Mechthild Prinz, Andrea Coleman, Asako Ishii, Howard J. Baum, and Robert C. Shaler

Department of Forensic Biology, Office of Chief Medical Examiner, New York City

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Forensic DNA typing using Y chromosome specific polymorphisms promises a new rapid approach to screening rape case evidence, and will also be valuable for unknown body identification and paternity cases involving male individuals.

From a panel of recently published Y chromosome specific STRs (M.A. Jobling and C.Tyler-Smith, Trends in Genet. 11 (1995) 449-456), we chose seven loci for a forensic validation following TWGDAM guidelines. The seven STRs were combined in two multiplex reactions consisting of a quadruplex containing DYS19, DS389 I, DYS389 II, and DYS390, and a triplex DYS391, DYS392, and DYS393 (M. Kayser, et al., Int. J. Leg. Med., in print).

The validation experiments dealing with standard specimen, consistency, reproducibility, and stain substrates yielded no surprising results, the expected donor type could be determined in all cases. For the non-human studies the results were also similar to autosomal STRs. DNA from male primates showed distinct allele peaks, while DNA from other animals like dog, cow or frog did not yield any amplification product.

For the evaluation of the Y STR multiplex systems using nonprobative evidence, 56 semen containing swabs or stains were differentially extracted, and typed with either HLADQA1/PM or an autosomal STR multiplex (vWA, TH01, F13A1, FES/FPS). Cuttings from the same samples were also extracted using a non-differential extraction method, and subsequently typed for the Y STR systems. While for the autosomal systems 41% of the samples did not show a male type, for the Y STRs only 29% did not yield any result. 20% of the samples typed after the one step extraction showed at least a partial profile. The biggest advantage of the Y STR approach revealed itself not in the overall success rate but in the final interpretation of the typing results. In 21% of the samples, the autosomal polymorphisms revealed a mixture, 34% of the samples had a clean, or at least predominant male type. While especially for the PM system, the occurrence of overlapping alleles between victim and suspect quite often forces the analyst to make a decision based on one locus, the Y STR results always provided unambiguous exclusions or inclusions for all tested loci. Based on the Y STR results it could also be determined that several of the severely mixed types, consisted of DNA from more than one semen donor. The latter will be extremely useful to know for interpreting DNA mixtures in vaginal swabs. Anticipated fields of application and statistical issues will be discussed.