Triallelic STR patterns in indian parentage testing: Observations from 15,000 consecutive STR profiles

Triallelic patterns at autosomal short tandem repeat (STR) loci are uncommon findings in forensic genetics but may complicate the interpretation of parentage and kinship analyses. In this retrospective study, we reviewed 15,000 consecutive autosomal STR profiles generated during routine parentage testing between January 2019 and July 2025 to characterize the occurrence and distribution of triallelic STR patterns in an Indian parentage-testing dataset. Fifteen STR profiles exhibited triallelic patterns, corresponding to an observed profile-level proportion of 0.1%. Triallelic patterns were detected at six STR loci: D21S11 (n = 7), D1S1656 (n = 3), D10S1248 (n = 2), and one case each at TPOX, CSF1PO, and D3S1358. Type II patterns predominated (10/15, 66.7%), whereas Type I patterns accounted for 5/15 (33.3%). Among the 15 triallelic-positive profiles, the corresponding parentage investigations comprised 7 maternity duo investigations, 6 paternity duo investigations, and 2 trio investigations. These findings highlight the importance of recognizing and appropriately interpreting triallelic STR patterns during routine parentage testing and provide additional reference data from an Indian parentage-testing dataset.