Empirical data on the transfer and persistence of trace DNA are crucial to the evaluation of forensic DNA evidence. This evaluation can be complicated by the occurrence of indirect DNA transfer; the possibility of which is well established, but research into such transfer is often focussed on unrealistic situations, e.g. handling of DNA-free items after participants have shaken hands for 1-2min. To simulate more realistic scenarios, this study investigated the deposition and persistence of both directly- and indirectly-transferred DNA on knives that had been artificially set up as 'regularly-used'. Each knife was handled in a prescribed manner by a specific participant over two consecutive days to simulate regular use. Each participant then shook hands for 10s with a fellow volunteer and immediately stabbed one of their knives into a foam block repeatedly for 60s. DNA was recovered by mini-taping from triplicate sets of knife handles from four pairings of volunteers after regular use, and at one hour, one day and one week after the handshaking and stabbing events. Total amounts of DNA recovered from the knives, regularly used by a single person, varied among individuals; one volunteer consistently deposited significantly greater amounts than the others, whilst another volunteer did not always leave complete profiles. DNA attributed to the regular user persisted for at least a week, declining with increasing time between DNA deposition and recovery. Non-donor DNA was co-deposited at <5% of the profiles recovered, except for one volunteer, who consistently left DNA from their romantic partner on their knives at ∼25% and ∼11% of the profiles before and after the handshaking and stabbing events, respectively. In three pairings of volunteers, after the handshaking and stabbing events, alleles that could be attributed to the respective handshakers' profiles were detected as partial minor profiles, equating to ∼10% of the profiles recovered. For the fourth pairing of volunteers, only complete single-source DNA profiles matching the regular user's profile were recovered. However, it is important to note that, when indirectly-transferred handshaker DNA was detected, it declined with increasing time between DNA deposition and recovery. These data provide an initial insight into the detection and persistence of directly- and indirectly-transferred DNA that extend the data already available on forensic DNA transfer. The results herein suggest that the sooner an item is sampled after an offence has occurred, the greater the chance of recovering indirectly-transferred DNA, which has implications for forensic reconstructions.
A B S T R A C TConsiderations concerning how DNA recovered from a crime scene was deposited are of increasing significance to forensic casework. While the possibility of indirect DNA transfer is well established, research into such transfer is limited and focused mainly on the handling of DNA-free items. This study investigated whether secondarily-transferred DNA can be detected on regularly-used items, and if so, for how long might it persist. Volunteers each used a set of knives regularly over a period of two days, after which, each of these 'regular users' shook hands with another person ('handshaker') and then immediately, without touching anything else, repeatedly stabbed one of their own regularly-used knives into foam for 60 s. DNA was recovered from the knife handles using mini-tapes approximately one hour, one day, and one week after the stabbings. In three of the four pairings of volunteers, complete and partial DNA profiles matching those of the regular user and handshaker respectively, at ratios of $10:1, were recovered from the knives within one hour. Alleles attributed to the handshaker were still detected after one week, but were significantly reduced in number and peak height for two of the three pairings. Unknown alleles were also recovered from the knives, suggesting other indirect DNA transfer events. These included repeated detection of alleles attributed to the DNA profile of a volunteer's partner. For the fourth pairing, only complete single-source DNA profiles matching the regular user's profile were recovered. This study demonstrates that, on regularly-used items, secondarily-transferred DNA can be detected and can persist for at least a week; this has implications for forensic reconstructions.
When evaluating trace DNA recovered from evidential items in forensic casework, it is crucial to consider how the DNA got there, and such evaluative interpretations should ideally be informed by published experimental data. A key activity-level question is whether the DNA obtained comes from the regular user, the last user (ostensibly the user at the time of the crime) or from indirect transfer events. The aim of this experiment was to provide data to contribute to answering this question, particularly when considering opportunistic crimes, in which an offender might grab the nearest item at hand required for their purpose, e.g. a weapon or tool, and therefore only handle it very briefly. Volunteers ('regular users') used knives in a prescribed manner to simulate regular use (one user per knife); DNA recovery by mini-tapes from these knives gave ~1-10 ng DNA, with <16% non-donor DNA from indirect transfer events. Different volunteers ('second users') then stabbed replicate sets of regularly-used knives into a foam block for either 2, 30 or 60 sec (on different occasions), with each timeframe in triplicate, and DNA was recovered from the knife handles using mini-tapes. For knives regularly-used by three of the four volunteers, the ratios of regular user to second user DNA were approximately 4:1, 2:1 and 1:1 for durations of use by the second user of 2, 30 and 60 sec, respectively. Analysis of the respective quantities of DNA showed that this trend resulted from a 2 decrease in regular user DNA via transfer to the second user's hands, rather than an increase in DNA deposition from the second user. However, for knives regularly-used by the fourth volunteer, DNA from the regular user remained at significantly higher quantities than DNA from the second user and unknown sources, irrespective of duration of use by the second user. Furthermore, one volunteer deposited a similar amount of DNA through regular use as the amount of indirectly-transferred unknown DNA deposited by another volunteer's hands. These observations indicate that caution should be taken when relying solely on absolute quantities of DNA to inform evaluative interpretations, and other parameters, such as profile quality and relative contributions to mixed profiles, should also be taken into account. To better assist activity level assessments, more extensive studies of this manner should be conducted to obtain probability distributions of different types of profiles resulting from this kind of activity.
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