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DNA mixtures

In this video, Dr Gavin Turbett and Dr Vivek Sahajpal discuss DNA mixtures and the complications which arise in their interpretation.

In case of a DNA mixture, there are two or more contributors to the DNA profile. The deconvolution process or the process by which the DNA examiner interprets the possible allele pairs or genotype in the DNA mixture can be highly subjective and complicated.

In 2016, the President’s Council of Advisors on Science and Technology, a council of expert’s to the US President, published a report called Forensic Science in Criminal Courts: Ensuring Scientific Validity of Feature-Comparison Methods (PCAST). The report examined a range of forensic disciplines that were based on ‘feature comparison’ methods, such firearm analysis, hair microscopy analysis, latent fingerprint analysis, bitemark analysis, footwear analysis and DNA analysis. The report found that forensic DNA analysis of single source DNA and simple mixture of two individuals was an objective method and foundationally valid. On the other hand, when it came to DNA analysis of complex DNA mixtures, the report found that the traditionally used combined probability of inclusion (CPI) method was subjective, prone to error and not foundationally valid. CPI method requires the scientists to make subjective choices about what can be considered alleles, possible allele pairings as well as major and minor contributors in a mixture. Each of these choices affect the final conclusion and opinion of the scientist. Due to the subjective nature of this method, there has been a move towards using probabilistic genotyping softwares (PGS) by scientists to deconvolute complex DNA mixtures. The PCAST report found that the process may be objective but there was a requirement for additional studies to examine its foundational and as applied validity.

In 2021, a report published by the National Institute of Standards and Technology, DNA Mixture Interpretation: A NIST Scientific Foundation Review, found that PGS utilises more information than traditional binary approaches (like CPI). The report is the first of its kind, where experts in the discipline have conducted a scientific foundation review of DNA mixture analysis methods. The report also found that interpretation of DNA mixtures varied across analysts and laboratories adding a lot of variance to the process. One of the key takeaways from this report is that there is not enough publicly available data to analyse the foundational validity of the PGS method of analysis.

Based on the reports mentioned above, it is important to keep the following things in mind:

  • Due to increased sensitivity of forensic DNA techniques, minute amounts of DNA can now also be detected. In such cases, it may be difficult to ascertain major and minor contributors, further complicating DNA mixture interpretation.
  • Biological evidence found at crime scenes may often be degraded and generate partial DNA profiles, which further complicates deconvolution of DNA mixtures
  • Interpretation of the raw data and electropherograms in a mixed DNA profile case requires the experts to make certain assumptions. It is therefore important that experts specify the assumptions made as part of this step in their reports.
  • It is important to note that experts often do not comment on all the possible genotypes found in the crime scene sample, instead only on the genotypes with greater statistical significance.

Research surrounding deconvolution of complex DNA mixtures is still at a nascent stage. Scientists are yet to validate the PGS method, while traditional binary methods have multiple elements of subjectivity. Therefore, from a legal perspective, it is important to assess how the interpretation of a DNA mixture has been carried out in a particular case and carefully evaluate it against established guidelines.

In this video, Dr Turbett and Dr Sahajpal further aid your understanding of the challenges in interpretation of DNA mixtures.

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Decoding Forensics for Legal Professionals

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