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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This module originally occurred on May 1, 2019
Duration: 4 hours


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

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
  • Conclusions

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

Funding for this Forensic Technology Center of Excellence webinar has been provided by the National Institute of Justice, Office of Justice Programs, U.S. Department of Justice.

The opinions, findings, and conclusions or recommendations expressed in this webinar are those of the presenter(s) and do not necessarily reflect those of the U.S. Department of Justice.

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