Case Studies

The current climate in forensic provision often limits the amount of forensic work commissioned and the work can be split across different providers resulting in a fragmented approach which may not fully address the case issues. There is often a focus solely on DNA profiling and whom the result matches. However a DNA match is only the start of the forensic evidence and in many cases it is vital to ensure there is an expert evaluation of its relevance within the context of the case, including what the DNA came from, how and when it was deposited.

It is crucial that the court is presented with a full picture of the evidence and what it means in relation to the both the prosecution and defence accounts.

Forensic experts have an overriding duty to the court and can fulfill this need. However Crown experts may only have been commissioned to assess the DNA match or may not be aware of the defence account, meaning that no evaluation is made of the ‘how and when’. Review by an independent expert can provide such missing evaluations or critically appraise any which are reported by the Crown scientist.

Sexual Assault- When only a few sperm are detected, does it necessarily mean that intercourse occurred?

Case scenario

Complainant C alleges the defendant D had vaginal intercourse with her. When only a few sperm are detected on a vaginal swab, does it necessarily mean that sexual intercourse occurred?

D denied intercourse but said they did have intimate contact. Examination of the vaginal swabs taken from C revealed a small amount of semen was present, which matched the DNA profile of D.

Expert evaluation

This involved assessing whether the small amount of semen found was more likely to have resulted given the account of D rather than if C’s account were true.

Our expert concluded that whilst the semen on the internal vaginal swabs could have been the result of sexual intercourse, the intimate activities that D described could also have resulted in a transfer of semen into the vagina. This differed from the Crown’s expert who viewed the latter as unlikely to result in a transfer of semen.

GBH Assault - What does a lack of blood on the accused's clothing mean?

Following an assault involving punching and kicking, what does a lack of blood on the accused’s clothing mean?

Case scenario

The defendant D is alleged to have carried out a prolonged assault on X, with punching, kicking and use of a baton. X received significant head injuries which bled profusely. D denied any knowledge of the assault or contact with X. D was arrested nearby shortly after the alleged assault because his clothing matched the description of the assailant. No blood was found on any of D’s clothing or footwear. The baton was not located.

Expert evaluation

When a person is injured during an assault, the amount of blood transferred to the perpetrator will be dependent on factors such as the nature of the assault, the extent and duration of contact between the perpetrator and the injured person after the onset of bleeding, and the amount of blood lost at the time of this contact. In many cases there is a lack of specific detail about the events occurring during an assault, including when the injuries were inflicted and the amount of bleeding during the period of contact. In this situation a lack blood on the accused’s clothing is often inconclusive in addressing whether or not the wearer caused the injuries. However where there is evidence of a prolonged assault during which significant bleeding occurred during the assault and multiple blows were dealt, it is extremely unlikely that no blood would be transferred to the perpetrator. The expectation would be for some blood staining to be deposited as a result of contact and also for the transfer of blood spatter from airborne droplets of blood produced during any blows into wet blood. An absence of blood on the accused’s clothing could therefore be very significant.

Murder - How and when was the DNA deposited?

Case scenario

A DNA profile matching the defendant D was detected on the handle of a screwdriver alleged to have been used to fatally stab a man.  What is the significance of the DNA result given D denies the murder and any knowledge of the screwdriver?

Expert evaluation

DNA profiling results can provide compelling evidence regarding the source of DNA, provided the quality of the match and the associated statistics are not in question, and there is no possibility that close relatives are involved. The core issue in this case, as in many others, was not whom the DNA came from but how and when was it deposited.

The DNA profile matching D was obtained from cells present on a swabbing taken from the handle of the screwdriver.  It was not possible to comment on how or when the DNA had been deposited.  As the case progressed witness evidence came to light that a cloth may have been wrapped around the handle – it is possible that D’s DNA present on the cloth had been transferred indirectly to the screwdriver handle without D ever having direct contact with it.  Wrapping in a cloth could also explain an absence of DNA from the true perpetrator on the handle, although this could also be due to other reasons such as wiping of the handle, or because different people shed different amounts of DNA.

DNA match evidence may seem compelling but evaluation of the scientific findings in the case context is vital!

Rape - Did the DNA come from semen?

Case scenario

The allegation was one of anal rape of female F by male M.  Traces of semen were present in the back of the crotch area of F’s knickers and DNA matching M had been obtained.  The issue was whether the DNA matching M came from the semen detected and if so whether it was from the alleged event or consensual sex which had taken place some weeks before.

Expert evaluation

The testing which had been carried out demonstrated that traces of semen were present, but our expert concluded that it was possible that this could have been from an event some weeks earlier.  Moreover, it could not be determined from what body fluid the DNA matching M had come and whether it was present on the inside or outside of the fabric of the crotch.  F and M had slept in the same bed following the alleged rape and there was potential for a transfer of DNA to the outside of the crotch of F’s knickers during that time.  The Crown’s scientist had concluded that the findings provided support for the rape having occurred as alleged; but our expert concluded that, when all the contextual information was considered, the scientific findings were neutral and did not support either the Crown or defence version over the other.