Tuesday, March 31, 2020

Touch DNA: The effect of the deposition pressure on the quality of latent fingermarks and STR profiles

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Michael Whyte
Crime Scene Officer and Fingerprint Expert with over 12 years experience in Crime Scene Investigation and Latent Print Analysis. The opinions or assertions contained on this site are the private views of the author and are not to be construed as those of any professional organisation or policing body.
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Latent fingerprints present unique, and sometimes the only, evidence found at a crime scene. Several factors affect their quality, including deposition pressure. Its effect on fingerprint size and quality, and on STR amplification success rate, is an emerging area of interest in forensic science.

A recent study published in ‘Forensic Science International: Genetics‘ investigated whether deposition pressure of a fingerprint had an impact upon the amount of DNA recovered.

This study examined 540 fingerprint samples, each consisting of index, middle and ring fingers, deposited by 30 donors on glass, polythene and paper under a range of weights from 0.1 to 10 kg. Both length and width of fingerprints increased with the increasing deposition pressure. Fingerprints deposited under lower (≤0.5 kg) deposition pressure varied in size (p < 0.01), while those deposited at higher (≥3 kg) deposition pressure were more consistent.

Fingerprint quality on glass and polythene, as determined by the AFIS minutiae count and by a fingerprint examiner on a scale from 0 to 4, improved with the increasing deposition pressure, but it deteriorated on polythene at deposition pressure of 10 kg. fingerprint quality on paper continued to improve from deposition pressure of 1 kg up to the maximum of 10 kg.

The effect deposition pressure has on the efficacy of DNA profiling from latent fingerprints was significant as shown by an increase in the DNA amount recovered, the number of amplified loci per sample, and the number of forensically useful DNA profiles (defined here as those with ≥8 full STR loci detected) as deposition pressure increased. This effect was most pronounced with polythene (R = 0.98) and paper (R = 0.96). Altogether, the success rate of DNA profiling varied from 16.3% in fingerprints deposited on paper to 21.2% and 22.5% of those on polythene and glass. The highest number of useful DNA profiles was obtained from glass under deposition pressure of 10 kg.

Forensically useful fingerprints obtained at low (≤1 kg) deposition pressure from all three substrates significantly outnumbered that of STR profiles, while an opposite, though less pronounced trend, was observed at high (≥3 kg) deposition pressure on polythene and paper.

Application of the simple device for collecting of fingerprints under controlled pressure designed for this study, and the palm-up mode of fingerprint deposition as described, allowed us to eliminate the undesirable effect of the hand self-weight and to objectively assess the actual effect of increasing deposition pressure on fingerprint size and quality, as well as on the efficacy of DNA profiling.

Read the paper here

Touch DNA: The effect of the deposition pressure on the quality of latent fingermarks and STR profiles, Forensic Science International: Genetics, https://doi.org/10.1016/j.fsigen.2018.10.016

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