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DNA Expertise

Special Reports

by James Clery, DNA Defence

I have personally been working in the forensic sciences at court reporting level since 1997 after completing my Master’s Degree in forensic science at the University of Strathclyde in Glasgow.

I specialise in the biological sciences involving bloodstain pattern analysis, scene of crime reconstruction, hairs and fibres, serology (which is the testing and interpretation of body fluid testing), forensic entomology (which is the study of insects) and of course DNA testing, interpretation of results, including complex mixtures in the ‘standard’ STR techniques as well as mitochondrial and Y-STR’s, and statistical interpretation. My personal forensic speciality is sex based cases since these tend to involve more complex testing’s and interpretations and have less empirical research data upon which to base opinions which means that more reliance on experience and past case involvement is vital. 

Having worked in New York City, Texas, South Africa and the UK in both Crown and defence capacities I have seen significant changes across the board. The roles of prosecution and defence expert witnesses has changed beyond all recognition. The closure of the Government Forensic Science Service and ACPO imposing a tendering system for the purchase of forensic testing has commoditised the industry in who can test a sample the cheapest. Based on financial considerations what is now expected of a Crown expert is different to the days where they had choices over the forensic strategy- what exhibits would be best to test to assist in helping to understand the level of support for a particular allegation. Nowadays it has become accepted that the Police will likely pre-screen the exhibits, either at the crime scene or at their laboratory, and submit only those items that they feel warrant further investigation at the Forensic Science Provider (FSP) laboratory. I don’t like to recall the number of times where I have been involved in trials where a Crown expert has not been given the full picture of the case circumstances and reported on limited information. One such example is where it was alleged the defendant (unnamed as was 16 at the time of the alleged offence), and another, had raped a woman in a park. Both males were said to have worn a condom. Both males agreed there had been some form of sexual consensual contact (that the complainant performed oral sex on them) and that condoms had been produced but no vaginal penetration had taken place. The condoms were duly submitted for forensic testing to the FSP and various DNA results obtained. I was instructed to review the DNA findings and comment on the level of support for the various scenarios put forwards. The DNA findings were found to support all scenarios since the expectation of finding DNA from the complainant on the condoms was equally expected. This was agreed by the Crown expert, that neither scenario could be favoured. However, one crucial aspect I picked up on was that she had stated in her taped interview that she had returned home afterwards and changed her sanitary towel. This was not picked up on by the Police, the taped interview had not been provided to the Crown expert and the condoms were not tested for the possible presence of blood. This was a crucial loss of data, a simple cheap test that is done hundreds of times a day in a FSP laboratory could have made all the difference to the case and at trial the Crown expert stated she was not asked to look for blood and agreed this was a “missed opportunity”. The forensic strategy in my opinion should routinely include testing for the possible presence of blood where any form of force is alleged since bleeding may occur even if the complainant is not on her case and which exhibits should be tested and what may be crucial. 

In the case of the Metropolitan Police the in-house laboratory will do the examination, and take samples and then submit the material for DNA testing to a forensic provider who will report on the DNA results and then the Metropolitan Police expert will report on the summary DNA findings (received in tabulated form not the raw data) in the context of the case. Again, at trial when the court is trying to understand which expert to call to give evidence, if it be necessary, and how both elements of the DNA results and this being placed in the context of the case specifics, can cause unnecessary confusion and delays. 

This of course does not mean the defence expert has remained immune to commercial influences. The closure of the Government Forensic Science Service has meant that many experts with a history of working for the Crown, and the nature of work this involves, have been forced to migrate into the defence arena with no experience of working for the defence. This is due to a lack of jobs in the FSP’s and essentially pricing themselves out of the market as it is cheaper to hire less experienced staff and train them in house. This has meant that experts, whilst technically qualified, may not possess the relevant skillsets required for working for the defence, be it in court presentation or in identifying the relevant questions to be asked in a case which has possibly not been as thoroughly explored as one would hope. 

More recently there has been a worrying trend in experts coming into the industry who are not fully qualified to act as an expert witness. Recently, after giving an invited guest lecture to the Society of Biology on assessment of data and understanding of the crime scene in forensic biology and forensic entomology at the University of Reading the valid point was made that cases are being put in jeopardy by the increasing level of unqualified experts providing reports and opinions at trial. This tends to be especially so in specialist areas such as forensic palynology that deals with the analysis of pollen. This is a combination of pound signs in their eyes combined with the Legal Aid Authority (LAA) pushing for the cheapest quote, and not for experience, which encourages inexperienced people to enter the arena and not be concerned with a relatively low fee for doing so. In my experience working as a defence expert witness over the years I have had countless occasions where I have been instructed on a matter which had already been dealt with by another expert, the testing and report of which had fallen short of the required standard. This is a deeply worrying situation for the courts to be in since the weight placed on expert evidence before the courts can have a big impact if the Crown’s case relies heavily on association through DNA testing alone, so called ‘single plank’ evidence. Also, due to its very nature the Court does not necessarily understand if an expert report is robust or valid as the expert is there to advise and assist the Jury. To ensure that all is done to qualify the Crown’s forensic evidence so that what goes before the Jury equally considers the defence positon is vital to my mind, and a position in which I am sure the courts would want in every case.

