Case Background
A DNA sample was collected from a firearm seized in a vehicle stop for the purposes of DNA testing.
A mixed DNA profile was obtained from the DNA analysis process. STRmix was used to deconvolute and statistically evaluate the DNA profile. The analysis of this DNA profile was subject to a motion to exclude, which was granted, given that STRmix had not been properly validated in the laboratory for use in a mixture with this many contributors.
STRmix™
STRmix™ provides the analysis and statistical evaluations for DNA profiles.
Forensic laboratories have internal validations that guide the use of STRmix. For most Australian laboratories, their use of STRmix is limited to DNA profiles that have originated from four or fewer individuals.
Published data indicates that DNA profiles that appear to have originated from four individuals have actually originated from five individuals 61% of the time.
The application to exclude this particular DNA evidence was put forward on the basis of two Rules of Evidence:
1. Federal Rule of Evidence 702
The Court is tasked with the “preliminary assessment of whether the reasoning or methodology underlying the testimony is scientifically valid and of whether that reasoning or methodology properly can be applied to the facts in issue.” Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579, 592–93, 113 S.Ct. 2786, 125 L.Ed.2d 469 (1993). Rule 702 thus establishes a standard of evidentiary reliability that focuses on the scientific validity of the expert’s methods rather than the soundness of his specific conclusions. United States v. Bonds, 12 F.3d 540, 566 (6th Cir. 1993).
2. Rule 403
The probative value of the DNA analysis is outweighed by the likelihood of undue prejudice.
It may have also been possible to make an application to exclude given the probative value of the DNA analysis is outweighed by the likelihood of undue prejudice based on the issues surrounding the transfer, persistence, prevalence, and recovery (TPPR) of trace DNA. With such a complex DNA profile, it would not be possible to evaluate how or when the DNA was deposited. In Australia, this would be made under the Evidence Act for the relevant jurisdiction.
Further information about the matter can be found here.
Concerns Over Laboratory Procedures
Australia is not immune from operating outside of their procedures. As recently as last week, I reviewed a matter where a DNA profile had been reported. This DNA profile clearly has the appearance of having originated from at least five individuals. However, this DNA profile has been statistically evaluated and reported as having originated from four individuals.
The analysis is outside the scope of the laboratory’s validated procedures, and it is recommended that this particular analysis is excluded from being presented to the court.
The Need for Independent Review
Unfortunately, there is no easy way to determine if the DNA profile has been appropriately interpreted. This determination requires a review of the DNA profile as well as the STRmix data. This means lawyers will need appropriately qualified assistance.
IFS is here to assist you in ensuring unreliable analyses are not presented in your matter.
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Jae Gerhard is a Forensic DNA Expert and the Principal Forensic Scientist at Independent Forensic Services, bringing over two decades of frontline experience in forensic biology. She has worked on some of Australia’s most complex and high‑profile cases with the Australian Federal Police and New South Wales Police, as well as international disaster victim identification operations in Bali and Thailand.
Specialising in DNA analysis, STRmix™, biological fluid examination, and bloodstain pattern interpretation, Jae now works independently with criminal defence teams to review and challenge forensic evidence. Her mission is simple: to ensure the science presented in court is accurate, balanced, and truly understood.