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Admissibility of DNA evidence

The Reaction is overwhelmingly positive to news that the first case to be featured on the BBC's Crimewatch programme has been solved after a DNA profile produced from evidence at the scene was linked to a family member of the killer on the national database leading the police to the defendant. Paul Stewart Hutchinson pleaded guilty to the murder of 16-year-old Colette Aram and was sentenced to life imprisonment with a minimum term of 25 years. The evidence against him included a DNA match, fingerprint, opportunity and confession.

However, as will be explained in this seminar, a DNA ‘match’ on the National DNA database might be wrong but the evidence could still be admissible in a criminal trial.


DNA is a complex chemical found in cells of the human body carrying genetic information (half from each parent) which determines a person’s physical characteristics. The DNA of any individual is the same in all their bodily fluids and tissues. Except for identical twins, each person’s DNA is unique although the science has not developed enough to every difference between every person so the science deals with regions of DNA. This means that DNA profiling techniques can be used to exclude someone as a donor of a particular sample but it cannot prove identity. In most cases, the scientists can provide a match probability dealing with whether a stain came from the suspect or from another individual with the same STR profile so, for example people who are related might share STR profiles so the match probability statistic can be calculated with that in mind.


The form of DNA analysis used to test samples in most criminal cases is called STR (short tandem repeat) profiling. A DNA profile from a body fluid stain (such as blood, semen or saliva) can be compared with the DNA profile obtained from a reference sample of any person. If the profiles are different, then that person is excluded as a possible source of the DNA. If the profiles are the same, then that person, together with anyone else who has the same DNA profile, can be considered as a potential source of the DNA. The evidential significance of a match can then be evaluated in relation to the probability of obtaining such a match by chance.


In most cases a sample is obtained from a scene and compared to a sample taken from a suspect. Where there is no known suspect, the comparison is done with those existing samples held on the national DNA database (persons previously arrested).

In some cases, defendants will argue that their DNA should not have been retained after arrest where no action was taken (See case of S. AND MARPER v. THE UNITED KINGDOM (Applications nos. 30562/04 and 30566/04) JUDGMENT STRASBOURG 4 December 2008.


Full profile

There is no doubt as to the probative value of a full profile DNA match taken from a scene sample such as a condom or a duvet. This is generally admissible evidence in the same way as fingerprint evidence.

Low template

As recently as December 21 2009, the Court of Appeal decided that low template DNA and partial profile DNA are also admissible in certain circumstances.

The first case was R v David Reed and Terrence Reed. On August 7 2007 David Reed and Terence Reed were convicted at Teesside Crown Court for the murder of Peter Hoe by multiple stabbing. The evidence against them was both motive and opportunity. Telephone and witness evidence put them in the area at the time of the killing and there was a relevant background to the incident. The defence case was one of alibi.

Low quantities of DNA in relation to each defendant were found on two pieces of plastic from knife handles found near Peter Hoe’s body. The components of the DNA had been revealed using the Low Copy Number (LCN) process. They were not from blood or other identifiable biological material, but simply cellular material that had been transferred to the pieces of plastic. There was no real dispute with a defence assertion that the cellular material could have been transferred from another source but the prosecution expert thought this unlikely. She gave evidence that in her opinion the DNA got there by handling.

The LCN process allows for the analysis of very low quantities of DNA. A tiny sample is recovered and copied then each component seen is given a number. It is as if a drop is turned into a pond then the experts go fishing for components and try to agree which components are present. Each dip might bring up a different component but the ones that appear the most are components which, according to expert opinion, can be included in the sample and, in some cases, given a statistic as to how many other people might be likely to have the same DNA. Even where such a statistic cannot be given, the Court of Appeal said that such low quantities could be used to say that the defendant was not excluded from the potential pool even though it could not be said how many other people could be included.

Partial profile

The second case was R v Neil Garmson. Neil Garmson was convicted at Stafford Crown Court on August 20 2007 of kidnap, rape and sexual assault which related to incidents that had occurred in April 2005 and March 2006.

In 2005, the complainant, and her partner had gone by car to a country park, a man tapped on the window then got into the car and demanded that he be driven to Newport. He ordered the partner out of the car, forced the woman to drive to a layby where he raped her. Afterwards she was ordered to drop him off in Newport. Garmson was identified by DNA taken in the 2006 incident.

In March 2006 a man forced himself into the back of a car in which a young woman was sitting. In the front passenger seat was another woman. The driver was a man. The perpetrator threatened them with a knife, demanded money and directed the man where to drive. He then sexually assaulted the woman in the back seat. Both women managed to escape. The driver was ordered to drop the man at a particular location. Garmson was subsequently identified by DNA taken from the lip of the victim. The DNA was merely a few components (known as a partial profile) but the Court of Appeal concluded it was admissible as it was not the only evidence. The prosecution also relied on the similarity of the attacks, fibres from the car which matched a jumper recovered from Garmson’s home, his knowledge of the area and that the drop off was near his mother’s house along with relevant comments he made on arrest .

