Interpretation of Y Chromosome STRs for Missing Persons Cases

Y chromosome STR haplotypes have been used in assisting forensic investigations primarily for identification and male lineage determination. The Scientific Working Group on DNA Analysis Methods (SWGDAM) Lineage Marker Committee published interpretation guidelines for Y-STR typing, which provide helpful guidance. However, these guidelines do not address the issue of kinship analysis with Y-STR haplotypes. Because of the high mutation rate of Y-STRs, there are complex missing person cases in which inconsistent Y-STR haplotypes between true paternal lineage relatives will arise and cases with two or more male references in the same lineage and yet differ in their haplotypes. Therefore, more useful guidelines are needed for interpretation of Y-STR haplotype data. Computational methods and interpretation guidelines have been developed specifically addressing this issue, either using a mismatch based counting method or a pedigree likelihood ratio method. However, these methods and guidelines have not been adopted by forensic laboratories, likely because of a lack of specific procedures and software to facilitate analyses.

The Missing Persons Unit under the Center of Human Identification at the University of North Texas Health Science Center (UNTCHI) specializes in the DNA analysis and identification of missing persons cases and processes >50% unidentified human remains of the missing persons cases in the US. It is common within UNTCHI to encounter complex cases that would be better served with enhanced Y-STR interpretation procedures. With the experiences and resources at UNTCHI, more sophisticated interpretation methods and guidelines for Y-STR applications are implemented, which include pairwise comparison with a mismatch-step-based counting method to quickly determine if two profiles are from the same male lineage, and a pedigree likelihood ratio (PLR) based method to evaluate the evidence weight of Y-STR profiles, particularly for complex missing persons cases. A software program is under development and will be validated to facilitate Y-STR haplotype interpretation, and the software will be made accessible to the forensic community free of charge.