DNA

Can we rely on the DNA match statistic from specialist statistical software?

DNA match statistic terms forming an image of a diceAs DNA profiling methods have become ever more sensitive, the detection of trace amounts of DNA has significantly improved. As a result, the number of DNA mixtures we encounter has increased.  This was especially highlighted following the introduction of DNA17 methods in 2014. In many situations the mixtures have multiple contributors and are complex to interpret.  Whilst previously many results were designated as too complex for interpretation, the last few years have seen the introduction of ‘probabilistic genotyping software’ packages which enable a statistical evaluation of complex results.

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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.

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The end of Dlugosz style DNA evidence in court?

Graphic of DNA molecule with DNA profile overlay

Last week the Forensic Science Regulator released long-awaited guidance in the area of DNA mixture interpretation. Whilst this document is still a draft for consultation and therefore not the final version, the main UK forensic providers have all had input and therefore changes are likely to be minimal.

 

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DNA transfer through non-intimate contact

Intertwined male and female symbols representing intimate contact

In sexual offence cases where no semen is detected, the presence of female DNA on penile swabs and underwear of the defendant may be put forward to address the issue of whether or not sexual intercourse took place.  However it is known that DNA can be transferred through non-intimate contact so research was commissioned to investigate the frequency and amount of DNA which might be transferred in this way.  The work was recently published in Science and Justice by the UK and Ireland Association of Forensic Science Providers’ Body Fluid Forum which our expert Julie Allard co-chaired at the time of the research.
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Demonstrating that penetration has occurred when no semen is found: the whys and wherefores of Y-STRs

Human chromosomes - Y-STR tests areas of the Y chromosome

Evidence to demonstrate that penile or digital penetration of the vagina has taken place can be difficult to find in some cases.  Traditional DNA profiling techniques can’t really help as the few male cells potentially deposited are overwhelmed by the large amount of female DNA present in the vaginal cells.  Y-STR typing is a form of DNA analysis that targets just the DNA present on the male Y chromosome allowing production of a male Y-STR profile even in the presence of seemingly overwhelming amounts of female DNA.READ MORE