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

Abstract

Considerations 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 60s. 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.