Interpreting a Major Component From a Mixed DNA Profile With an Unknown Number of Minor Contributors

• NCJ Number: 254169
• Author(s): Todd Bille; Steven Weitz; John Buckleton; Jo-Anne Bright
• Date Published: May 2019
• Length: 10 pages
• Grant Number(s): 2017-DN-BX-0136
• Agencies: NIJ-Sponsored
• Publication Type: Research (Applied/Empirical); Report (Study/Research); Report (Grant Sponsored); Program/Project Description
• Award: Population Genetic Issues for Forensic DNA Profiles

Annotation

This study examined mixed DNA profiles of varying complexity and interpreted them by altering the assigned number of contributors (N).

Abstract

Modern interpretation strategies typically require an assignment of the number of contributors (N) to a DNA profile. This can prove to be a difficult task, particularly when dealing with higher order mixtures or mixtures where one or more contributors have donated low amounts of DNA. Differences in the assigned N at interpretation can lead to differences in the likelihood ratio (LR). If the number of contributors cannot reasonably be assigned, then an interpretation of the profile may not be able to be progressed. The current project assigned LRs for true- and non-contributors and compared the results given different assignments of N over a range of mixture proportions. When a component of a mixture had a proportion of at least 10 percent, a ratio of at least 1.5:1 to the next highest component, and a DNA amount (as determined by STRmix) of at least 50 rfu, the LR of the component for a true contributor was not significantly affected by varying N and was therefore suitable for interpretation and the assignment of an LR. LRs produced for minor contributors were found to vary significantly as the assigned N was changed. These heuristics may be used to identify profiles suitable for interpretation. (publisher abstract modified)