Y-STR testing is useful in criminal casework, particularly for sexual assault cases where the amount of male DNA is low, or masked by a dominant female contribution.

Despite probabilistic genotyping (PG) methods being widely accepted as the preferred interpretation method for autosomal short tandem repeat (STR) profiles, the interpretation of Y-STR profiles remains largely a manual process following a binary model where haplotype combinations are either accepted or not. Resolved haplotypes then have a weight of evidence assigned for the person of interest, typically using either the profile frequency estimators based on counts observed in databases such as the YHRD, or based on Discrete Laplace parameters calculated from the database [1].

The interpretation of Y-STR data presents challenges, especially in complex mixtures with multiple male contributors. These challenges are compounded by the lack of consensus-driven interpretation frameworks, and the variability in how laboratories handle stochastic effects, PCR artefacts, and multi-copy loci, leading to subjectivity in reported casework.

In this study we explore whether a probabilistic genotyping method can address these issues with the interpretation of Y-STR profiles. After interpretation using PG, likelihood ratios were assigned for known and non-donors using a Haplotype Centred (HC) method [2].

We provide early results of a sensitivity and specificity study for a range of mixed Y-STR profiles of varying quality and quantity.

Authors

• Emma S. Marie (New Zealand Institute for Public Health and Forensic Science, New Zealand)
• Jo-Anne Bright (New Zealand Institute for Public Health and Forensic Science, New Zealand)
• Paul Stafford-Allen (New Zealand Institute for Public Health and Forensic Science, New Zealand)
• Zane Kerr (New Zealand Institute for Public Health and Forensic Science, New Zealand)
• Hannah Kelly (New Zealand Institute for Public Health and Forensic Science, New Zealand)
• John S. Buckleton (New Zealand Institute for Public Health and Forensic Science, New Zealand)

References

[1] L. Roewer, M.M. Andersen, J. Ballantyne, J.M. Butler, A. Caliebe, D. Corach, M.E. D’Amato, L. Gusmão, Y. Hou, P. de Knijff, W. Parson, M. Prinz, P.M. Schneider, D. Taylor, M. Vennemann, S. Willuweit, DNA commission of the International Society of Forensic Genetics (ISFG): Recommendations on the interpretation of Y-STR results in forensic analysis, Forensic Sci. Int. Genet. 48 (2020) 102308.

[2] D. Taylor, J. Curran, J. Buckleton, Likelihood ratio development for mixed Y-STR profiles. Forensic Sci Int Genet. 35 (2018) j.fsigen.2018.03.006.