DNA and forensic genetics expert
Expert class library · Forensic science · Either side
A forensic DNA analyst or statistician who interprets DNA mixtures and parentage results and explains the statistics: random match probabilities, combined probabilities of inclusion and the likelihood ratios produced by probabilistic genotyping software such as STRmix and TrueAllele. Courts have broadly accepted probabilistic genotyping, with the Sixth Circuit reversing an order excluding STRmix and New York's highest court upholding TrueAllele, but they exclude results from software used outside the conditions it was validated for, and reverse convictions where an analyst said a defendant 'could not be excluded' without meaningful statistics.
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What this expert is retained to answer
- How many contributors did the analyst assume, and was the software validated for a mixture of that size and quality?
- What does the likelihood ratio actually mean, and how will it be explained to the jury without overstating it?
- If the analyst says the defendant cannot be excluded, what is the probability that a random person also could not be excluded?
- Did the laboratory validate the software internally for its own equipment and protocols?
- Has the defense had access to validation data or source code, and was that access needed?
Methods
- STR profiling and mixture interpretation
- Probabilistic genotyping with software such as STRmix and TrueAllele
- Likelihood ratio, random match probability and combined probability of inclusion statistics
- Estimation of the number of contributors
- Internal validation and laboratory quality assurance
How the testimony is attacked
- Outside the validated range. Software is only as reliable as its validation. In United States v. Williams the court excluded probabilistic genotyping results because the program was validated only for mixtures of up to four contributors and the analyst did not reliably establish that only four people contributed.
- 'Cannot be excluded' without statistics. An inclusion is meaningless without its probability. In State v. Phillips the South Carolina Supreme Court reversed a murder conviction where the analyst testified the defendant could not be excluded although, for one sample, one in two random people also could not be excluded.
- Novel software. Defendants attack new genotyping programs as untested. In United States v. Gissantaner the Sixth Circuit reversed an order excluding STRmix, finding it testable, peer reviewed, widely used and accepted.
- Source code secrecy. Defendants seek proprietary source code to test the software. In People v. Wakefield the New York Court of Appeals upheld TrueAllele after a Frye hearing and found no error in denying source code discovery.
- Misleading statistics. Likelihood ratios can be misunderstood as the probability of guilt, and courts police how they are described to juries.
What the public record shows
Courts have filed at least 180 opinions since 2015 that address this class of testimony under Daubert or Rule 702, as of October 4, 2026. 4 rulings that show how courts handle this class of testimony:
| Outcome | Case | Court | Year | Why |
|---|---|---|---|---|
| Exclusion reversed | United States v. Gissantaner990 F.3d 457 | 6th Cir. | 2021 | The Sixth Circuit reversed a district court order excluding STRmix probabilistic genotyping evidence linking a defendant to a gun, holding the method satisfied Rule 702 because it can be and has been tested, has been peer reviewed and is generally accepted, while noting Rule 403 may require care in describing the likelihood ratio. |
| Admitted | People v. WakefieldNo. 3 | N.Y. | 2022 | The New York Court of Appeals held the trial court did not abuse its discretion in finding, after a Frye hearing, that TrueAllele's continuous probabilistic genotyping approach, including its use of data below the stochastic threshold, is generally accepted, and that denying the defendant discovery of the source code was not error. |
| Excluded | State v. PhillipsOpinion No. 27978 | S.C. | 2020 | The South Carolina Supreme Court reversed murder and weapon convictions because the trial court should have sustained objections to a DNA analyst's testimony that the defendant could not be excluded from mixtures, where for one sample the chance a random person also could not be excluded was one in two and the State failed to explain the underlying concepts and presented some wrong information. |
| Excluded | United States v. Williams382 F. Supp. 3d 928 | N.D. Cal. | 2019 | In a double murder case, the court excluded an analyst's probabilistic genotyping conclusion that there was very strong support that the defendant contributed DNA to a car door handle, because the software was validated only for mixtures of up to four contributors and the analyst did not reliably establish the number of contributors. |
Each case links to the free opinion text on CourtListener.
Under amended Rule 702
Since December 1, 2023, Rule 702 says expressly that the party offering an expert must show the court it is more likely than not that the testimony meets each requirement: that it rests on sufficient facts or data, uses reliable methods, and reflects a reliable application of those methods to the case. Questions about the basis of an opinion are no longer automatically matters of weight for the jury. For how the circuits have applied the amendment, see the Rule 702 tracker, which follows each court of appeals; for what that means for preparing or attacking this class of expert, see the guide on amended Rule 702.
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Last reviewed October 4, 2026. How this page is built: rulings are found by searching court opinions on CourtListener, and each one is read in the opinion before it is summarised here; outcomes are labelled by what the court did with the expert's testimony. No individual expert is named. This page summarises public decisions for orientation and is not legal advice; read the opinion before relying on any ruling.