Forensic utilization of NGS-STRs and evaluation of system efficacy for different kinship identifications

The accurate identification of complex kinship relationships remains a significant challenge in forensic practice. Traditional kinship identification methods, which primarily rely on length polymorphisms of short tandem repeats (STRs), often face difficulty in achieving sufficient discriminatory power for complex relationships due to the limited genetic information they provide. While next-generation sequencing (NGS) enables the detection of allelic sequence polymorphisms within STRs, its practical value for different kinship analyses in specific populations requires comprehensive evaluation. To this end, the present study investigated the genetic polymorphisms of 52 STRs, with a focus on both length and sequence variations, aiming to evaluate the system efficacy and forensic application value of the amplification system in the forensic identification of complex kinship analyses.

The 52 STRs were highly polymorphic in the studied Baoan group. The acquisition of sequence polymorphism information significantly enhanced the genetic polymorphisms of STRs, and the number of alleles increased by 61.00% compared to length-based polymorphisms alone. Kinship performance was evaluated by simulating 1,000 kinship pairs and 1,000 unrelated individual pairs on the basis of the allele frequencies of 52 STRs, and the influence of different combinations of STR loci on the identification efficacies of different kinship was assessed by using the likelihood ratio (LR) method and identical by state (IBS) method, respectively. When the LR was greater than 10,000 or less than 0.0001 as the judgment threshold, the system efficacy of the 52 STR loci based on sequence polymorphisms for forensic identifications of full siblings and unrelated individuals was 99.85%, and those of half-siblings, grandparents-grandchildren, uncle-nephews and unrelated individuals were 61.50%, 60.95%, and 61.00%, respectively.

The availability of sequence polymorphism data and the increased number of STR loci could enhance the efficacy of the detection systems for kinship identifications to a certain extent. These results highlight the potential of combining length and sequence polymorphisms of STRs in forensic practice, offering a valuable tool for addressing the challenge of complex kinship identification. Future research should validate these thresholds in casework samples and expand the number of STR loci to enhance the detection efficacy for distant relationships.