Forensic DNA Analysis

DNA profiling can provide apparently very powerful evidence when a profile matching an individual is found on an item or a person associated with a crime.  However given the very high sensitivity of current methods in detecting minute traces of DNA (from just a few cells), it is crucial that an evaluation is undertaken by an expert regarding how and when it may been deposited.

It is important for an expert to consider the following aspects alongside whether the DNA match itself can be relied upon:

  • Is the DNA attributable  to a specific body fluid, such as semen, blood, saliva?
  • Is the presence of a body fluid from an individual significant in the context of the case information whereas the presence of their DNA (not attributable to a specific body fluid) may have an ‘innocent’ explanation?
  • Could the DNA or body fluid be the result of an indirect transfer (secondary, tertiary etc) or is it more likely to have been deposited due to direct contact with the relevant surface?
  • Where was the sample taken from, is its location significant?

Scrutiny of the Crown’s report

When the defendant is presented with evidence of a DNA match that apparently links him/her to a crime, and this is disputed, it is paramount that scrutiny of the Crown’s  results and interpretation are undertaken. Whilst it is rare to find issues with the production of the actual DNA profile, our experts often find that the Crown’s report just gives details of what body fluids and profiles have been detected and whom it matches (the source), but provides no confirmation (or otherwise) of whether the DNA detected came from the body fluid observed.  For example, just because a blood stain has been detected, it does not necessarily mean that the DNA profile was obtained from the blood. If the DNA came from underlying material deposited by ‘touch’, there may be an innocent explanation; whereas if the DNA is assigned to blood, it may support involvement in an assault.

Streamlined Forensic Reports (SFR)

Frequently DNA evidence is only presented in the form of a Streamlined Forensic Report (SFR1) which usually gives minimal details of the DNA match. SFR1s cannot be used evidentially so if the defendant disputes the evidence, the defence legal team should request a stage 2 SFR report from the Crown. It will usually also be necessary for an  expert to be instructed on behalf of the defendant to consider the match in detail and the means by which the DNA could have been deposited.

If only an SFR1 report is available, we can  provide an initial short report to outline the limitations of the SFR1 and provide potential avenues for consideration by the defence team.

How was the DNA transferred?

Where the transfer of the DNA and/or a body fluid has been considered in the Crown’s report, it is often only the prosecution account which is addressed, with no clarity as to how likely the findings are given the defendant’s account. An evaluation considering both versions of events and the possible mechanisms for the transfer may often change a powerful “1 in a billion match” to a neutral conclusion.

Review of the statistical analysis

Forensic Context’s experts are able to review the statistical analysis of DNA results. The current DNA17 and DNA24 methods in routine use are highly sensitive and frequently detect mixtures of DNA (a mixed profile). Some of these are straightforward to interpret but many mixed results are complex and require the use of specialist ‘probabilistic’ software to provide a statistic to evaluate the match.


Y-STR profiling which detects male specific DNA on the Y chromosome is increasingly being used, especially in sexual offence cases as it helps to separate out the male DNA from mixtures which contain a large amount of female DNA.

Cold case reviews and historic methods

In older cases different examination methods and DNA analysis techniques may have been used.  Our experts have experience in a range of legacy techniques including SLP, Quad, SGM and SGMPlus.

If you want to explore issues with the DNA profiling results in your case, contact us and you can speak directly to one of our DNA experts to discuss your requirements.