The change in the role of the defence expert can be illustrated by the amount of, what I would call, primary work being pushed onto the defence expert. So much so that it can be the case where I will have examined more exhibits than the Crown expert in a particular case. In discussing the issues with my counterpart it appears to be more due to a lack of funding to do the work rather than a lack of desire in performing the work. One such case, as way of example, occurred in R-v- John Taylor. The case was one of a single count of rape and the facts of the case up to the point the two parties met on the 22nd February 2012 was largely undisputed by the Crown and Defence. The two had gone on a date at a Chinese restaurant and they left around 7.30pm and the complainant agreed to go back to his home address. The flat consisted of a single bedroom, bathroom, small lobby which leads into a small lounge with an open plan kitchen. There is no dispute that on arrival both sat on the sofa in the lounge and a bottle of wine was opened. At this stage the version of events differed considerably. The complainant states that Mr Taylor had  pushed himself onto her whilst on the couch, removing her clothing. She states he then penetrated her vagina with his penis and had sex without her consent. She did not know if he ejaculated. The key point was that intercourse took place on the sofa. Mr Taylor’s account was that there was kissing on the sofa, and he removed her top with her consent and she agreed to go into the bedroom. Whilst there she removed her remaining clothing and he performed oral sex on her. He states that they both agreed to sexual intercourse and he put on a condom and began coitus. After a short while she complained she was too hot and later again said she had to go home to feed her cat and coitus stopped. Mr Taylor was arrested 2 days after the alleged rape. The allegation was that the rape occurred on the sofa.

At this stage Mr Taylor insisted the sofa covers be collected as well as the bedding, sheets, pillow cases etc. He also produced the condom he had used on the night in question. The Police decided not to test the bedding or the sofa covers. This was despite Mr Taylor stating he would pay for the forensic testing personally if needs be. It was at this point I was instructed on behalf of the defence to test the bedding and sofa covers for body fluids and DNA to determine who may have had contact with them. The sofa did not contain any visible staining or body fluids after serological screening. Minitapes, small strips of sterile adhesive plastic, were used to collect any cellular DNA. Poor quality DNA profiles were obtained which did not particularly assist with possible contact of the complainant with the sofa. It is important to note that the absence of DNA on an item does not mean that someone must not, or could not, have had contact with it. Remember this was the first time she had been the flat and an absence of her DNA was not entirely surprising given his version of events, but less so considering her account since in her version she was naked and raped on the couch, in his version she only had her top off. The bedding was a different story. One of the pillow cases contained a visible saliva stain which matched the complainant. The sheets did not contain any visible body fluids but minitapes taken from the head and foot end (the Police failed to designate which side was the ‘head’ and ‘foot’ end so both ends were sampled) yielded a strong DNA match with the complainant. The same was true of the duvet cover. These findings strongly supported the Hd over the Hp. The complainant stated she had not been in the bedroom. The Crown FSP reviewed all of the examinations and DNA results and agreed all test data. At trial Prosecuting Counsel dropped the case against Mr Taylor based on the overwhelming forensic evidence. For completeness the condom was also tested but this did not assist given the Hp and Hd scenarios. This case is an example of the strength and value of forensic testing in assisting a Police investigation and the thoroughness that should be brought to bear with this valuable tool.

In a broader context due to my role working for the defence, I have listed the types of questions I would pose to any defence Counsel in a DNA case. Of course not all of these questions will apply equally in every case because the points in issue will necessarily vary.

General points:

• Has the continuity of an exhibit and any DNA samples created from it been maintained from the point of collection through to production of the DNA results? This will involve addressing the statements from the police collecting the evidence and whilst at the FSP to show traceability in the laboratory. 

• Has the exhibit been kept separate from other exhibits that could potentially contaminate it? This applies to the crime scene before collection takes place- have any other surfaces, objects or people been touched prior to collecting?- and not just in the laboratory where the exhibits from different people must be examined in a separate location and any subsequent DNA samples kept separate. 

