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Centre for the Forensic Sciences Blog



My Master’s Dissertation: Secondary Transfer of Trace DNA

By uctzsh0, on 17 April 2016

One of the forefronts of forensic DNA research is the secondary transfer of trace DNA. Secondary transfer is an example of passive transfer and occurs when one person’s DNA is deposited onto an object by someone (or something) else. Trace DNA is usually found in small amounts from sources such as skin cells which are deposited on an object after it has been touched or casually handled.  If this object becomes evidence in a criminal investigation, the possibility of placing an innocent individual at the crime scene arises. The occurrence of secondary transfer of trace DNA has only come to light in the last several years and so there is currently not much known about it. The study I have proposed for my dissertation will evaluate if relative pressure of contact has any effect on the transfer of trace DNA. Knowing this will reveal what factors are required for secondary transfer to happen, and help uncover its relevance in a forensic context.

Tobias, Samuel

Me dusting for prints in a CSI practical session

Forensic DNA technology has progressed to the point where genetic profiles can be generated from samples where only a few cells are present. For this reason, steps have to be taken to prevent any contamination of the samples. The research undertaken by the UCL Centre for Forensic Science is carried out in the collaborative UCL division of Biosciences, UCL CFS and UCL Institute of Archaeology Ancient DNA facility. The ultra-clean lab provides a safe and contamination-free space to extract the DNA from the low-template samples of trace DNA.

As the use of DNA in forensic science continues to expand, the need for scientific literature on the subject will only grow as well. The occurrence of secondary DNA transfer is on the leading-edge of forensic biology research; therefore, my ultimate goal is to create a publishable study that will contribute to the field of forensic DNA research in a beneficial way.

CFS Seminar Series – Julie Allard PFS

By uctzgam, on 27 February 2014

February 6, 2014

This seminar was from one of Principal Forensic Services’ forensic biologists, Julie Allard, who specialises in body fluids and their potential to provide DNA profiles for use in a criminal investigation. The types of body fluids discussed included blood, semen and saliva, all of which have presumptive tests for their detection:

  • Blood – Leucomalachite Green (LMG) test
  • Semen – Acid phosphatase test
  • Saliva – Phadebas test

These tests produce a colour change indicating the presence of the body fluid in question, and can be used on different materials including clothes and weapons. Once areas presumed to contain these body fluids have been identified, samples can be sent for DNA profiling. This is where it starts to get a bit tricky, not only can different levels of interpretation be addressed in relation to the DNA evidence obtained, the DNA evidence itself can be troublesome! For example, is there enough DNA present to produce a profile? Is there a mixture of two or more people’s DNA present? Has the DNA profile come from the body fluid in question? Whose DNA and therefore body fluid is present in the sample? Can the presence of a given person’s DNA determine the actions of that individual?

Julie emphasised the importance to take the context of the case into account when answering these questions. A forensic scientist may address the activity level of the evidence, e.g. Person A kicked Person B but must not address the question of whether or not a person committed a crime or not, since this is the job of the jury. The CAI (case assessment and interpretation) model aids a forensic scientist in evaluating the evidence they are given in a balanced, logical, robust and transparent manner.

However, the interpretation of evidence for use in court is a contentious subject, Julie pointed out that as new information arises concerning a case, the evidence must be re-evaluated in light of this new information. Furthermore, it is likely that the information that the prosecution scientist has received does not correlate with the information received by the defence scientist, leading to much disagreement between experts!

Finally, for forensic scientists to effectively use the CAI model to clearly show how a given conclusion was derived from the evidence provided, in light of the case information, Julie stressed that the logical assessment and interpretation steps need to be based upon empirical data that mimics the forensic context.