Analysis and interpretation of mixed profiles generated by 34 cycle SGM Plus^® amplification

Abstract 

Interpretation rules for standard 28 cycle PCR have been described previously for the analysis of mixed STR profiles. In this study the same guidelines are applied to 200 mixtures derived from pairs of known donors combined in ratios of 1:1, 2:1 and 5:1 which have been profiled in duplicate with SGM Plus^® at total inputs ranging from 1ng to 50pg. The paired profiles were distributed among 35 FSS (Forensic Science Service) reporting officers trained in low copy number (LCN) interpretation who analysed them blind following standard casework procedures. Based upon the results from initial duplicate 34 cycle PCR reactions, the reporting officers made appropriate decisions regarding the benefits of processing the reserved third aliquot. Using the combined results, 49 consensus profiles were successfully resolved into major and minor contributor peaks. This demonstrates the reliability of the interpretation rules used in standard 28 cycle SGM Plus analysis when applied to 34 cycle generated profiles by trained and experienced reporting officers. No minor contributor peaks were assigned to a major profile in the final reported results. Those profiles which did not show sufficiently marked and consistent differentiation into major and minor peaks would have been correctly resolved if the profile of one contributor (e.g. the “victim”) was known.

Keywords: Low template DNA (LT-DNA), Low copy number (LCN), Mixture analysis, Mixture interpretation, Drop-in contamination