So, as has been described, the Court of Appeal's judgment allows for low samples of DNA and partial profiles to be admissible in a criminal trial even where the accuracy of the findings depends on expert opinion and/ or there is a possibility of transfer.

Bear in mind that running a part profile (some components but not all) through the database can give a ‘match’ for those components but if only a part profile is recovered from a crime scene it will never be known if the remaining components match the DNA of the suspect.

Mixed sample

The analysis of a crime scene sample, even in relation to DNA, is not necessarily an exact science. The admissibility of a complex mix of DNA from more than one person was considered in R v Ishaq at Leicester Crown Court in January 2010. This was a cold case gang rape from 2000 where there was DNA evidence which could prove presence but some of which was also consistent with transfer. In addition, experts eventually agreed on the day of trial that the possibility of a person other than the defendant being responsible could not be excluded as the computer had given an indication of components present in a partial profile, which had initially been discounted by the Forensic Science Service (FSS). At least one of those components was not in the profile taken from Mr. Ishaq in 2008 and could not be attributed to others implicated or convicted suggesting someone else with some corresponding DNA components could have been responsible and not Mr Ishaq. He was acquitted.


The finding of DNA at a crime scene is a useful tool in proving the identity of the offender particularly in serious sexual offences and murder. In all of the above cases the defendants suggested they had an alibi for the events. The DNA evidence was used to put them at the scene and therefore to prove the commission of the crime. However, as few as four components were used in relation to Garmson, the possibility of transfer was accepted in the Reed case and Mr. Ishaq’s case made it plain that the FSS experts will exclude components which the computer indicates are present and which could exonerate a suspect.



Whilst making it clear there should be other evidence, the Court of Appeal put no limit on how few components can be considered as useful.


Guidance given in R. v Alan James Doheny [1997] 1 Cr. App. R. 369

The following procedures should be adopted where DNA evidence is involved:

• 1. The scientist should adduce the evidence of the DNA comparisons between the crime stain and the defendant's sample together with his calculations of the random occurrence ratio.
• 2. Whenever DNA evidence is to be adduced the Crown should serve on the defence details as to how the calculations have been carried out which are sufficient to enable the defence to scrutinise the basis of the calculations.
• 3. The Forensic Science Service should make available to a defence expert, if requested, the databases upon which the calculations have been based.
• 4. Any issue of expert evidence should be identified and, if possible, resolved before trial. This area should be explored by the court in the pre-trial review.
• 5. In giving evidence the expert will explain to the jury the nature of the matching DNA characteristics between the DNA in the crime stain and the DNA in the defendant's blood sample.
• 6. The expert will, on the basis of empirical statistical data, give the jury the random occurrence ratio— the frequency with which the matching DNA characteristics are likely to be found in the population at large.
• 7. Provided that the expert has the necessary data, it may then be appropriate for him to indicate how many people with the matching characteristics are likely to be found in the United Kingdom or a more limited relevant sub-group, for instance, the Caucasian, sexually active males in the Manchester area.
• 8. It is then for the jury to decide, having regard to all the relevant evidence, whether they are sure that it was the defendant who left the crime stain, or whether it is possible that it was left by someone else with the same matching DNA characteristics.
• 9. The expert should not be asked his opinion on the likelihood that it was the defendant who left the crime stain, nor when giving evidence should he use terminology which may lead the jury to believe that he is expressing such an opinion.
• 10. It is inappropriate for an expert to expound a statistical approach to evaluating the likelihood that the defendant left the crime stain, since unnecessary theory and complexity deflect the jury from their proper task.
• 11. In the summing-up careful directions are required in respect of any issues of expert evidence and guidance should be given to avoid confusion caused by areas of expert evidence where no real issue exists.
• 12. The judge should explain to the jury the relevance of the random occurrence ratio in arriving at their verdict and draw attention to the extraneous evidence which provides the context which gives that ratio its significance, and to that which conflicts with the conclusion that the defendant was responsible for the crime stain.
• 13. In relation to the random occurrence ratio, a direction along the following lines may be appropriate, tailored to the facts of the particular case: “ Members of the jury, if you accept the scientific evidence called by the Crown this indicates that there are probably only four or five white males in the United Kingdom from whom that semen stain could have come. The defendant is one of them. If that is the position, the decision you have to reach, on all the evidence, is whether you are sure that it was the defendant who left that stain or whether it is possible that it was one of that other small group of men who share the same DNA characteristics.”


Much depends on what is in dispute but if the outcome of a trial depends exclusively or almost exclusively on a serious disagreement between distinguished and reputable experts, arguably it will be unwise, and therefore unsafe, to proceed.


Although the Court of Appeal has made it clear that much depends on the other evidence in the case, in relation to mixed, partial or low DNA, one can quickly see that there is a real danger of an incorrect DNA "match".

As to whether a court will allow expert evidence to be given to a jury in relation to any sample recovered and compared to the defendant, this is a matter for judicial discretion. Whether juries will leap to the wrong conclusion based on the admission of low or few DNA components remains to be seen.

Felicity Gerry
Author of The Sexual Offences Handbook





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