An Evaluation of the Transfer and Persistence of Deoxyribonucleic Acid (DNA) Evidence

DNA analysis is now a sufficiently sensitive technique to enable identification of an individual from an extremely small amount of biological material. Exhibits are routinely submitted to forensic laboratories for recovery and analysis of ‘touch DNA’, in order to link an offender to the crime scene. One such exhibit type is spent cartridge cases, where DNA transferred from the handler to the exterior surface of the casing may be the only evidence available for identification of the handler. Alternatively the firearm itself may be recovered, which could also have potential for uncovering the identity of the shooter by means of ‘touch DNA’ profiling. However, the analysis of minute amounts of DNA introduces additional interpretational challenges. The ability to identify the source of a low level DNA sample and the relevance of a recovered DNA profile to the crime scene are not comprehensively understood. The variations in DNA deposition, recovery, transfer and persistence were examined, through a series of controlled laboratory experiments. Volunteers were asked to take part in DNA deposition studies that involved handling items for set periods of time, to determine the variability in the quality of DNA deposited. They were also asked to take part in handshaking studies, where the persistence of DNA, as well as the primary and secondary transfer of DNA, was studied. Additional variables were considered in relation to DNA recovered from spent cartridge cases, including the effect of firing and gunshot residue on DNA quality.
DNA was extracted using QIAamp® DNA Mini Kit (Qiagen) and Chelex® (Bio-Rad) protocols and amplified with the AmpFlSTR® SGM Plus® Kit and the AmpFlSTR® Identifiler® Kit (both Applied Biosystems). DNA profiles were analysed on the ABI PRISM™ 310 Genetic Analyser and the ABI PRISM™ 3500 Genetic Analyser (both Applied Biosystems).
It was possible to recover a usable DNA profile from a handled item and the quality of DNA deposited after repeated contacts was comparable. The quality of DNA recovered from ‘touch DNA’ samples from different individuals varied, and specific methods for recovery based on surface type were found to increase the likelihood of generating a successful DNA profile. Where an item was handled by more than one individual, the major contributor to the profile was not always that of the final handler. Furthermore, secondary transfer of DNA was observed to some degree in every test sample. This research also highlighted the challenges of interpreting mixed profiles, especially with low levels of DNA present.
Identification of the handler of a spent cartridge case was not possible using DNA profiling techniques, due to the increased DNA degradation as a result of conditions experienced during the firing process. However, where a higher yield of DNA was present prior to firing, there was the possibility of recovering an interpretable DNA profile from this type of evidence.
The findings of this research should be considered when submitting items for DNA analysis, when considering best practice for recovery of ‘touch DNA’ samples and when attempting to interpret ‘touch DNA’ evidence profiles.