DNA transfer: A question of how and when

Cartoon of Mr Red and Mr Yellow shake hands and transfer their DNA to the other personIn recent years, forensic science has seen a number of significant improvements in the field of forensic DNA analysis.  One of the drivers has been the need to detect ever decreasing amounts of DNA.  The DNA17 technique, in routine use in the UK since July 2014, is much more sensitive than its predecessor SGMPlus, and therefore the spotlight is shining brightly on the issues around DNA transfer and persistence.

What does the presence of an individual’s DNA on an item related to a crime actually mean in the context of the case circumstances?  The presence of a DNA match to a suspect’s profile tells you nothing about how or when the DNA got there.  Scenarios incorporating multiple DNA transfer steps, rather than a single direct transfer, are increasingly being raised in court as potential means for the presence of the defendant’s DNA at the crime scene or on an evidential item.

An expert’s evaluation of how DNA may have been transferred requires consideration of the case circumstances and the likelihood of the findings being due to DNA being transferred to the surface in question by direct contact with an individual or by an indirect transfer via an intermediary (secondary, tertiary transfer etc).  Such opinions are based on the nature and strength of the DNA result and any body fluids present, information and data in published papers and the expert’s knowledge and experience.

Every case situation is likely to be unique and data obtained from a limited number of research situations under controlled conditions does not inform precisely about the likelihood of DNA transfer in the huge variety of case scenarios encountered. Ultimately this means that opinions on DNA transfer may vary between experts because their experience and views in this area differ and whilst this is not an ideal situation, it does highlight the need to obtain reliable expert advice.

In addition, streamlined forensic reporting systems, the need to reduce turn-around times and to cut costs means that the consideration of the transfer and persistence issues in a DNA case are often absent or are reduced to a generic paragraph in the forensic provider’s statement which blandly states the basic tenets of DNA transfer.  This does not provide the courts with the necessary information to understand DNA transfer and without the assistance of an expert it is impossible for the court to make an informed decision about DNA transfer in the context of the case in hand.

The introduction in recent years of new software packages for the statistical evaluation of complex DNA profiling results provides a much-needed objective evaluation for the type of complex mixed results which are often seen with DNA17.  However, it is important that the focus does not rest solely on whether a statistical evaluation of the defendant’s DNA being present can be given.  How that DNA got where it is, ultimately has to be the question that it is vital to address.

Updated version of article originally posted on 8 October 2015