Evaluation of multiple transfer of DNA using mock case scenarios

doi:10.1016/j.legalmed.2011.09.006

Legal Medicine

Volume 14, Issue 1, January 2012, Pages 40-46

https://doi.org/10.1016/j.legalmed.2011.09.006 Get rights and content

Abstract

DNA transfer and its possible role in explaining the presence of a biological sample at a crime scene is becoming more prevalent in criminal investigations and related court proceedings. To assist understanding of DNA transfer and assess the extent to which we can utilise already available information regarding transfer of DNA we compare transfer rates determined from mock multi-step transfer scenarios with transfer rates predicted by the application of currently available transfer rate data. The transfer results obtained from the scenarios tested were, in some instances, different (both lower and higher rates) from those predicted. These discrepancies are most likely the result of the impact of as yet untested variables. These may include the variations in substrate type, transfer area size and environmental factors such as temperature and humidity among others. Whilst detailed re-enactments of proposed transfer scenarios, that take into account the many possibly relevant aspects affecting transfer are desirable, to provide an accurate likelihood estimate, these are not always possible. The application of detailed transfer rate tables that include data on the many factors affecting transfer could provide a useful substitute for evaluating the likelihood of specific transfer events. The value and accuracy derived from applying such tables will improve as more research in this area is conducted and the tables expanded and refined.

Introduction

While the concept of secondary transfer occurring during illegal activity has been part of criminal investigation since the early 20th century [1], [2], secondary DNA transfer has only recently become an issue in criminal proceedings [3], [4], [5]. The likelihood of such transfer and the factors affecting it remain largely unknown, leaving forensic practitioners vulnerable to the scrutiny of lawyers on this issue. Recent publications [6], [7] have presented the first, albeit simplistic, methodology for estimation of the percentage of blood, saliva and skin DNA transferred under a specific combination of variables, including type of substrate (cotton, plastic and wool), type of contact (passive, pressure and friction) and sample moisture content (wet and dry).

The value of the data obtained by the above methodology is now tested by conducting three experiments, the design of which was suggested by court cases, in which secondary and further transfer events were proposed as an explanation of the accused’s DNA being present at the crime scene. It should be emphasised that the aim in these experiments was not to replicate aspects of the case scenarios as presented in court, but instead take certain components and variables relevant to the issue of transfer in the particular case and investigate them under laboratory conditions. The results obtained from these experiments are compared with those from the blood and skin transfer data published previously [6], [7]. In this way, the value of these published data in estimating the likelihood of the proposed transfer events can be evaluated. Further, similarities between observed and expected transfer rates would strengthen the application of the latter. Additionally, such comparison between the expected and observed results may reveal the existence of other, unknown, variables that can have a significant influence on transfer that require further investigation. The research findings presented here are by no means exhaustive of the issue, but may offer some insight into the complex nature and behaviour of trace DNA under specific and controlled conditions.

Section snippets

Proposed events

A woman was found strangled and partially burned in her house approximately a day after the offence took place. A tape-lift of the lapel of the victim’s pyjamas provided a DNA profile of her ex-partner who claimed he had not been in her house or in contact with the victim for several months. The ex-partner was charged with murder based on DNA as well as other circumstantial evidence.

At trial, defence proposed secondary DNA transfer scenarios to explain the presence of the accused’s DNA on the

DNA quantities

All but one tested surface contained detectable DNA quantities, ranging from 0.009 to 9.2 ng, and were derived from several sources, including perpetrator (donor), victim (recipient) as well as unknown (Table 1). In all but 1 of 20 coats examined, DNA from the original source (donor) was observed (after contact with the vector (singlet or toy)). Total average DNA quantities deposited on the singlets (2.27 and 5.1 ng for immediate and delayed (24 h) transfer, respectively) were greater than those

Case 1

Observed transfer rates were generally 2–4 times greater than expected for the case 1 scenario. When each of the five replicates was looked at separately, however, several had transfer percentages similar to the expected values. It is possible that if we increase the number of replicates a better measure of variation will be obtained. Additionally, the relative areas of contact on both toys/singlets and the coats, as well as the type of the experimental material used (plastic (toy) and fabric

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Since the introduction of DNA typing, the analysis of DNA has become a daily occurrence in forensic laboratories worldwide. Over the past few years, the technology to analyze DNA has become sophisticated and expanded such that touch DNA (or contact trace DNA) can be recovered from a person or object “touched” by another person or object. Such direct and/or indirect transfer of biological evidence or DNA has been demonstrated in numerous instances that have led to convictions of those committing the crimes and to those where the deposition of an individual's DNA is in question. Studies have shown that the amount of DNA transferred from an individual or an inanimate object depends on, but is not limited to, several factors such as surface substrates and the shedder status of the individual. More recently, studies have shown that biological material such as semen can be transferred to clothing during laundering. Such situations of DNA transfer can lead to misinterpretations or questions about the deposition of the DNA. These situations of “misplaced” DNA are discussed in this chapter together with the significance of these findings.
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Citation Excerpt :
Many studies demonstrate the propensity for DNA to be directly transferred to a surface [3–6] and then further transferred indirectly, through subsequent contacts with other substrates [7–12]. In previous research, primary, secondary, tertiary and quaternary transfer has been shown to occur within a variety of scenarios involving numerous substrates [3,4,7,9,10,12–14]. It is also known that DNA can persist on surfaces for extended durations, as demonstrated by the detection of DNA which has persisted on an item for several years in numerous cold cases [15–17].
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Current forensic DNA profiling kits and techniques enable the detection of trace amounts of DNA. With advancements in kit sensitivity, there is an increased probability of detecting DNA from contamination. Research into DNA transfer within operational forensic laboratories provides insight into the possible mechanisms that may lead to exhibit contamination. To gain a greater understanding of the potential for evidence bags to act as DNA transfer vectors, the level of DNA accumulating on the exterior of evidence bags during the exhibit examination process was investigated. The exterior of 60 evidence bags were tapelifted before and after the examination of the exhibit inside of the bag resulting in 120 DNA profiles. These DNA profiles were compared to DNA profiles of staff working within the building and samples taken from the exhibit inside the bag. Common DNA profile contributors from each sample were also identified through STRmix™ mixture to mixture analysis. The average DNA quantity and number of profile contributors was higher in samples taken from the bag before exhibit examination than after examination. Fifty six percent of all samples taken identified a match between DNA recovered from the evidence bag and at least one staff member. On 11 bags, a common contributor was identified between the exhibit in the bag and the exhibit package post-examination. In one instance a DNA profile, matching that of a donor, on the exhibit bag before examination was also detected on a sample taken from the exhibit, raising the possibility of outer bag-to-exhibit DNA contamination. This study demonstrates that operational forensic laboratories must consider exhibit packages as a potential source of DNA contamination and evaluate their exhibit handling and storage procedures accordingly.
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Brief CommunicationEvaluation of multiple transfer of DNA using mock case scenarios

Abstract

Introduction

Section snippets

Proposed events

DNA quantities

Case 1

Forensic Sci Int Genet

Leg Med

Leg Med

Int Congress Ser

Forensic Sci Int Genet Suppl Ser

Forensic Sci Int Genet Suppl Ser

Forensic Sci Int Genet

Forensic Sci Int

Brief Communication
Evaluation of multiple transfer of DNA using mock case scenarios