The deposition and persistence of indirectly-transferred DNA on regularly-used knives

Abstract

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.

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

“…Cleaned knives were then handled in a prescribed manner across two days and three regularly-used knives from each of the four volunteers were mini-taped to examine the quantities of DNA and the nature of the DNA profiles initially deposited, prior to the handshaking and stabbing stage of the experiment. As reported previously, the average total quantities of DNA recovered from these knife handles were 3.4 ± 0.5, 0.9 ± 0.8, 1.2 ± 0.5 and 10.4 ± 3.7 ng for volunteers W, X, Y and Z, respectively [21].…”

“…However, it was surprising that no unknown non-donor DNA was observed, given that unknown alleles had been detected on the regularly-used knives by volunteer Z, prior to the handshaking and stabbing events ( Table 1). With regards to the non-donor DNA in the other volunteer pairings, as reported previously, DNA that could be attributed to the respective handshakers' profiles were detected at ratios to regular user DNA of approximately 1:10, 1:7 and 1:11 for the pairings of volunteers W:Y, X:Z and Y:W, respectively [21], equating to approximately 10% of the total profiles for all three pairings (Fig. 6).…”

“…It was also shown that the indirectlytransferred DNA could persist for at least a week and that DNA from other noncontrolled indirect DNA transfer events was detected. This paper extends these findings and the aims of Meakin et al [21] by investigating the detection and persistence of DNA from the regular user on the knives and by providing a more thorough analysis of the indirectly-transferred DNA.…”

“…It is therefore important to examine whether DNA indirectly-transferred via a handshake can still be detected on items when the contact time of the handshake is much shorter, such as just 10 seconds, and when the items have an initial background layer of DNA present. Using knives that were artificially set up as 'regularly-used', an initial study by Meakin et al [21] showed that indirectly-transferred DNA from a 10 second handshake could indeed still be detected. It was also shown that the indirectlytransferred DNA could persist for at least a week and that DNA from other noncontrolled indirect DNA transfer events was detected.…”

Cale CM, Earll ME, Latham KE, Bush GL. Could Secondary DNA Transfer Falsely Place Someone at the Scene of a Crime? J Forensic Sci 2016;61(1):196–203

“…Further, Cale et al (1) posit that the transfer of DNA through an intermediary has not been systematically evaluated with current technology. Studies have provided empirical evidence of secondary (and further, including tertiary and quaternary) transfer (9,10,16,27), have investigated variables that may affect the transfer and subsequent detection of touch DNA (7,8,13,18,20,(22)(23)(24)28,29), have provided casework relevant examples and simulations to model the possible effect of transfer on conclusions (9)(10)(11)14,16,17,21,(30)(31)(32), and have provided statistical models to evaluate the possibility of transfer (33)(34)(35). Such studies have been conducted with various autosomal STR methods employed in forensic biology, including the current sensitive technologies, and, more recently, with alternate methods such as mRNA profiling (18,21).…”