Investigation into the variables affecting the transfer and detection of background DNA in forensic casework

Current research has explored some of the variables impacting DNA transfer, prevalence, persistence and recovery (DNA-TPPR) from surfaces and items during criminal activity, and has highlighted the possibility of collecting other DNA, termed ‘background’ DNA (bDNA), that may be present on the surface or item from previous use. Because of this, bDNA poses an issue when sampling for targeted DNA as it may interfere with the collection, detection, and interpretation of targeted DNA, and more information is needed on the presence of bDNA, and who might contribute, under different scenarios. While there are many surfaces and items used in criminal activities that may harbour bDNA, here we investigate the prevalence of bDNA on flooring in different rooms within homes, the impact of household cleaning methods on bDNA presence, and the transferability of bDNA to contacting surfaces with a known history of use and applying different contact methods. Variation in DNA quantity, number of contributors, and composition of DNA profiles was observed for samples collected directly from flooring within and between houses and rooms. Household cleaning methods had minimal impact on the quantity of DNA recovered from flooring and the minimum number of contributors assigned to DNA profile, while DNA profile composition was impacted to varying degrees. The transfer of bDNA from flooring to surfaces with a known history of use was observed and occurred to a greater degree following contact methods where movement and pressure were involved. This study builds on current DNA-TPPR knowledge, allowing forensic experts to better inform the trier of fact when commenting on the activities surrounding how the DNA evidence was deposited on the substrate from which it was collected.