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.

 

What can the scientist say from looking at a DNA mixture result?

  • The minimum number of people that could have contributed to the mixture, but NOT the maximum.
  • Whether DNA from a male and/or female is present, but not how many of each gender. However Y-STR profiling can assist regarding the minimum number of males present in a mixture
  • In some results, it is clear that the contribution from one person is much stronger – this is called the major profile or a predominant contribution. The weaker portion is called the minor.  Often the minor portion of the result will be an incomplete profile and it can be as little as one DNA component.
  • Whether or not a named person could be a potential contributor to the mixture.
  • Some mixed DNA profiles are complex because DNA from three or more people is present, often in unequal amounts.  Some of the  DNA components  from one or more of the contributors may be missing from the result, and parts of the result may be weak.   Even when there appear to be prominent components in complex results, it must be borne in mind that such components may reflect a contribution of the same DNA component from multiple contributors.  For these mixtures, it is usually not possible to separate the result out into individual profiles.

 

Statistical evaluation of DNA mixtures

The terms ‘standard’ and ‘specialist’ are often used by forensic scientists in relation to the statistical evaluation of DNA profiling results.  Until relatively recently, a statistical evaluation of a mixed DNA profiling result could only be carried out for a limited number of results, generally those which had no more than two contributors of DNA to the result.  This type of statistical evaluation is referred to by the term ‘standard’ and is generally carried out using a simple computer program.

Sophisticated software packages are now available which enable  the statistical evaluation of more complex results (those with contributions of DNA from three or more individuals).  In the UK, forensic providers routinely use a number of different ‘probabilistic genotyping’ software packages which are based on complex mathematical modelling.  The Forensic Science Regulator and the international ISO standards to which Forensic Science Providers are accredited, require that such software undergoes validation prior to use. In the UK a commercial software package called STRmix™ is widely respected and is used by several laboratories. Some Forensic Providers use software developed in-house instead of, or in addition to, STRmix™ – LiRa is used by Eurofins Forensic Services and DNA Resolve by Cellmark Forensic Services. However, all software packages still have limitations with regards to the type of results with which they can be used.

Such software will provide a statistical evaluation for the likelihood ratio (LR) , the figure that forensic scientists give in their reports to provide an estimate of how much more likely the DNA findings are if individual A contributed rather than if they did not.

 

What is the relevance of a DNA match and is the DNA related to the crime?

This is the crucial question that the court wants answered. The presence of a DNA match to a suspect’s profile tells you nothing about how or when the DNA got there. A DNA expert may be able to provide the court with an evaluation of how DNA may have been transferred by consideration of the case circumstances and the accounts of each party.

For example, consider a case where a firearm is recovered from a vehicle in which the defendant was travelling. The prosecution allege he has used the firearm and charge him with illegal possession, which he denies. A complex mixed DNA profile is recovered from the firearm.  The Crown’s scientist reports that the defendant could have contributed DNA to the mixture detected, but provides no other evaluation about DNA transfer.

An expert reviewing the DNA result on behalf of the defence will firstly consider the minimum number of people that could have contributed DNA to the mixture; then whether there is any clear major profile from one contributor. They will assess the robustness of the reported match with the defendant and the associated match statistic.  But importantly there is other information within the DNA result which will assist.  The firearm was wrapped in an item of clothing when it was recovered from the vehicle.  If DNA matching the defendant is present on the firearm, is it at a level which might be explained by an indirect transfer of DNA from the clothing item or other surfaces within the vehicle where his DNA may be present?

 

This article is intended as an introduction to DNA mixtures,  if you would like to learn more, take a look at the DNA explainer on the NIST website or contact us to discuss provision of a CPD session in your chambers.