Short tandem repeats, or STRs, are stretches of DNA in which a short sequence of two to six base pairs repeats over and over. The number of repeats at any given locus varies from person to person, which is precisely what makes STRs the backbone of modern forensic DNA profiling. Because these repeat counts are inherited, one copy from each parent, they allow investigators to compare a crime scene sample with a suspect’s profile, or to establish biological relationships in paternity cases. But the statistics underpinning those comparisons are only valid if laboratories know how common each repeat variant, or allele, is in the relevant population. A profile match that looks overwhelming in one population may carry different weight in another, which is why population-specific frequency databases are a cornerstone of ethical and accurate forensic practice.

The research team, led by Brajesh Kumar and Amitabh Biswas of Galgotias University’s School of Forensic Sciences, together with colleagues from the Central Forensic Science Laboratory in Chandigarh, Ingenomics India, and the Regional Forensic Science Laboratory in Jabalpur, used the Ingenomics AutoProfiler STR Kit to amplify all 24 markers simultaneously. This six-dye multiplex system, which was independently validated for forensic applications, covers the expanded set of loci now favored internationally, including the highly variable SE33 and Penta E markers alongside familiar names such as FGA, vWA, and TH01. Genotyping was carried out by capillary electrophoresis, and the resulting allele calls were subjected to statistical analysis using established population genetics software, including Arlequin, GenAlEx, and PowerStats.

The headline numbers are striking. Across the 24 loci, the probability of match, which expresses the chance that two randomly selected unrelated individuals share the same genotype, ranged from 0.012 to 0.186. Inverted into its more familiar form, the power of discrimination ranged from 0.814 to 0.988, meaning the best markers could distinguish between any two individuals in the population with near certainty. SE33 topped the list with a discrimination power of 0.988, followed closely by Penta E at 0.983 and FGA at 0.962. At the other end of the spectrum, CSF1PO at 0.814 and D2S441 at 0.844 were comparatively modest performers, a pattern consistent with their lower allelic diversity in populations worldwide.

Polymorphism information content, a measure of how informative a marker is for linkage and identity analysis, ranged from 0.601 to 0.933, with the majority of loci exceeding 0.7, a threshold conventionally regarded as indicating high polymorphism. Gene diversity values, which capture the expected heterozygosity at each locus, spanned 0.666 to 0.939, confirming that the Santhal sample carries substantial genetic variability. Observed and expected heterozygosities were calculated for every locus, and tests against Hardy-Weinberg equilibrium were performed to verify that allele frequencies behave as expected under random mating, a critical quality check before any forensic statistic can be considered reliable.

For paternity testing, the study’s parameters matter even more directly. The power of exclusion, the probability that a randomly selected man would be excluded as the biological father of a child, ranged from 0.428 to 0.913 across individual loci, with SE33 and Penta E showing the strongest exclusion capacity. The typical paternity index varied from 1.667 to 11.667, again led by the most polymorphic markers, including SE33, Penta E, and D12S391. When the loci were combined, the results became extraordinary: the combined power of exclusion reached 0.999999999991, the combined paternity index stood at 3.51 multiplied by ten to the tenth power, and the combined probability of match fell to 2.41 multiplied by ten to the power of negative 28. The combined power of discrimination, rounded, was effectively 1. In practical terms, the chance that two unrelated Santhal individuals would share an identical 24-locus profile is so vanishingly small that a matching profile constitutes evidence of identity at a level few other forms of forensic evidence can approach.

Why does population-specific data of this kind matter so much? Forensic statistics rest on the product rule: the rarity of a full profile is estimated by multiplying the frequencies of the individual alleles, an operation that assumes statistical independence within the population under consideration. If a population has a history of endogamy, consanguinity, or founder effects, as many isolated or semi-isolated communities do, allele frequencies can drift in ways that inflate or deflate match probabilities. The Santhal, an Austroasiatic-speaking tribal community with a distinct demographic history in eastern India, are precisely the kind of population for which generic national databases may not be adequate. By generating a dedicated allele frequency dataset, the study gives forensic practitioners in Bihar and neighboring states a defensible statistical foundation when the source of a sample may belong to this community.

The work also contributes to the broader scientific picture of Indian genetic diversity. Previous studies have documented STR polymorphism in Chhattisgarh, Himachal Pradesh, northwestern India, and among the Sindhi population, and genomic research on Austroasiatic speakers has highlighted the role of landscape barriers and sex-specific admixture in shaping their genetic structure. Each new population dataset refines the map of how genetic variation is distributed across the subcontinent, and the Santhal data will allow comparisons that illuminate both forensic and anthropological questions. The authors note that the 24 loci are highly informative not only for individual identification but also for kinship analysis, extending their utility to disaster victim identification, missing persons investigations, and complex relationship testing.

