DNA

Contactless DNA Transfer

Contactless DNA transfer symbolised by the contactless payment icon and DNA moleculesAs DNA profiling techniques have become more sensitive, it has become increasingly apparently that indirect transfers of DNA routinely occur which might result in an individual’s DNA being detected in a place that they have never been or on a surface with which they have never been in physical contact.  Recent research has taken this a step further by looking at the potential for a transfer of DNA when there has been no contact at all, sometimes referred to as ‘contactless DNA transfer’.

 

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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

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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