General Guidelines for Categorization and Interpretation of Mixed STR DNA Profiles

• NCJ Number: 217859
• Journal: Canadian Society of Forensic Science Journal Volume: 39 Issue: 4 Dated: December 2006 Pages: 179-216
• Author(s): Ray Wickenheiser
• Date Published: December 2006
• Length: 38 pages
• Publication Format: Article
• Publication Type: Guideline
• Language: English
• Country: Canada

Annotation

This article presents SWGDAM's (Scientific Working Group on DNA Analysis Method's) guidelines for interpreting mixed DNA profiles (more than one individual has contributed biological material to a sample).

Abstract

The author discusses the LSD (Least Squares Deconvolution) objective mathematical approach to interpreting two-person mixtures, as well as the Combined Probability of Inclusion/Probability of Exclusion method. The guidelines recommend separating mixed DNA profiles into six specific types, which simplifies the process of identifying individual contributors. One type consists of a set of two mixed DNA profiles that contain a male and a female contributor, specifically a sperm (male) and an epithelial (female or nonsperm). The second mixture type is a mixed DNA profile obtained from a single sampling of a specimen (usually nonsemen-related), to which a forensically valid assumption can be applied. The third type of mixture has a distinct major contributor and a trace minor contributor, and the fourth type of mixture is a mixed profile with one major contributor and one minor contributor. The fifth mixture type is a mixed profile with one major contributor and more than one minor contributor. The sixth type of mixture is a mixed profile with no distinct major and/or minor contributors. The separation of mixed DNA profiles into distinct types retains significant case-specific information by considering relevant known DNA profiles. If a forensically valid assumption can be made regarding the presence of a contributor expected to be present, then consideration of the contribution of a known sample can be used in deducing another contributor. If no assumptions can be made based on case-specific knowledge, a more conservative interpretive approach is warranted. Where appropriate, indication of a sole source may be possible. 8 figures, 2 tables, and 34 references