An interpretive, population-structure-centred forensic analysis demonstrating how Y-STR population differentiation affects evidential robustness and database choice

Y-chromosomal short tandem repeats (Y-STRs) are widely used in forensic genetics, yet the robustness of evidential interpretation depends not only on marker diversity but on the degree of population differentiation underlying reference database selection. This study evaluates Y-STR population structure among South African population groups using the 27-locus Yfiler® Plus system, treating population differentiation as the primary analytical endpoint. Population structure was quantified using analysis of molecular variance (AMOVA), pairwise Nei’s genetic distance, and principal coordinates analysis (PCoA). To translate population structure into forensic relevance, a sensitivity framework was applied to assess how population misassignment alters haplotype frequency estimation and the resulting likelihood-based evidential weight. Standard forensic diversity parameters are reported only for contextual comparison and are provided in the Supplementary Material. The findings demonstrate how population-structure metrics can be operationalized to support transparent database selection, mitigate interpretive overstatement, and strengthen the robustness of Y-STR evidence in forensic reporting and testimony.