Helping to distinguish primary from secondary transfer events for trace DNA

•When trace DNA evidence is presented in Court, a common question is the mechanism of transfer.•Complex mechanisms of transfer showing two, three or more steps require consideration of many different factors.•We show how these factors can be considered within a Bayesian Network to evaluate trace DNA evidence considering competing activities.•The Bayesian Network presented consists of blocks of Network structure that can be configured to suit the case circumstances.•We identify a number of areas in which target research could assist in elements of the presented object oriented Bayesian Network.

DNA is routinely recovered in criminal investigations. The sensitivity of laboratory equipment and DNA profiling kits means that it is possible to generate DNA profiles from very small amounts of cellular material. As a consequence, it has been shown that DNA we detect may not have arisen from a direct contact with an item, but rather through one or more intermediaries. Naturally the questions arising in court, particularly when considering trace DNA, are of how DNA may have come to be on an item. While scientists cannot directly answer this question, forensic biological results can help in discriminating between alleged activities. Much experimental research has been published showing the transfer and persistence of DNA under varying conditions, but as of yet the results of these studies have not been combined to deal with broad questions about transfer mechanisms. In this work we use published data and Bayesian networks to develop a statistical logical framework by which questions of transfer mechanism can be approached probabilistically. We also identify a number of areas where further work could be carried out in order to improve our knowledge base when helping to address questions about transfer mechanisms. Finally, we apply the constructed Bayesian network to ground truth known data to determine if, with current knowledge, there is any power in DNA quantities to distinguish primary and secondary transfer events.