Impact of relevant variables on the transfer of biological substances

There is a paucity of data on the relative amounts of DNA containing material that is likely to be transferred given specific casework scenarios that incorporate multiple transfer steps. Availability and application of such data could be helpful in estimating the probability of a given set of circumstances actually occurring. Here we utilise data on transfer percentages, given knowledge of specific variables (type of biological substance, level of moisture of the biological substance, the substrate on which the sample is located, the substrate with which it comes into contact, and the manner of contact), as determined by Goray et al. [1,2] to extrapolate the amount of DNA expected to be retrieved from a targeted sample area, under particular multi-transfer step scenarios. We demonstrate that, in many scenarios incorporating multiple transfer steps, unrealistically large amounts of biological material would need to be present at source to generate a detectable level of DNA from the targeted crime scene surface. These findings will assist in comparing the likelihood of postulated alternative crime scenarios involving DNA transfer, at the investigation stage and/or during court proceedings.