Review articleOn DNA transfer: The lack and difficulty of systematic research and how to do it better
Introduction
In 1910 the French criminologist Edmond Locard formulated what today is known as Locard’s exchange principle and which may be paraphrased as “every contact leaves a trace” [1]. Applied to traces in forensic biology this basic principle points to the fact that biological agents will leave small amounts of DNA containing material by any contact with objects or other biological agents. As biological agents may be individualized by simultaneous analysis of a sufficient number of DNA polymorphisms, the presence of DNA containing material from a specific individual may be proven beyond rational doubt given the power of contemporary forensic DNA analysis.
However, this DNA based individualization does not inform on the mode of contact causing the biological material to be transferred to from where a sample was collected, i.e. it does not allow for contextualization of the trace material and hence to reconstruct how it had been deposited. This context, though, may be critical to assess the actual involvement of an individual in the crime in question.
Consequently, since DNA from touched items and surfaces (“touch DNA”1) can successfully and reliably be analyzed [3,4] the question as to how a particular DNA containing sample came to be from where it was recovered is of increasing forensic interest and several studies have shown that trace amounts of DNA do not necessarily have to be a result of direct skin contact but can also be transferred to an object via indirect, secondary or even higher order transfer scenarios [3,5,6]. In fact, the number of samples from “swabbed surfaces” intending to recover invisible touch DNA is soaring in most forensic routine laboratories [[7], [8], [9]] and expert witnesses in court are increasingly challenged to assess for instance whether an incriminatory DNA sample matching to a suspect could have been transferred to the crime scene in an innocent manner [10] and to guess at the probability of such an occurrence. Given that current forensic DNA analytical methods are highly sensitive and capable of detecting DNA in samples containing only tens of or even single cells [11] the probability of such contaminations by transfer is far from zero. However, it is salient from the tenor of available data that neither the quantity of DNA from a given sample nor a resulting profile’s quality allow for a reliable inference of the mode or order of transfer that conveyed the material in question to its final resting place [2].
Notably, to the present date (October 2018), there is extensive literature including a couple of reviews [[2], [3], [4],[12], [13], [14]] on primary, secondary, tertiary and even higher order DNA transfer, its possibility, plausibility, dependency on an array of variables and factors and vast numbers of permutations thereof (see Appendix A). A general and very rough summary of the data amassed so far would be: DNA transfer of even higher order is possible and while its probability cannot reliably be calculated it is inversely correlated to the number of transfers. However, from our point of view there is a lack of systematic data on DNA transfer with existing research widely varying in quality and relevance. Also, there is an uneven distribution of focus on the various aspects of DNA transfer with certain aspects being investigated very thoroughly (e.g. shedder status, see Section 2) – in some cases even redundantly (e.g. surface characteristics, see Section 4) - and other aspects largely neglected (e.g. casework-relevant persistence scenarios or assessment of the relative weight of different impacting variables) (see Section 4 “A critique of DNA transfer research”).
The basis of this review was a self-devised database of available scientific reports concerning DNA transfer in which we included all essential studies starting in 1993 (see Appendix A) to the best of our knowledge. Our aim was, starting from a comprehensive survey of the status quo and appreciating its increasing importance, to in the first part raise consciousness toward the underestimated and insufficiently accounted for complexity of DNA transfer and thus appendant research of forensic scientists serving as expert witnesses in court but also acting in the role of a journal referee to point them to areas of criticism when reviewing a manuscript on DNA transfer. In the second part, we present propositions how to systematize and integrate future research efforts concerning DNA transfer.
Section snippets
Variables influencing DNA-transfer
Previous research on DNA transfer has demonstrated that the transfer, persistence and recovery of trace DNA on/from persons and/or objects is a highly complex and multifactorial process and thus a multitude of parameters has to be taken into consideration when evaluating the probability of the constitution of the trace in question resulting from a suggested scenario.
In the following we give a brief introduction of variables that have plausibly been demonstrated to influence transfer and
Variables influencing the persistence of DNA between and after transfer steps
Considering a series of DNA transfer events, there might be relevant delays between individual transfer steps. Furthermore, surfaces of interest in certain contexts of criminal activities are rarely sampled directly after the biological material has been deposited. For every delay, the persistence of DNA on the surface of interest needs to be evaluated.
A critique of DNA transfer research – what makes understanding DNA transfer so hard
A large body of research contributing to our knowledge on the mechanisms and intricacies of DNA transfer notwithstanding, our capacities of evaluating and calculating the likelihood of specific DNA transfer scenarios are still very limited [2]. Goray et al. [101] performed experiments mimicking transfer scenarios conceivably suggestive of a casework context and applied previously published literature data to predict DNA transfer rates for these scenarios. They found that while some general
A repository of what is known
In a typical scenario a forensic scientist in the role of an expert witness testifying in court is presented by the defense with a theory for an arbitrarily complicated alternative explanation intended to exonerate their client of how a trace in question came to be in the location from where it had been recovered and will often be challenged not only to assess whether this explanation is possible in the first place but also to quantify its probability2
How to do it better – A proposal
Considering the difficulties presented in Section 4.1–4.3 it becomes evident that more thoroughly designed, highly controlled as well as more realistic studies are needed to better understand the intricacies of DNA transfer variables including their relative impact and their interdependencies. Section 4.4 in combination with Section 3 further demonstrate the need for measures to reduce non-transferential variability when studying and evaluating DNA transfer scenarios.
Based on these observations
Conclusion
Testifying in court considering case scenarios that include possible instances of DNA transfer and thus research on DNA transfer, its conditions, limits and probabilities is of increasing importance in forensic science. Many different variables influence how much DNA is transferred and if and how it can be detected and analyzed.
The available literature on DNA transfer lacks quality and systemization and does not, in our view, measure up to the need to provide for an in-depth understanding of
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