Exploring the Persistence and Transfer of DNA Under Fingernails in Mock Physical Assaults

Trace DNA analysis has been an important forensic method for investigating DNA left at the scene of a crime, but few studies have explored the mechanisms of two-way transfer and the persistence of trace DNA.1,2 The persistence and transfer of trace DNA is difficult to quantitatively measure particularly when there is a mixed profile.3 Analyzing trace DNA is pertinent in cases of physical assaults where DNA is often deposited on the body of the victim.4 The touch DNA recovered from a victim’s body preceeding an assault may aide in the identification of the offender.5 This study is designed to investigate the persistence and two-way transfer of trace DNA under the fingernails and on the forearms in a physical assault scenario. Simulations of physical assaults were conducted with a female mock victim lightly scratching the forearm of a male mock offender. Twelve pairings of volunteers (6 females and 6 males) were recruited for either 5 scratches or 10 scratches. The “victim’s” dominant hand and the forearm of the “offender” were sampled at 0 and 3 hours in distinct simulations. Each pair of volunteers were involved in each simulation combination for a total of 16 samples/ pair. Reference samples were collected from buccal swabs and extracted utilizing Chelex resin. Trace samples from the fingernails were collected with Copan subungual dry swabs, and samples from forearms were collected by Puritan cotton swabs. The samples were extracted using the QIAmp DNA Investigator Kit and QIAshredder Kit and quantified using the Trio Quantification kit on a QuantStudio ProFlex 96-well Reverse Transcription Polymerase Chain Reaction (RT-PCR) system. Samples were amplified using the GlobalFiler Amplification kit. The Short Tandem Repeat (STR) fragments were separated using the SeqStudio Genetic Analyzer. The Yfiler Plus Kit will be utilized after the initial fragmentation analysis to amplify the Y-STR markers of the male profile, if necessary, to improve resolution of DNA mixtures that include small quantities of male DNA. The STR profiles will be analyzed with the GeneMapper ID-X software. Overall, the preliminary results on an initial set of 32 trace samples showed that the quantification values of samples collected from the victim’s fingernails ranged between 0.007 ng/ μl and 0.486 ng/ μl. The trace samples collected from the offender’s forearms ranged between 0.003 ng/ μl and 0.351 ng/ μl. A mixed STR profile was observed in 62.5% (15/24) fingernail samples and 75% (6/8) forearm samples. A general decrease in the proportion of the offender’s DNA in relation to the victim’s DNA was observed over time from trace samples collected from the victim’s fingernails, which is in line with the previous study conducted by Damour et. al.2 It was also noted that there was a general decrease in the proportion of the victim’s DNA in relation to the offender’s DNA over time from two-way trace samples collected from the offender’s forearms. In conclusion, a comprehensive investigation of 140 trace samples is in progress to reveal persistence and the two-way transfer of DNA from the mock offender to mock victim’s fingernails and vice versa.