Trace DNA evidence dynamics: An investigation into the deposition and persistence of directly- and indirectly-transferred DNA on regularly-used knives

Abstract

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.

DNA detection of a temporary and original user of an office space

“…Profile interpretation showed that whilst in most of samples the office owner was the only or the major/majority contributor to the samples, the temporary occupier was detected as a major/majority contributor on several items in both laboratories, despite the relatively shorter duration of use ( Table 2). Detection of the last person temporarily touching or using an item or space, in differing degrees relative to the previous handler/owner, concurs with findings of previous studies [8][9][10][11][12][13][14]. Several of the samples where the temporary occupier was a major/majority contributor were from relatively small surface areas, suggesting that with prolonged use, the DNA of the owner may be more readily removed and replaced with the DNA of the temporary occupier.…”

“…While the DNA proportions of the last user and the owner of the item (if the last user and the owner are two different people) has been investigated for commonly used items [8][9][10][11][12][13][14], there are a number of other scenarios that require further investigation. A common scenario where this may be relevant involves a person (or a few people) temporarily occupying a space known to be predominantly occupied by a single person (or a few people), in the absence of the original occupant (s) e.g.…”

DNA transfer in forensic science: A review

“…DNA from person A residing on the hand of person B is then transferred to object 1 when contacted by person B with bare hands. DNA from person A is detected on object 1 [51][52][53][54]. -DNA residing on a specific area of a collected item (object 1), that has been packaged along with a second item (object 2) in the same packaging, is transferred to another area of object 1, to another item (object 2), or to the inside of the packaging [55].…”

A review of trace “Touch DNA” deposits: Variability factors and an exploration of cellular composition

“…a sunlit windowsill) has been shown to lower this persistence significantly, with recovered DNA displaying clear degradation in typing results after 2 weeks [51]. Total DNA recovered from knives immediately after use compared to a week later, was reported to be significantly higher in only 50% of subjects [53]. There is limited systematic investigation of the time course of DNA degradation, particularly in either casework-like touch deposit samples or over significant amounts of time.…”