J.M. Laird, B.M. Monaghan and P.J. Newall
Centre of Forensic Sciences, Toronto, Canada

× Ø × Ø × Ø × Ø × Ø × Ø × Ø × Ø × Ø × Ø × Ø × Ø × Ø × Ø × Ø

An examination of non-probative casework was undertaken to assess the occurrence of differential amplification in subordinate profiles from DNA mixtures. Genomic DNA from multiple-source samples was amplified in the range of 1 ng to 62.5 pg. Allelic dropout was observed in subordinate profiles at various target levels and at different loci, depending on the sample. The occurrence of this stochastic phenomenon is not unexpected, since many mixtures go undetected until the typing state and because the relative and absolute quantities of the lesser source of DNA in a mixture often cannot be determined prior to amplification. We also report the findings of experiments undertaken to examine the effects of combining dual sources of DNA in various ratios prior to multiplex PCR. The ratio (dominant: subordinate) of DNAs up to which a full secondary profile was detected, decreased with decreasing template quantity. Furthermore, peak height ratios between dominant and subordinate profiles decreased with increasing template quantities. Results of these studies highlight the benefits of amplifying a series of target amounts when dealing with forensic DNA mixtures and of interpreting the ensuing profiles relative to each other. We include a probative casework example of this approach using the STR quadruplex.