Touch DNA: “Touch” time and resiliency

With every touch of the skin on a surface, cells are left behind; with every cell, the genetic code can be found. People touch doors, tables, and so many other surfaces every day, often with multiple people touching the same object through the course of a day1. New research has found that Touch DNA analysis can erroneously implicate a person who never even laid eyes on the crime scene as the main contributor of the DNA on its handle. When a swab is used for evidence collection from a surface, the investigator may not know how many people may have touched this evidence in the past or what level of persistence DNA may have on touched objects over time. Though every touch leaves cells containing DNA, most contact leaves only a few cells if minimal pressure is used. These trace levels of DNA may remain undetected or, if detected, may be at such low levels that only stochastic effects and low levels of allele drop-in are seen 2. The goals of this work are:- to evaluate how much time (in term of seconds) is necessary to leave "Touch DNA" on dress; - what kind of the technique maximizes the DNA recovery, evaluating among swabs, cut or adhesive tape to sample an area of interest. To acquire a greater knowledge of the rate of detectable wearer, touch and background DNA, 30 females wore their brassiere for 12 h. The lateral regions were handled by one of 5 male volunteers with a different timing: for 60 s, 45 s, 30 s, 20 s, 10 s. Every experiment was carried out in triplicate: in the first case collecting sample was done by the swab, the second by cut of area if interest and the third sample by adhesive tape. The quantity of recovered DNA was determined using real-time Polymerase Chain Reaction (PCR) with Alu-based targets and SYBR green detection. The samples were also analyzed using capillary electrophoresis-based Short Tandem Repeat (STR) typing to determine the percentage of recoverable alleles. Touch DNA is an emulation of the Locard exchange principle, in that any time a person is in a location, that person may leave evidence of their presence. The evidences showed that the best technique to recovery of "Touch DNA" is the cut of the interest area. The toucher was detected as a single profile in samples beetween 60s and 45 s. The wearer was present in a mixture from 30s to 10s; the toucher was always observed as the major contributor. Greater knowledge of the frequency of detection of reportable wearer DNA and/or toucher allows scientists to evaluate the likelihood of observing a matching profile if an individual wore a garment rather than touched it in disputed case scenarios. Everyone in the medico-legal community — forensic scientists and technicians, DNA analysts, potential jurors, judges and lawyers for both the prosecution and defence— must know and understand the potential for mistakes.