Mixture Interpretation: Defining the Relevant Features for Guidelines for the Assessment of Mixed DNA Profiles in Forensic Casework

• NCJ Number: 227702
• Journal: Journal of Forensic Sciences Volume: 54 Issue: 4 Dated: July 2009 Pages: 810-821
• Author(s): Bruce Budowle Ph.D.; Anthony J. Onorato M.S.F.S., M.C.I.M.; Thomas F. Callaghan Ph.D.; Angelo Della Manna M.S.; Ann M. Gross M.S.; Richard A. Guerrieri M.S.; Jennifer C. Luttman M.F.S.; David Lee McClure B.S.
• Date Published: July 2009
• Length: 12 pages
• Publication Format: Article
• Publication Type: Issue Overview
• Language: English
• Country: United States of America

Annotation

This paper addresses various scenarios that should be considered in the development of clearer guidelines for interpreting and documenting assumptions in the assessment of mixed DNA profiles in forensic casework.

Abstract

Although a standardized approach for interpreting DNA mixtures is not possible or desirable, more guidance is needed in order to ensure the reliable interpretation of results. There are various ways that mixed DNA profiles occur. The aspects of mixture interpretation discussed in this paper should be considered as requisites for any documented mixture-interpretation guidelines. This ensures that any qualified forensic scientist would be able to understand the process followed within a laboratory for the purpose of evaluating any specific interpretation for its validity. The recommendations that the International Society of Forensic Genetics (ISFG) has offered provide basic considerations for mixture interpretation. The current paper provides additional considerations for interpreting mixtures. Interpretation of DNA mixture profiles requires consideration of a number of aspects of a mixed profile, many of which should be established by onsite, internal validation studies conducted by a laboratory's technical staff prior to performing casework analysis. The relevant features include criteria for the identification of mixed specimens, the establishment of detection and interpretation threshold values, the defining of allele peaks and nonallele peaks, the identification of artifacts, consideration of tri-allelic patterns, estimation of the minimum number of contributors, resolution of the components of a mixture, determination of when a portion of the mixed profile can be treated as a single-source profile, consideration of potential additive effects of allele sharing, the impact of stutter peaks on interpretation in the presence of a minor contributor, comparison with reference specimens, and issues related to the application of mixture calculation statistics. 1 table, 17 figures, and 15 references