‘Stochastic’ effects at balanced mixtures: A calibration study

DNA mixtures are challenging not only at low template DNA level but also at highly balanced quantitative ratio. In this latter case, interpretation may be complicated by the joint action of combinatorial uncertainty and stochastic effects of the PCR. We explore this particular and so far little noticed aspect of mixture interpretation by first providing a complete quantitative combinatorial analysis of the two-person mixture model (2PM) at highly balanced ratio of contributors, and then by carrying out a calibration study of the 2PM model on good quality experimental mixtures. The calibration tests provided the evidence for the existence of irregular distribution of peak heights, that can misguide the correct genotype assignment at high template ratios too. Repeating the experiment, performing Bayesian analysis to the whole evidence and developing a careful joint prediction of all plausible genotype datasets is highly mandatory in these cases, prior to set evidentiary LRs and use them in court.