DNA statistics Tag

DNA mixtures – Do you understand them?

DNA mixture profile overlaying a structural image of a DNA molecule A ‘DNA mixture’ or ‘mixed DNA result’ is one in which DNA from more than one person is present.  The prevalence of DNA mixtures has escalated in recent years due to the use of extremely sensitive DNA profiling methods which can detect just a few cells.

Alongside this, the complexity of mixture results has increased as it is commonplace to detect mixtures containing DNA from three or more people, which are more difficult to interpret. With an unmixed ‘single DNA profile’ it is straightforward for the forensic expert to interpret the result and say whether the profile matches an individual or not, and to provide a statistical assessment for the match. However whilst the expert can assess whether an individual could have contributed to a complex DNA mixture result  specialist statistical software is required to assess the likelihood of whether (or not) that person could have contributed to the mixture.

 

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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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Streamlined Forensic Reporting – the hidden truth

Dictionary entry for evidence - forensic evidence is now commonly reported using streamlined forensic reportsStreamlined Forensic Reporting (SFR) was rolled out nationally in April 2013.  The CPS guidance document states that  “SFR is a revised case management procedure for producing forensic evidence at court, which seeks to reduce unnecessary costs, and delay in the criminal justice system. The process takes a more proportionate approach to forensic evidence through the early preparation of a short report that details the key forensic evidence the prosecution intend to rely upon.  The aim is to achieve early agreement with the defence on forensic issues but where this cannot be achieved in the first instance, to identify the contested issues.”

The first stage SFR1 report is not a witness statement or report to which the Criminal Procedure Rules apply.  It is, at this stage, difficult for the defence to determine whether they dispute the evidence being provided as the information that the SFR1 provides is often nothing more than a bland description of the analytical result, with little if any background or interpretational information.  READ MORE