Animals as DNA Reservoirs and Vectors of DNA Transfer
The results of these scientific studies indicate that animals can act as reservoirs of human DNA and vectors for human DNA transfer that may need to be considered during evaluative DNA reporting.
Furthermore, if an interaction between an animal and a perpetrator is suspected, consideration should be given to collecting DNA evidence from suspected contact areas on an animal.
You can find links to the studies below:
- The Role of Cats in Human DNA Transfer
- Presence of Human DNA on Household Dogs and Its Bi-Directional Transfer
Activity Level DNA Interpretations
These studies indicate that considerations may need to be given to DNA transfer facilitated by pets in any activity level interpretations if there are pets in the household or environment.
Activity Level Interpretations and Bayesian Networks
Activity Level Interpretations in relation to DNA should be performed using what is known as a Bayesian Network, which is informed based on scientific data. The result should be a numerical value and not a verbal description, such as “the evidence is more likely to have been deposited by direct DNA transfer.”
A Bayesian Network looks like the evaluation below, where a reasonably straightforward matter with the following two propositions is considered:
H1: The POI wore his/her own T-shirt as a balaclava.
H2: An unknown individual wore the POI’s T-shirt as a balaclava.
From Luke Volgin (2019) The Importance of Evaluating Findings Given Activity Level Propositions in Order to Avoid Misleading Evidence, Australian Journal of Forensic Sciences, 51:sup1, S10-S13, DOI: 10.1080/00450618.2019.1569131
Each aspect of this network needs to contain a value that is derived from appropriately relevant scientific studies to produce an overall likelihood ratio for the probability of the evidence.
Additional Considerations in Activity Level DNA Evaluation
Additional nodes could be added to this evaluation that consider other possible aspects of the evidence, including but not limited to:
- DNA transfer by pets
- DNA transfer/relocation as a result of evidence packaging
- Errors in the forensic testing process
Work on activity level DNA interpretations remains ongoing. However, especially when it comes to trace DNA, I am skeptical if scientists will ever be able to come up with a suitably robust network that accounts for all the uncertainties around DNA. This is only reinforced when papers such as those listed above are published. It seems there is no end to the possibilities by which DNA can be transferred and deposited.
Please have any activity level report you receive carefully scrutinised.
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