Module 1: The Elements of DNA Profile Interpretation and Probabilistic Genotyping

Module 1: The Elements of DNA Profile Interpretation and Probabilistic Genotyping

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Probabilistic Genotyping of Evidentiary DNA Typing Results – An Online Workshop Series
Module 1: The Elements of DNA Profile Interpretation and Probabilistic Genotyping

Wednesday, May 1st, 2019 from 12:00:00 PM ET – 4:00:00 PM ET
Duration: 4 hour(s)

Based on the review of various forms of STR typing results and factors that impact the presence or condition of DNA on evidence, instructors in this installment of Probabilistic Genotyping of Evidentiary DNA Typing Results lay a foundation for the interpretation of forensic DNA typing results. The session will begin with background information on forensic DNA analysis, including terminology and mechanisms, to help all participants comprehend the material covered in this series. A retrospective view of approaches to interpretation and statistical analyses will be reviewed for the purpose of understanding the origin of strategies, methods, issues and solutions that comprise the history of forensic STR testing.

A framework to interpret evidence will be detailed as follows:

An introduction to propositions and the likelihood ratio
Common pitfalls in giving statements – prosecutor’s fallacy
The hierarchy of propositions – beyond the DNA profile (how, why, when?)
Pitfalls – the problem of carry-over of the LR to higher levels in the hierarchy
How can DNA transfer – an introduction
An example statement
The need for experiments
Where it goes wrong and why – the trial of Amanda Knox

Interpretation at the sub-source level will be addressed from a historical perspective as follows:

What is low template DNA?
The consensus method
Why a universal method of interpretation is desirable
Drop-out, drop-in and stochastic thresholds
Heterozygote balance
Inclusions and exclusions
Qualitative and quantitative continuous models

Particular challenges, including those encountered by the Texas Department of Public Safety, will be highlighted as they continue to be relevant as improved technologies and commercial products have evolved. In preparation for subsequent modules, the tenets and origins of probabilistic genotyping will be introduced.

Detailed Learning Objectives:

1) Interpret STR typing results that originate from a single DNA contributor and specify the features that can create complexity in the interpretation of mixtures of DNA from multiple contributors

2) Distinguish STR typing results that originate from limited DNA template and recognize the intricacies to interpretation introduced by variation in allelic peak heights, allele dropout, stutter, peak sharing, inhibition of amplification and DNA degradation

3) Identify factors in the transfer and persistence of DNA on evidence and constraints to conclusions that may be drawn

4) Conceptualize the strategy as well as the benefits of probabilistic genotyping in the interpretation of forensic DNA typing results.

Dr. Tamyra Moretti – Federal Bureau of Investigation, Quantico, Virginia
Dr. Peter Gill – University of Oslo, Oslo, Norway
Lynn Garcia – Texas Forensic Science Commission, Austin, Texas

Recommended Reading:

Bieber, F.R, Buckleton, J.S., Budowle, B., Butler, J.M. and Coble, M.D. Evaluation of forensic DNA mixture evidence: protocol for evaluation, interpretation and statistical calculations using the combined probability of inclusion. BMC Genetics, 2016. 17: 125-39.

Coble, M.D. and Bright, J.-A., Probabilistic genotyping software: An overview. Forensic Sci. Int. Genet., 2019. 38: 219-24.

ENFSI guideline for evaluative reporting in forensic science, Strengthening the Evaluation of Forensic Results across Europe (STEOFRAE). European Network of Forensic Science Institutes 2015.

Evett, I.W., et al., Interpreting small quantities of DNA: the hierarchy of propositions and the use of Bayesian networks. J Forensic Sci, 2002. 47(3): p. 520-30.

Evett, I.W. and B.S. Weir, Interpreting DNA Evidence – Statistical Genetics for Forensic Scientists. 1998, Sunderland: Sinauer Associates, Inc.

Gill, P., Analysis and implications of the miscarriages of justice of Amanda Knox and Raffaele Sollecito. Forensic Science International: Genetics, 2016. 23: p. 9-18.

Gill, P., Misleading DNA evidence: Reasons for miscarriages of justice. 2014, London: Elsevier.

Gill, P., Haned, H., Bleka, O., Hansson, O., Dorum, G. and Egeland, T. Genotyping and interpretation of STR-DNA: Low-template, mixtures and database matches-Twenty years of research and development. Forensic Sci. Int. Genet., 2015. 18: 100-17.

Making Sense of Forensic Genetics 2017,

Moretti, T.R. An attorney’s guide to forensic DNA testing and testimony. U.S. Department of Justice (in press).

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Funding for this Forensic Technology Center of Excellence event has been provided by the National Institute of Justice.


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