DNA transfer: Review and implications for casework

DNA-bearing cellular material can come to be present on a surface by either direct or indirect transfer. Direct transfer includes contact, but also includes activities within the vicinity of an item that may result in the transfer of DNA directly from an individual without any contact, such as speaking, coughing, and sneezing. Indirect transfer of DNA is when DNA from an individual comes to be on an item via an intermediary surface. It is important to consider indirect transfer in the evaluation of trace DNA in casework. The term ‘trace DNA’ in this review refers solely to DNA that cannot be attributed to an identifiable body fluid.This review presents and considers data from trace DNA experiments to establish whether the quantity of DNA recovered from a crime stain and/or the quality of a DNA profile obtained can be used to infer the likely mechanism of transfer. The data show that varied results are obtained from apparently similar trace DNA samples, presumably due to the many factors that affect the detection of trace DNA. The nature and effect of these varying factors and the application of the data to casework is considered generally and with specific reference to DNA transfer to skin, DNA beneath fingernails, ‘wearer DNA’, and various contamination considerations.