The effect of pressure on DNA deposition by touch

Abstract

Casework exhibits are routinely examined for DNA that might have been deposited by touch, although the success of downstream profiling can vary. Many variables affect DNA deposition by touch, such as 'shedder status', surface type, and nature of contact. This may include pressure, which has been shown to increase the transfer of DNA between two surfaces, although whether pressure can impact DNA deposition directly from skin has yet to be examined. Therefore, this study uses a novel method to investigate whether pressure can affect the amount and quality of DNA directly deposited by touch. With the fingertips of one hand, volunteers exerted pressure for one minute onto a DNA-free polycarbonate board placed on top of a balance; all five fingermarks were then swabbed and combined as one sample for DNA extraction, quantification and profiling. For each hand, the area of the combined fingertips was used to determine the weight value to which to push the balance to give pressures of 4, 21 or 37 kPa. Volunteers used both their right and left hands at each pressure in a randomised order on each day of three non-consecutive days. Increasing the pressure between skin and surface significantly increased the amount of DNA deposited, which resulted in the detection of more alleles, from both the donor and unknown sources. No significant differences were observed in the amounts of DNA deposited between hands and among different days for each volunteer. DNA amounts significantly varied between individuals at 21 and 37 kPa, but not at 4 kPa. These findings provide insights into the impact of pressure on touch DNA deposition, and suggest that pressure is a key variable for crime scene investigators and forensic examiners to consider when prioritising items/surfaces that are likely to produce successful touch DNA results during a criminal investigation.

Opportunistic crimes: Evaluation of DNA from regularly-used knives after a brief use by a different person

“…Volunteer C deposited significantly less DNA than the other volunteers, also as previously observed when this volunteer participated as volunteer X in a prior study [17]. These observations provide further support for the concept of 'shedder status', which was first proposed by Lowe et al in 2002 [19] and is gaining wider acceptance in recent years [9,17,[20][21][22][23]. DNA from unknown sources was also recovered, which had been indirectly-transferred to the knife handles via the hands of the participants.…”

DNA transfer in forensic science: A review

“…As indicated in section 2.1, Goray et al [37,38] demonstrated that in most situations, when two objects come into contact with each other, more DNA tends to be transferred when pressure with friction is applied compared to passive contact or pressure contact without friction. Tobias et al [121] confirmed that when surfaces were contacted by fingertips, increasing the pressure significantly increased the amount of DNA deposited, which resulted in the detection of more alleles from both the donor and unknown sources.…”

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

“…One category of studies investigate DNA on items following specific controlled actions, such as knife handling or hand-washing [56,81]; increased pressure of finger contact, for example, has been shown to result in higher levels of DNA recovery [82]. A second genre of studies analyse a series of more "realistic" scenarios with less control but potentially more immediate relevance to casework [51,83,84,85].…”