MaSTR™: an effective probabilistic genotyping tool for interpretation of STR mixtures associated with differentially degraded DNA

The recently developed probabilistic genotyping software package MaSTR™ (SoftGenetics LLC) was used to develop statistical weight estimates for a variety of two-person STR mixture profiles with differentially degraded sources of DNA. A total of 864 analyses, on 144 two-person profiles, were performed. Mixture ratios ranged from 1:1 to 1:10, including pristine sources of DNA and various combinations of artificially degraded DNA (average size fragments of 150 or 250 bps). Quantities of DNA template were varied (0.1 to 0.5 ngs of total input) and MaSTR™ analysis was performed with eight chains of 10,000 or 40,000 iterations, with or without a conditioning profile to generate likelihood ratio (LR) values. Overall, the software performed as expected. The resulting log(LR) values for pristine mixture profiles were typically greater than 10³⁰. Lower-quality mixture data associated with sources of DNA at ~ 0.05 ngs for each contributor resulted in peak imbalance and allelic dropout which reduced the weight in support of a contributor. This was exacerbated by higher levels of degradation, with some instances resulting in log(LR) values in support of an exclusion. These studies provide additional support for the use of probabilistic genotyping software solutions in forensic investigations, addressing concerns raised by the President’s Council of Advisors on Science and Technology (PCAST).