News & Views

Has the defendant’s DNA come from semen?

 

Microscopic preparation from a vaginal swab stained with semen (400x magnification)

 

The Crown’s forensic report states that semen has been detected and that a DNA profile matching your client has been found.

 

You might assume that the DNA came from the semen.
But that may not always be correct!

 

The reason is that the DNA could originate from another body fluid or cells present in the same area as the semen and therefore any attribution of the DNA profile to the semen present may not be robust.

Semen consists of a fluid (seminal fluid) which contains spermatozoa (sperm cells). Although the DNA profiling process usually includes a step that preferentially extracts DNA from the sperm rather than from any other cells, this is not always 100% effective. In other cases, because the amounts of semen are so low, this preferential extraction step is not used.

 

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Forensic Medical Examination Standards

latex gloves worn to minimise contamination during a forensic medical examination

Two very pertinent documents regarding standards for forensic medical examination in cases of alleged sexual assault in England and Wales were published on 27 May 2020 by the Forensic Science Regulator (FSR).    These were developed  by our expert Julie Allard  and other experts from Principal Forensic Services (PFS), in conjunction with experts from Lime Culture CIC and the FSR’s Medical Forensics Specialist Group.The standards underwent public consultation prior to final publication in May 2020.

Examination Standards

FSR-C-116 Sexual Assault Examination: Requirements for the Assessment, Collection and Recording of Forensic Science Related Evidence
FSR-G-212 Guidance for the Assessment, Collection and Recording of Forensic Science Related Evidence in Sexual Assault Examinations

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Torn or cut? – Examining damage to clothing

cut or tear damage to woven fabric

In some cases, the need to examine damage to clothing may be evident, for example in a stabbing scenario. Here the scientist might examine stab cuts and potentially consider whether they may have been made with a specific weapon for example.  However, there are other situations where the examination of damage might be overlooked but may provide useful information.  In an alleged sexual offence it is usual to examine underwear and other clothing for the presence of semen (and other relevant body fluids depending on the scenario) and to attempt to obtain a DNA profile of the offender. But even where it is alleged that clothing has been removed forcibly, damage is less often considered.

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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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A Blueprint for Change

House of Lords Logo white on red backgroundToday, the Lords Science and Technology Committee published their report following their inquiry into Forensic Science.  This was probably the most comprehensive and inclusive inquiry to date taking evidence from all stake-holders in the forensic science arena.  And this time, parties that had previously been tentative in responding to questions were now far more candid and hard-hitting in their responses than in previous inquiries.  Maybe this reflected the urgency of change that is now necessary to prevent the potential collapse of the forensic market and the repercussions that would have for the criminal justice system.  The Lords appear to have realised that we are at a critical point:

“This report follows others that have raised similar concerns, yet the changes that are necessary have not been made, despite acknowledgements that they would be. Forensic science in England and Wales is in trouble. To ensure the delivery of justice, the time for action is now.“

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