News & Views

Within spitting distance – is saliva a significant forensic evidence type?

Saliva bubbles coming from a baby's mouthWhilst blood and semen are the two most commonly encountered body fluids, saliva can often arise as a significant forensic evidence type, for example:

  • on a mask worn during a robbery can help to identify the offender
  • drinking vessels or cigarette ends at scenes of crime
  • genital swabs in a sexual offence
  • upper areas of clothing where it can help identify the wearer of the garment

However difficulties occur because saliva cannot be easily identified. The test to identify whether it is present  relies on the detection of a substance called amylase which is usually present in large amounts in saliva.  Complications arise because amylase also exists in other body fluids although with the exception of faeces and vaginal secretions it is usually present at much lower levels than in saliva.

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Contamination – should my client be concerned?

What is contamination?

Forensic experts reserve the term contamination for transfers of trace evidence which occur after the alleged crime event or incident and are not related to the crime.  Transfer before the incident is called ‘adventitious transfer’, for example someone handling a knife whilst doing some cooking before the knife is used by another person a few hours later to stab someone.

It does not just apply to DNA, any substance of forensic interest can be a contaminant, for example body fluids, fibres, hairs, gunshot residue, paint and glass fragments.

How does DNA contamination occur?

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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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