Extending biogeographic ancestry inference to minor contributors in forensic DNA mixtures

DNA mixtures are common in forensic samples and limit the effectiveness of recently developed intelligence tools based on novel sequencing technologies, such as biogeographic ancestry prediction. DIP-STR markers, which combine a low-mutation insertion-deletion (DIP) with a closely linked high-mutation short tandem repeat (STR), enable allele-specific detection of minor contributors in two-person DNA mixtures and contain information on population structure. In this study, we evaluated a set of 46 DIP-STR markers across globally representative populations to assess their potential for biogeographic ancestry inference. Using both clustering and likelihood-based assignment approaches, major continental groups remain distinguishable even when the genetic information is limited to the small fraction of markers detecting the minor component of a mixture. These data show that DIP-STRs offer a complementary solution by providing both minor DNA detection and ancestry information in scenarios where standard panels struggle, such as challenging mixed DNA samples. Furthermore, modelling the distinct evolutionary dynamics of DIPs and STRs within haplotypes has the potential to improve both the resolution and robustness of ancestry inference. Together, these findings establish DIP-STRs as a viable tool for forensic ancestry analysis and provide a global reference dataset for future method development.