DNA Mixture Interpretation: A NIST Scientific Foundation Review

Published

December 18, 2024

Author(s)

John Butler, Hariharan Iyer, Richard Press, Melissa Taylor, Peter Vallone, Sheila Willis

Abstract

Improvements in DNA testing methods have allowed forensic scientists to reduce the quantity of DNA required for profiling an individual. Today, DNA profiles can be generated from a few skin cells. This increased sensitivity has extended the usefulness of DNA analysis into new areas of criminal activity beyond homicides and sexual assaults – but also the complex DNA mixtures often seen in casework. Distinguishing one person's DNA from another in these mixtures, estimating how many individuals contributed DNA, determining whether the DNA is even relevant or is from contamination, or whether there is a trace amount of suspect or victim DNA make DNA mixture interpretation inherently more challenging than examining single-source samples. These issues, if not properly considered and communicated, can lead to misunderstandings regarding the strength and relevance of the DNA evidence in a case. This report explores DNA mixture interpretation with six chapters, a bibliography of cited references, an appendix with glossary and acronyms, and two supplemental documents. Chapter 1 introduces the topic of DNA mixtures, the difficulties behind their interpretations, and discusses the relevance of issues explored in the other chapters of this scientific foundation review. Chapter 2 provides background information on DNA and describes principles and practices underlying mixture measurement and interpretation. The likelihood ratio (LR) framework and probabilistic genotyping software (PGS) are also discussed. Chapter 3 lists data sources used in this study and strategies to locate them. Chapter 4 and Chapter 5 cover the report's core concepts: reliability and relevance issues in DNA mixture interpretation at the sub-source and activity levels of the hierarchy of propositions. Chapter 6 explores the potential of new technologies to assist mixture interpretation and considerations for implementation. Two supplemental documents provide context on how the field has progressed (NISTIR 8351sup1) and summarized information from publicly accessible validation and proficiency test results covering DNA mixture interpretation (NISTIR 8351sup2). There are 497 references cited in this report.

Citation

NIST Interagency/Internal Report (NISTIR) - 8351

Report Number

8351

NIST Pub Series

NIST Interagency/Internal Report (NISTIR)

Pub Type

NIST Pubs

Download Paper

https://doi.org/10.6028/NIST.IR.8351

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Keywords

activity level propositions, binary models, case assessment and interpretation, case context, cell separations, combined probability of inclusion, complex DNA mixture, contamination, continuous (fully continuous) models, discrete (semi-continuous) models, DNA, DNA mixture, DNA mixture interpretation, DNA transfer and persistence, forensic science, hierarchy of propositions, interlaboratory studies, internal validation studies, interpretation, likelihood ratio, massively parallel sequencing, measurement, microhaplotypes, next generation sequencing, peer-reviewed publications, principles, probabilistic genotyping, probabilistic genotyping software, proficiency tests, relevance, reliability assessment, receiver operating characteristic (ROC) curves, scientific foundation review, software reliability, technical merit evaluation, technology, validation studies

Citation

Butler, J. , Iyer, H. , Press, R. , Taylor, M. , Vallone, P. and Willis, S. (2024), DNA Mixture Interpretation: A NIST Scientific Foundation Review, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://doi.org/10.6028/NIST.IR.8351, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=959142 (Accessed April 6, 2025)