The Development and Use of Mock Fingerprints in the Analysis of Touch DNA Evidence

Touch DNA evidence is left behind when a person comes into contact with an item and deposits dead skin cells on the item or surface. These shed epithelial cells contain DNA that has the potential to aid in criminal investigations. One of the major issues surrounding touch DNA samples is the fact that the quantity of DNA in these samples is low. This can affect downstream DNA processing and yield in incomplete or unusable results. Conducting research on ways to improve these results is important, but can also be difficult. This is because the initial quantity of DNA in the fingerprints is unknown and the amount of DNA present in the sample varies between individuals. In order to resolve some of the issues regarding touch DNA samples, this project has developed mock fingerprint samples that can be used as positive controls for true fingerprints. These samples contained a known quantity of DNA, which reduced the chance of variability and allowed for analysis regarding percent recovery to be conducted. These mock fingerprints were utilized in the optimization of collection and extraction protocols by testing different swabbing solutions and techniques, as well as modifications to the incubation phase during extraction. From these experiments it was determined that the addition of an overnight incubation and a single wet swab, wetted with 2% SDS, resulted in the largest percent of DNA recovered. A standard curve was also generated using the mock fingerprints, which then allowed for true fingerprints from various surfaces to be plotted on the curve. The final experiment employed the mock fingerprints in order to determine where DNA is lost throughout the collection and extraction process. By pipetting the mock fingerprints directly into extraction tubes, onto swabs or onto glass slides it was revealed that a large percentage of DNA is left on the swab after extraction. Overall, the mock fingerprints were effectively used as positive controls throughout this project, and therefore it is recommended that future research continues to use them to obtain a better understanding of the nature of true touch DNA samples.