Examination of the variability in mixed DNA profile parameters for the Identifiler multiplex

The interpretation of mixed DNA profiles is often undertaken by determining to what extent possible genotype combinations may be eliminated from consideration. This requires an understanding of the variability in height or area for the two alleles of a heterozygote (heterozygote balance), and in the variability of mixture proportion or mixture ratio across loci. We present here empirical data to help assess the magnitude of this variability for Identifiler profiles. We find that heterozygote balance is affected by peak height as expected and that at heights above 267RFU the two peaks of a heterozygote demonstrates a peak height ratio between 0.6 and 1.66. For mixtures we introduce a concept, APH, the average peak heights of the active alleles. Above an APH of 110RFU mixture proportion demonstrated variation no more than 0.2 from the average across loci. Mixture ratio variability was affected by both the mixture ratio, M(r) per se and by the APH. The function (2APH/(1+M(r))) appears to be a reasonable predictor of the variability and approximates the expected peak height of the minor alleles. At (2APH/(1+M(r)))> or =300RFU the mixture ratio at a locus is expected to be within a factor of 2 of the average across loci.