Validation of a combined autosomal/Y-chromosomal STR approach for analyzing typical biological stains in sexual-assault cases

Abstract

DNA testing is an established part of the investigation and prosecution of sexual assault. The primary purpose of DNA evidence is to identify a suspect and/or to demonstrate sexual contact. However, due to highly uneven proportions of female and male DNA in typical stains, routine autosomal analysis often fails to detect the DNA of the assailant. To evaluate the forensic efficiency of the combined application of autosomal and Y-chromosomal short tandem repeat (STR) markers, we present a large retrospective casework study of probative evidence collected in sexual-assault cases. We investigated up to 39 STR markers by testing combinations of the 16-locus NGMSElect kit with both the 23-locus PowerPlex Y23 and the 17-locus Yfiler kit. Using this dual approach we analyzed DNA extracts from 2077 biological stains collected in 287 cases over 30 months. To assess the outcome of the combined approach in comparison to stand-alone autosomal analysis we evaluated informative DNA profiles. Our investigation revealed that Y-STR analysis added up to 21% additional, highly informative (complete, single-source) profiles to the set of reportable autosomal STR profiles for typical stains collected in sexual-assault cases. Detection of multiple male contributors was approximately three times more likely with Y-chromosomal profiling than with autosomal STR profiling. In summary, 1/10 cases would have remained inconclusive (and could have been dismissed) if Y-STR analysis had been omitted from DNA profiling in sexual-assault cases.

Forensic use of Y-chromosome DNA: a general overview

“…Due to the achieved high haplotype diversity, these tools allow for the characterization of a paternal lineage with high, albeit not maximal, degree of certainty, especially when the tested sample donor comes from an outbred population (Purps et al 2014; Vermeulen et al 2009). Moreover, these commercial Y-STR kits allow the detection and characterization of DNA from males in mixed stains with high excess of DNA from females, also in cases with very low quantities of DNA from the minor male contributor as typical in material from sexual assault (Purps et al 2015). Recommendations on forensic analysis of Y-STRs have been established by the DNA Commission of the International Society of Forensic Genetics (Gill et al 2001; Gusmao et al 2006), and the Y-STR kits have forensically been validated (Gopinath et al 2016; Krenke et al 2005; Mulero et al 2006; Thompson et al 2013).…”

“…This allows forensic practitioners not only to exclude male suspects from being involved in a crime via non-matching Y-STR haplotypes, but also to identify the paternal lineage that a trace donor belongs to via matching Y-STR haplotypes (Roewer 2009). For example, a recent study of hundreds of sexual assault cases, where Y-STR haplotyping had been applied together with standard autosomal STR profiling, showed that one tenth of these cases would have remained inconclusive without the use of Y-STRs, and furthermore, Y-STR haplotyping was three times more suitable to identify multiple male contributors than autosomal STR profiling (Purps et al 2015). …”

Joint Genetic Analyses of Mitochondrial and Y-Chromosome Molecular Markers for a Population from Northwest China

“…Another marker system used in this study is Y-STRs, which contribute to plenty of Y chromosomal haplotypes for forensic applications, historical researches, and genealogical investigation of paternal relatives [13,14]. Based on that, there is a large public database, named the Y-STR Haplotype Reference Database (YHRD, https://yhrd.org), which contains a large number of Y-STR haplotypes from different populations all over the world, can be used as reference population data.…”

Case report: Coincidental inclusion in a 17-locus Y-STR mixture, wrongful conviction and exoneration

“…These Y-STRs are particularly useful in cases where an evidentiary mixed DNA sample has a very low concentration of male DNA, and a high concentration of female DNA (i.e. sexual assaults) [1][2][3]. Y-STR typing can also be used to resolve complex relationship testing [4].…”