Y-chromosomal short tandem repeats, or Y-STRs, are stretches of DNA in which short sequence motifs repeat several times. Because the Y chromosome passes almost unchanged from father to son, these markers are invaluable for tracing paternal lineages, resolving mixed male-female DNA samples at crime scenes, and identifying male relatives. For decades, forensic laboratories have typed Y-STRs using capillary electrophoresis, a technique that determines the number of repeat units by measuring the physical size of the amplified DNA fragment. The problem is that two alleles with the same total length can carry completely different internal repeat arrangements, and capillary electrophoresis cannot tell them apart.

Massively parallel sequencing, often called next-generation sequencing, changes this equation fundamentally. Instead of relying on fragment size, MPS reads the nucleotide sequence of each amplified marker directly, revealing repeat pattern variations inside the core repeat region as well as single-nucleotide substitutions and small insertions or deletions in both the repeat and flanking regions. When the researchers included flanking regions in their analysis, they detected 757 distinct alleles at the sequence level across the 86 loci, compared with only 491 alleles identified by length-based typing alone. That increase of more than 50 percent in allele count translates directly into greater discriminating power for forensic casework.

Among the 86 loci examined, 37 showed an increase in allele counts when sequence information was added, and eight of those exhibited increases exceeding 100 percent. Haplotype diversity, a measure of how genetically varied the set of Y-chromosomal profiles is within the population, rose from 0.9984 to 0.9986. Discrimination capacity, which reflects the proportion of unrelated individuals who can be distinguished from one another, improved from 0.8510 to 0.8642. While these numerical gains may appear modest, in forensic practice they mean that more innocent individuals can be excluded and more true contributors can be confidently identified when sequence data supplement length data.

The sequence analysis revealed three main categories of hidden variation. Repeat pattern variations occur when the arrangement of repeat units within the core repeat region differs among alleles of the same total length, for example when a continuous run of four-base repeats is interrupted by a different motif. Single-nucleotide substitutions involve the replacement of one DNA base with another, either inside the repeat array or in the sequences immediately adjacent to it. Insertions and deletions, collectively called InDels, involve the gain or loss of one or more bases in the repeat or flanking regions. Together, these variant types accounted for the observed increase in polymorphism and demonstrated that flanking regions, which are often overlooked, carry valuable forensic information.

Beyond the forensic gains, the study provides a window into the paternal genetic structure of the Yunnan Han population. Population genetic analysis indicated that the Yunnan Han are most closely related to Han populations from Sichuan and Chongqing, neighboring provinces in southwestern China. At the same time, all Han groups clustered separately from the ethnic minority populations examined in the comparison, reflecting the distinct paternal genetic heritage of the Han majority relative to surrounding groups. This fine-scale resolution of population structure is possible precisely because sequence-based typing captures more variation than length-based methods, allowing researchers to detect subtle differences among populations that would otherwise appear identical.

The implications for forensic practice are significant. In sexual assault cases, where male and female DNA are mixed, Y-STR profiling is often the only way to identify the male contributor. When two male suspects share the same Y-STR profile by length, sequence-based typing can now distinguish them, potentially preventing wrongful accusations. In paternal kinship testing, where the goal is to establish whether two men share a common male ancestor, additional sequence variation reduces the probability of coincidental matches between unrelated men. The study also contributes to the growing body of population-specific allele frequency data that forensic statisticians need to calculate match probabilities accurately in Chinese courts.

The research team, led by Liping Hu and Jiamei Jiang as co-first authors under the supervision of Shengjie Nie, Ming Zhao, and Linlin Liu, collected samples from 302 unrelated Han Chinese men in Yunnan Province. The work was supported by the Science and Technology Program of the Yunnan Provincial Department of Science and Technology. Reference population data for comparative analysis were obtained from publicly published literature, ensuring that the new Yunnan Han dataset could be placed in proper context alongside existing Chinese and international populations.

As massively parallel sequencing continues to replace capillary electrophoresis in forensic laboratories worldwide, studies like this one are building the empirical foundation for sequence-based Y-STR interpretation. The Yunnan Han findings confirm that reading the Y chromosome at the nucleotide level reveals hidden genetic variation, improves forensic identification of individuals, and illuminates the fine-scale paternal genetic structure of a population that has long been understudied at this resolution. For forensic scientists, the message is clear: the Y chromosome still has secrets to give up, and sequencing is the key.

Subject of Research: Sequence diversity and forensic efficiency of Y-STRs in the Han population of Yunnan Province, Southwest China

Article Title: Sequence diversity and forensic efficiency of Y-STRs in the Han population of Yunnan Province, Southwest China

Article References: Hu, L., Jiang, J., Zeng, Y., Ye, H., Ruan, J., Yang, T., Li, X., Lei, Y., Huang, R., Zhao, M., Liu, L., & Nie, S. (2026). Sequence diversity and forensic efficiency of Y-STRs in the Han population of Yunnan Province, Southwest China. International Journal of Legal Medicine. https://doi.org/10.1007/s00414-026-04027-1

Image Credits: AI Generated

DOI: 10.1007/s00414-026-04027-1

Keywords: forensic genetics, Y-STR, massively parallel sequencing, Yunnan Han, DNA profiling, population genetics, Sequence, diversity, forensic, efficiency, Y-STRs, population