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