secondary transfer Tag

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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Washing out the stains

Clothing in a washing machine - can washing remove semen stains?

Locating semen stains on clothing usually relies initially on a chemical screening test followed by a microscopic examination for spermatozoa (sperm cells) to confirm the presence of semen. But are the tests used sensitive enough to detect semen after an item has been washed?

The term ‘washed’ can mean many different things ranging from a quick rinse, a full wash cycle in a washing machine or soaking and then washing for example.  How long the item is immersed in water, the type of detergent and temperature used will have some effect on whether semen can be detected afterwards. A scientist will need to be given as much information as possible about the type of washing used so that they can advise on the likelihood of detecting any staining and decide what tests and strategy they should employ.  This is also relevant when an expert is asked to comment on whether stains are likely to be found on a washed item, and whether the laboratory testing was appropriate. 

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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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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