Experts are increasingly concerned by issues regarding the activity level of DNA stains. A case from our burglary-related casework pointed out the need for experiments regarding the persistence of DNA when more than one person touched a tool handle. We performed short tandem repeat (STR) analyses for three groups of tools: (1) personal and mock owned tools; (2) tools, which were first "owned" by a first user and then handled in a burglary action by a second user; and (3) tools, which were first owned by a first user and then handled in a moderate action. At least three types of tool handles were included in each of the groups. Every second user handled the tool with and without gloves. In total, 234 samples were analyzed regarding profile completeness of first and second user as well as properties like detectable major profile or mixture attributes. When second users simulated a burglary by using a tool bare handed, we could not detect the first user as major component on their handles but attribute him to the stain in 1/40 cases. When second users broke up the burglary setup using gloves, the first user matched the DNA handle profile in 37% of the cases. Moderate use of mock borrowed tools demonstrated a material-dependent persistence. In total, we observed that the outcome depends mainly on the nature of contact, the handle material, and the user-specific characteristics. This study intends to supplement present knowledge about persistence of touch DNA with a special emphasis on burglary-related cases with two consecutive users and to act as experimental data for an evaluation of the relevance of alleged hypotheses, when such is needed in a court hearing.
In forensic DNA analysis, salivary traces at crime scenes are a promising way to identify a person. However, crime scenes are oftentimes investigated a while after the crime and recovered samples might have been degraded leading to poor PCR amplification. Probably due to decomposition and negative visual impression of spoiled food, bite mark samples make up only a small part of our casework routine. In this study, bite marks on apples and chocolate bars as well as on an inert surface (microscope slide) were stored up to 3 weeks indoors and outdoors during different seasons and analyzed for amylase activity and DNA quantity and quality. The results underlined the stability of human nuclear DNA not only on inert but also on biological surfaces and their forensic usefulness even when bite marks are stored 21 days under adverse but realistic conditions at a crime scene. Overall, amylase activity as well as DNA quantity decreased over time depending on storage environment with a certain inter- and intrapersonal variation. But amylase activity testing was not found to be an appropriate screening tool for further analysis. Apple bite marks resulted in generally higher DNA amounts than chocolate bars and microscope slides. Although mold reduced the DNA quantity, complete STR profiles could be analyzed. High air humidity and cold temperatures were found to act preservative on raw food with high water content but caused loss of information over time for smooth inert surfaces and hygroscopic foods like sweets. Many factors are involved in the stability of DNA in bite marks and its resulting quality and quantity available for an STR analysis. However, since there was a substantial proportion of informative STR profiles even from bite marks stored for 21 days, the results encourage the analysis of those even if their visual appearance seems unfavorable.
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