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

Area of Science:

• Forensic Science
• Genetics
• Molecular Biology

Background:

• Standard interpretation rules for 28-cycle PCR analysis of mixed short tandem repeat (STR) profiles exist.
• Low Copy Number (LCN) DNA analysis presents challenges in distinguishing contributors within mixed profiles.

Purpose of the Study:

• To evaluate the reliability of established 28-cycle PCR interpretation guidelines when applied to 34-cycle PCR profiles.
• To assess the effectiveness of these rules in analyzing mixed STR profiles generated from known donor pairs at various ratios and DNA inputs.

Main Methods:

• 200 DNA mixtures from pairs of known donors (ratios 1:1, 2:1, 5:1) were analyzed using SGM Plus® with total DNA inputs from 1ng to 50pg.
• 35 Forensic Science Service (FSS) reporting officers, trained in LCN interpretation, blindly analyzed duplicate profiles following standard casework procedures.
• Decisions on processing a third aliquot were based on initial duplicate 34-cycle PCR results.

Main Results:

• 49 consensus profiles were successfully resolved into major and minor contributor peaks.
• No minor contributor peaks were incorrectly assigned to a major profile in the final reported results.
• Profiles lacking clear differentiation could be resolved if one contributor's profile (e.g., victim) was known.

Conclusions:

• The interpretation rules for standard 28-cycle SGM Plus analysis are reliable when applied to 34-cycle generated profiles by experienced analysts.
• The study demonstrates the robustness of these interpretation guidelines for complex DNA mixture analysis.
• Successful resolution of major and minor contributors supports the validity of the established rules in forensic casework.