DNA 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

Semen transfer – sexual offence or innocent transfer?

Semen transfer from complainant laying on bedWhilst the presence of semen matching the DNA profile of the defendant may seem compelling evidence when it is detected on intimate swabs or clothing of the complainant, there are other mechanisms of semen transfer which might account for the findings.  A forensic sexual offences expert can consider these and evaluate the likelihood of the semen findings given the case circumstances, the allegation made and the defendant’s account.

It is often the case that when the Crown expert prepares their report, they have no knowledge of the defendant’s account and so can only comment on the findings in respect of the complainant’s version.READ 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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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.

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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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