The complexities of DNA transfer during a social setting
Introduction
In recent years the forensic community has seen a number of significant improvements in the field of forensic DNA. One of these is the ever decreasing quantities of target DNA that can now be detected and successfully amplified to generate profiles [1]. Often, there is no dispute to the identity of the individual from whom the DNA in question has originated, yet the mode by which the DNA got to where it was collected from is frequently questioned. Scenarios incorporating multiple DNA transfer steps, rather than direct transfer, are increasingly being raised in court as potential means for the presence of the defendants DNA at the crime scene or on a piece of evidence. This has prompted several investigations into DNA transfer under very controlled and semi-controlled conditions [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]; however little is published about DNA transfer in “uncontrolled” or real life situations. The benefit of conducting controlled experiments is that it allows for the impact of specific variables to be investigated independently. However, the added effects of other, unknown variables and all their interactions will also impact transfer outcomes. Whilst utilizing general information on the effects of specific variables to predict the likelihood of DNA being transferred in particular situations can be helpful, the resultant picture could be over-simplified and potentially result in miss-representation of the results in court when estimating transfer possibility. There is thus a need to evaluate DNA transfer in a more holistic manner.
Two or more individuals coming together and communicating forms the basis of any social interaction and, thus, is one of the most common situations in which DNA is transferred. Here we focus on a small social everyday setting incorporating three participants sitting at a table sharing a drink (one central jug and individual glasses) over a 20 min period. We examine the effects of multiple direct and indirect transfer of DNA amongst this small group of people and objects. This was facilitated by video recordings of all interactions, in combination with the DNA results obtained, so that the occurrence or lack thereof, of specific sources of DNA from test surfaces collected post sitting could be effectively interpreted. Preliminary results of a subset of these tests were presented previously within conference proceedings [16] and here we present a fuller analysis of an extended investigation as well as provide the supporting data.
Section snippets
Experimental design
Three individuals participated in a social interaction of having a drink of juice together for the duration of 20 min. Participants were invited to have a drink and chat while participating in a blind experiment that was going to be video recorded. Participants were invited to take a seat; however the seats were not allocated but chosen at random by the participants. The participants sat on chairs with plastic armrests around a table and drank from individual glasses while using a communal jug
Test surfaces
No DNA was detected on the jug, glasses or table segments prior to each experiment. This was also the case for the chair arms in the last four experiments; mixed DNA profiles from four people were detected on two of the three chairs in the first experiment. All participants were excluded as contributors to these mixtures. This is most likely due to the less effective cleaning when using lower percentage hypochlorite that was used in the first experiment. Components of these DNA mixtures were
Discussion
The results of these experiments show that, in many instances the last person or the only person to come in contact with an object was the main or the only contributor of the DNA detected on it.
Interestingly, in some instances the last person to touch the tested surface was not the main contributor to the mixtures obtained. These results support the hypothesis that some individuals may deposit more DNA than others when touching a surface [7], [9], [18] even though other factors such as
Acknowledgment
We thank Bianca Szkuta for assistance with graphical design of figures and constructive comments on earlier drafts of the manuscript.
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