Validation of a top-down DNA profile analysis for database searching using a fully continuous probabilistic genotyping model.

Slooten described a method of targeting major contributors in mixed DNA profiles and comparing them to individuals on a DNA database. The method worked by taking incrementally more peak information from the profile (based on the peak contribution), and using a semi-continuous model, calculating likelihood ratios for the comparison to database individuals. We describe the performance of this "top down approach" to profile interpretation within probabilistic genotyping software employing a fully continuous model. We interpret both complex constructed profiles where ground truth is known and casework profiles from non-suspect crimes. The interpretation of constructed four- and five- person mixtures demonstrated good discrimination power between contributors and non-contributors to the mixtures. Not all known contributors linked, and this is expected, particularly for minor contributors of DNA to the profile, or when the DNA from contributors was in relatively equal contributions. This finding was also reported by Slooten for the semi-continuous application of the approach. The maximum observed LR was shown to not exceed the LR obtained after a standard interpretation approach outside of that expected due to Monte Carlo variation. The interpretation of 91 complex profiles from no-suspect casework demonstrated that approximately 75% of profiles returned a link to someone on a database of known individuals. With a yearly average of 110 no-suspect cases that fall into this too-complex category at Forensic Science SA, the top down analysis, if applied to all such profiles, would represent an increase of 83 links per year of investigative information that could be provided to investigators.

Assessment of the Yfiler® Plus PCR amplification kit for the detection of male DNA in semen-negative sexual assault cases.

The ability to detect male epithelial cells deposited during digital penetration or penile penetration without ejaculation is limited by the sensitivity of the Y-STR profiling kit. In this study, the relative profiling success of the Thermofisher Yfiler® Plus kit was compared to its predecessor, AmpFlSTR Yfiler®, for 104 semen-negative sexual assault samples from casework at Forensic Science SA, Adelaide, South Australia. Yfiler Plus generated allele information in 25% more samples than Yfiler and gave a higher recovery of informative alleles in all but two samples where detectable male DNA was present. Where a profile was obtained in both kits, 92% of samples gave a higher percentage of informative loci with Yfiler Plus compared to Yfiler. Yfiler Plus also resolved DNA mixtures in 15 samples as compared to 1 sample with Yfiler. Detection of male DNA with the Quantifiler™ Trio DNA Quantification kit was shown to correlate with a successful profiling outcome with Yfiler Plus. The success of profiling with Yfiler Plus was independent of the time elapsed between the alleged offence and the sample being collected, the type of sexual penetration which occurred, and the anatomical origin of the sample.