• Prior to submission to the FSP have the Police examined the exhibit e.g. in a fumigation or drying cabinet and was this done in a DNA clean environment? Was this done in the same laboratory as other exhibits?

• Given the scenarios put forward, and questions posed, have suitable stains been targeted for examination? I very often see drug wraps being tested in their entirety and it is therefore then not possible to determine if the resulting DNA obtained is linked to the inside or outside surfaces which can have a big impact on what can be said as to activity.

• Should any other people, e.g. shared house or relatives be considered?

• Has the DNA profile been screened on the DNA database? A ‘spec search’ can be performed even if the DNA result does not meet permanent loading criteria. If so what matches were obtained and have these possible matches been checked?

• Was the examination done in laboratory conditions under a UKAS 17025 quality system?

Partial/ incomplete DNA results

• How many components are present that actually match the defendant? A partial profile means anything from 1 to 19 matching DNA components under the SGMPlus system, (where 20 components are expected for a full match).

• Is it possible that the DNA profile is in fact comprised of DNA components from more than one person?

• Are there any unconfirmed DNA components present? Whilst they do not meet reporting guidelines they can indicate the presence of DNA from another/ more person(s). 

• Do these unconfirmed DNA components match the defendant?

• Does the profile show evidence of any degradation? Whilst it is not possible to age DNA a degraded sample can assist with understanding if the DNA is consistent with being aged.

Mixture profiles 

•How many contributors are present? This is reported as “at least x number people have contributed”. It is accepted that the greater number of contributors to a mixture the greater the chance that someone could match purely by chance. Again the issue of statistical or verbal scale weight is important i.e. how likely is it that a person had contributed than they have not?

• Is there support for more than the designated contributors based on peak heights and areas (and not simply due to a count of DNA components alone)?

• Is it possible to designate a single major contributor? Is it possible that an apparent single source major or prominent contributor is actually an assemblage of DNA from multiple people?

• Has the correct statistic been applied- Match Probability (MP) versus Likelihood Ratio (LR)?

• Have assumptions been made based on the mixture and case circumstances- are they fair and do they assist the court? Have relatives been considered?


• How frequent are the DNA components found in the population? This is the basis for the statistical calculation.

• Is it possible to separate out a single contributor from another if a mixture is present? Is a Match Probability (MP) calculation able to be supported?

•Is it possible to perform a Likelihood Ratio (LR) on a mixed sample with only two contributors? An LR can only be used where there is DNA from two people. It is possible to do statistics on complex mixtures from more than two people but this is limited.

• In the absence of an MP or LR is it possible to determine if a more prominent contributor can be reported under Court of Appeal ruling of R-v-Dlugosz where in the absence of a statistical assessment a verbal weight as to a possible contribution is able to be stated?

• Ultimately is the result suitable for statistical interpretation? Drop in, where apparent DNA components are present which are not true DNA components, or drop out, where DNA components present in a person’s DNA profile are not present but would be ‘expected’ if they had contributed should be considered. 

• If there is additional data present in a DNA profile been considered and would further work be suitable to determine possible other contributors? I am often asked to repeat Crown DNA testing to determine if DNA components seen at a lower level could be improved. 


What level of interpretation has been made?

There are two main levels in the Hierarchy of Propositions- source and activity- that an expert witness can address. The former addresses who the DNA has potentially come from (and biological source at sub source level) and the latter how it could have got there. Often the Crown expert is only asked to consider the source level activity which may be suitable for the case, but may leave important questions unanswered such as the issue of secondary DNA transfer where DNA can be innocently transferred by another person.

• What biological source has the DNA come from? Has body fluid testing been performed? Has the biological source been confirmed, rather than just via a presumptive test and presumption of source e.g. saliva on a balaclava or drinks can?

• Is it possible to say, based on the amount of DNA is it more likely that the DNA was from skin cells or a body fluid? For any given amount of DNA it might be a greater amount of skin cells or a lesser amount of a body fluid. This can obviously assist with understanding how the DNA was transferred.

• Does the quality of the sample provide any support for a recent or older deposition?

• How was the DNA deposited?

• Is there support for transfer via another person or object? It has been shown empirically that DNA can be transferred to an object without a person ever having been in contact with it.

• Is there further work that could have been done to assist in interpretation? Is this due to time and cost constraints imposed on the Crown expert? What does the Police submission form state on what is required to be examined and questions explored?

• Have alternative scenarios been considered? We can use Bayes Theorem to interpret the findings, the balance between the Crown hypothesis (Hp) and Defence hypothesis (Hd). Are the findings supportive of one or the other or both, or neither?