Evaluation of multiple transfer of DNA using mock case scenarios.

DNA transfer and its possible role in explaining the presence of a biological sample at a crime scene is becoming more prevalent in criminal investigations and related court proceedings. To assist understanding of DNA transfer and assess the extent to which we can utilise already available information regarding transfer of DNA we compare transfer rates determined from mock multi-step transfer scenarios with transfer rates predicted by the application of currently available transfer rate data. The transfer results obtained from the scenarios tested were, in some instances, different (both lower and higher rates) from those predicted. These discrepancies are most likely the result of the impact of as yet untested variables. These may include the variations in substrate type, transfer area size and environmental factors such as temperature and humidity among others. Whilst detailed re-enactments of proposed transfer scenarios, that take into account the many possibly relevant aspects affecting transfer are desirable, to provide an accurate likelihood estimate, these are not always possible. The application of detailed transfer rate tables that include data on the many factors affecting transfer could provide a useful substitute for evaluating the likelihood of specific transfer events. The value and accuracy derived from applying such tables will improve as more research in this area is conducted and the tables expanded and refined.