Activities between activities of focus—Relevant when assessing DNA transfer probabilities
Introduction
Consideration of the likelihood of various indirect transfer scenarios occurring, as opposed to direct transfer scenarios, is increasingly requested in case deliberations. Such deliberations regarding indirect transfer often focus on the initial activities leading to the DNA in question being picked up, and on the activities surrounding its deposition at the location where it is sampled. Between these activities however, there is often a time lapse and insufficient attention is given to activities that may have occurred during this period, or the impact they may have on profiles generated.
During any brief moment of time a person is often touching something. This could include touching themselves, another individual or an object/surface. The latter may be an item they are wearing, using or have their hand resting on, and could be personal, shared with others or solely belonging to/used by others. Depending on its use and ownership, the sources and quantities of DNA on each item will vary.
Most contacts between them surfaces will result in an exchange of material between them. This could result in a loss and/or gain of DNA from or to the object from which a DNA sample is collected for further investigation. DNA transfer is influenced by factors such as the nature of the biological material, the composition of the surfaces coming into contact, the freshness of the sample/s at time of contact, the manner of contact and other unidentified factors [1], [2], [3], [4], [5], [6], [7], [8]. Detecting transferred DNA is not only dependent on the quantity transferred, but the quantity and quality relative to other sources of DNA that may be present on the contacting surfaces that are co-collected during sampling.
As more items are contacted by the originally deposited biological sample, the greater the likelihood of it no longer being detected on the surface it was originally deposited on, or on the surface it was considered ultimately to have been transferred. The level of loss will be dependent on the history of the substrates contacted in terms of who, how and when they were previously touched, as well as the manner of contacts.
So, even seemingly irrelevant everyday activities, that do not appear to have any direct association with the criminal act under investigation, could have a profound impact on the likelihood of detecting a relevant quantity of DNA that is assumed to have been deposited during a specific action and later transferred during a crime related activity.
To help gain an understanding of the frequency and manner of the items and surfaces touched during everyday activities, we observed individuals performing a range of general activities and assessed a number of potentially relevant factors.
Section snippets
Materials and methods
160 videos relating to individuals performing a range of common activities were viewed and analysed for a range of factors (Table 1). The videos fit the following general criteria: readily accessible; duration of ∼15–40 s; the apparent dominant hand visible at all times; able to readily recognise what was touched.
Results and discussion
Our findings show that several items are often touched during various general daily activities over relatively short periods of time (Table 1). On average, across all activities, a person performs ∼5.7 touches with their dominant hand per 22.1 s, translating to ∼15 touches per 60 s. The touches were of personal objects (including self) likely to contain the toucher’s DNA and non-personal objects (including other people) likely to be DNA-free or contain DNA from other individuals. The objects
Conclusion
Appreciation and consideration of general activities that may have occurred during the time between key focus activities (i.e. those associated with the pickup of DNA in question at point A and deposit at collection site B) and their potential impact on what is deposited at the final collection site, are imperative when weighting alternative transfer scenario propositions of how DNA became present at a location.
Conflict of interest
None.
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