DNA statistics Tag

Using Y-STR DNA Profiling in Sexual Offence Cases

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

Use of Y-STR DNA profiling techniques has noticeably increased in the last year or two. Scientific evidence to demonstrate that penile or digital penetration has taken place during a sexual offence is often difficult to obtain.  Despite the sensitivity of standard ‘autosomal’ DNA profiling techniques, such as DNA17,  the few male cells potentially deposited can be overwhelmed by the large amount of female DNA present on intimate swabs taken from a complainant. However Y-STR profiling is another highly sensitive form of DNA analysis which 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.  It is also useful when there are multiple male contributors to a sample and for paternity relationship testing since Y-STR profiles are inherited from father to son and all male descendants have essentially the same Y-STR profileREAD MORE

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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