any action as a result of its content.