DNA transfer: a question of when and how

DNA transfer shaking hands

In 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 designed to be more sensitive than its predecessor SGMPlus, and therefore the spotlight is shining more 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?  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.  As noted in David Bentley QC’s article in the Law Gazette entitled ‘DNA and case preparation’ (12 January 2015), in the case of low level DNA samples, the presence of a match to a suspect’s profile tells you nothing about how or when the DNA got there.

Much of the data that the forensic community currently has to rely on regarding DNA transfer was obtained using SGMPlus. The question therefore arises about how relevant this data is to current casework using the newer DNA17 system.  Moreover, the research tends to use controlled conditions and little is published about DNA transfer in real-life ‘uncontrolled’ situations.  Some recent research has been carried out using the more up to date DNA technology, but the data is limited. And of course, every case situation is likely to be unique and data obtained from a limited number of research situations cannot hope to inform precisely about the likelihood of DNA transfer in the huge variety of case scenarios encountered.

More research would undoubtedly help, but budgetary constraints make it unlikely that research to address DNA transfer in specific case scenarios will be considered in all but the most serious cases. 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 reduced to a generic paragraph in the forensic provider’s statement which blandly states the basic tenets of DNA transfer.  This is hardly providing 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 as informed a decision as possible about DNA transfer in the context of the case in hand.

David Bentley’s article also discusses the introduction of new software packages for the statistical evaluation of complex DNA profiling results.  These provide much needed objective evaluation of the type of complex mixed results which will be seen even more with DNA17.  However, it is important that the focus does not rest solely on whether a statistical likelihood 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 explore.