Deconstructing DNA

DNA profiling techniques are constantly advancing – here are five tips which should help criminal practitioners to catch up with the latest developments, and to engage more productively with DNA evidence.

Of the many forensic science techniques which lie at the disposal of the criminal justice system, DNA profiling is pre-eminent. Each day the criminal courts encounter expert opinion evidence, in which a DNA ‘match’ purports to demonstrate a connection between an accused, and a particular locus, person, or object. The palette of bio-identification techniques that utilise the enormous discriminatory power of DNA have advanced rapidly, since their introduction thirty years ago[1]. However, during that time, it must be admitted that the criminal justice system has occasionally struggled to keep abreast of a rapidly advancing body of science which relies upon genetic sequencing, Bayesian reasoning, and expressions of probability.

For the legal profession, as well as the bench, this has been a challenging journey. However, the current consensus is that the more difficult challenges (such as contamination, and biased evaluations) have now been resolved, and that forensic DNA – with its unparalleled powers of selectivity – now stands as an ‘infallible gold standard’ of bio-identification. As a result, much DNA evidence is not disputed.

However, whilst the science behind forensic DNA is undoubtedly sound, and issues of contamination have largely been addressed, the content of expert opinion in this field still requires careful scrutiny. Indeed, it may come as a surprise to many that – whilst DNA profiling is a robust process – the interpretation and evaluation of DNA profiles, is a fallible process, based upon experience and expertise.

The purpose of this brief article is not to disclose ways in which practitioners may exploit these fallibilities in order to neutralise DNA evidence. Rather, it is to offer some beneficial guidance; five topics which may help practitioners to engage with scientific evidence in a more meaningful way. In short, when DNA appears to offer all of the answers, we have to learn to ask better questions. Therefore, it is hoped that the points below may provide the starting points for further enquiry, and professional development.

  1. Differentiate ‘source level’ from ‘activity level’ analyses

The reputation of DNA profiling is largely deserved, and frequently bolstered by the instrumental role of DNA in breaking ‘cold cases’. Without the support of forensic DNA profiling, the so-called ‘Golden State killer’, ‘Ramsey Street Rapist’, and ‘Worlds End’ murderer, would never have been brought before the courts. These convictions constitute welcome successes for their respective criminal justice systems. However, it should be noted that the majority of these cases share one notable feature. They all involve offences, in which the criminal activity can readily be inferred from the presence of the recovered DNA sample; the paradigm example is where DNA has been recovered from semen, belonging to an unknown male, found on the body of a murder victim who has been sexually assaulted. In such cases, the ‘activity’ may (to some degree) be inferred from the ‘source’.

However, the really important question for the courts is not ‘Who’s DNA is it?’, but ‘How did it get there?’ (see R v Reed & Garmson [2009] EWCA Crim LR 2698). It is clear that, in the majority of cases, it requires a greater interpretative effort to answer the second question i.e. to infer a particular activity from the recovery of a DNA sample alone. Practitioners should therefore be aware of the significant difference between ‘source’ and ‘activity’ level analyses, prepared to question bare ‘source-level’ analyses, and – as will be shown below – alive to the relative difficulty of addressing questions relating to ‘activity’.

  1. Take a holistic view

From the point of its introduction into the criminal justice system, the Crown Prosecution Service recommended that suspects not be charged on the basis of DNA evidence alone. However, that position has evolved and, in cases such as Tsekiri, R v [2017] EWCA Crim 40 (17 February 2017) the court of Appeal now state that there is no principle which prevents a jury from hearing a case based upon a sole piece of DNA evidence. In light of the above topics this approach may prove problematic. Practitioners would therefore be advised, where possible, to take a more holistic approach to the evidence presented. For example, while the presence of an accused’s DNA on an assault victim, may be indicative of a violent altercation, it may also be the result of innocent contact, or of going to the aid of a victim. In such circumstances, interpretation of the pattern, weight, and position of the DNA may (in conjunction with an analysis of other evidence types such as blood pattern analysis, gunshot residue, or fingermarks) contribute to a more holistic account of the evidence, providing more aid the court.

  1. Understand the problems associated with ‘mixed samples’

At its inception, DNA ‘fingerprinting’ relied on the recovery of DNA samples from visible stains, such as blood, or semen. However, DNA profiling techniques have are now so refined in sensitivity that they routinely pick up additional trace material, from individuals who may have had no physical connection to the crime under investigation. The resulting ‘mixed’ profile often consists of one major contributor and one (or more) minor contributors. This configuration often results in a ‘major/minor’ sample, whose peaks are readily separated (think ‘trees’ and ‘undergrowth’). However, it would be wrong to conclude that a major profile is the result of more recent, or prolonged, contact. Such profiles require careful interpretation, often using validated algorithmic software. Even then, the results may be less than fully conclusive. Therefore, in all cases, practitioners should be prepared to explore the possibility of further individuals being present in a given sample.

  1. Challenge ‘transfer and persistence’

The interpretative challenges highlighted above are not limited to attribution. For whilst DNA profiles may be ‘matched’ to individuals on the DNA database – often with strong probabilities, in the order of one-in-one-billion – the resulting evidence may yet be effectively neutralised through the issue of DNA ‘transfer and persistence’; the study of the transfer of DNA deposits across multiple surfaces, and their longevity. This issue constitutes a major stumbling block for forensic DNA, given that tests are now so sensitive that they routinely pick up invisible traces of epithelial ‘touch’ DNA from multiple individuals. Given that different individuals shed at different rates; that different surfaces retain DNA at different rates; and that DNA may be deposited by secondary, or even tertiary, intermediaries (not to mention the presence of environmental factors), the task of answering activity-related propositions with any degree of certainty may now prove difficult, if not impossible. Legal practitioners should be ready to explore these issues with the reporting scientist. 

  1. Exercise caution around streamlined reports

All of the issues highlighted above assume a greater urgency when evidence is presented in the form of a Streamlined Forensic Report. These reports, introduced by the CPS and MoJ, were intended to achieve efficiencies in both time and expense. However, many lead biologists have voiced misgivings with regard to the scheme. The initial report [MG22(a)] is often compiled by a non-expert police administrator and should be treated with particular caution (indeed, due to the fact that these non-experts are unable to interpret mixed samples, the latter are frequently reported as ‘inconclusive’). Whilst the scheme is intended to facilitate better case management, it may be argued that the requirements of the Criminal Procedure Rules can only be achieved through a more nuanced engagement with the underlying issues; interpretation of which are notably absent from these types of report.

Hopefully the above topics will provide some practical guidance to practitioners, allowing for enhanced effectiveness. However, it could be argued that there is a further impetus for engagement with scientific evidence; a broader responsibility grounded in the solicitor’s ethical duties. For whilst all solicitors are required to reach a given standard of technological competence, the requirement is generally interpreted as relating solely to data storage and communication. Nonetheless, in some jurisdictions (see, for example, the State Bar of Georgia, United States) that requirement has been extended to encompass technological competency when dealing with the types of expert ‘scientific’ evidence routinely encountered in the CJS. Whilst no comparable duty is explicitly stated in England & Wales, this approach is recommended; a modest amount of engagement – with statistics, and technological processes – can deliver great leaps in proficiency. Such proficiency cannot help but translate to better service, to the client, the court, and the ends of justice.


The Author of Deconstructing DNA

Karen Richmond is a legal researcher with the Leverhulme Research Centre for Forensic Science. Her doctoral study focusses on the experience of marketisation in the Forensic Science sector in England and Wales.