The study was approved by the Institutional Ethics Committee of Apex Hospital in Varanasi, and the authors report no conflicts of interest and no external funding. All participants were unrelated volunteers who consented to contribute samples, and the authors acknowledge their role in enabling the research. The sampling strategy, restricted to unrelated individuals, is essential for Hardy-Weinberg testing and for ensuring that the frequency estimates reflect the general population rather than clustered family lineages.

As DNA evidence continues to expand its role in the Indian judicial system, studies like this one quietly underpin every statistic that appears in a courtroom report. A match probability of ten to the negative 28 is only as credible as the allele frequency table behind it, and that table must reflect the population to which a suspect or victim belongs. With the Santhal community now represented in the forensic literature by a rigorously characterized 24-locus dataset, investigators and courts in Bihar gain a resource that makes DNA statistics for this population both more accurate and more defensible. It is a reminder that the power of forensic genetics depends not just on the chemistry of amplification and detection, but on the patient, community-by-community work of measuring human variation itself.

Subject of Research: Forensic genetic profiling of autosomal STR markers in the Santhal tribal population of Bihar, India

Article Title: Allele frequency distribution and forensic efficiency parameters of 24 autosomal STR markers in the santhal tribal community of Bihar, India

Article References: Kumar, B., Biswas, A., Dixit, S., Sharma, S., & Shrivastava, P. (2026). Allele frequency distribution and forensic efficiency parameters of 24 autosomal STR markers in the santhal tribal community of Bihar, India. The Science of Nature, 113(6), Article 125. https://doi.org/10.1007/s00114-026-02176-2

Image Credits: AI Generated

DOI: 10.1007/s00114-026-02176-2

Keywords: forensic genetics, STR markers, Santhal, allele frequency, paternity testing, population genetics, India, DNA profiling, Hardy-Weinberg equilibrium, power of discrimination, tribal population, polymorphism

The team began with a computational question: does polystyrene physically interact with the molecular machinery of DNA analysis? Using in-silico docking, they modeled how polystyrene binds to Taq DNA polymerase, the heat-stable enzyme that copies DNA during the polymerase chain reaction, or PCR. The predicted binding energy was a favorable −4.57 kilocalories per mole, suggesting the polymer does not merely float passively beside the enzyme but engages with it in ways that could disrupt its catalytic work. The researchers then turned to the DNA itself, docking polystyrene against the mitochondrial DNA HVI region, a workhorse target in forensic sequencing, and against four short tandem repeat markers that anchor human identification: D8S1179, D13S317, D21S11, and D2S1338. The interaction energies ranged from −1.95 to −1.53 kilocalories per mole, with D8S1179 showing the strongest association. In plain terms, the plastic showed a measurable chemical affinity for the exact genetic regions forensic laboratories amplify every day.

Computational predictions, however, are only as good as their experimental confirmation. To probe what happens to DNA at the molecular level when polystyrene is present, the team used attenuated total reflectance Fourier-transform infrared spectroscopy, or ATR-FTIR, on calf thymus DNA mixed with the polymer. The spectra told a precise story. The deoxyribose C–O stretching peak shifted from 1066 to 1047 wavenumbers per centimeter, the asymmetric phosphate stretch moved from 1242 to 1260, and the purine ring C–N stretching band drifted from 671 to 737. Each of these shifts indicates that polystyrene is not a bystander: it perturbs the sugar-phosphate backbone and the nitrogenous bases of DNA, the very structures that primers must recognize and polymerases must read. A forensic assay depends on those molecular contacts happening cleanly and repeatedly; a plastic particle wedging itself into the chemistry undermines the entire chain of events.

The consequences showed up immediately in DNA quantification, the step that tells a laboratory how much human DNA a sample contains and therefore how to process it. When the researchers spiked known quantities of control DNA with polystyrene at concentrations between 25 and 100 micrograms per milliliter, the readings collapsed. A sample containing 0.5 nanograms of DNA was estimated at just 0.027 nanograms, and a 1.0-nanogram sample registered as 0.046 nanograms—underestimations by factors of roughly eighteen and twenty-two respectively. Part of the explanation lies in fluorescence interference. Real-time PCR instruments quantify DNA by reading fluorescent signals, and polystyrene is known to fluoresce and to absorb or scatter light in ways that scramble those readings. Notably, the internal positive control’s cycle threshold value barely moved, shifting from 27.83 in controls to 27.72 in treated samples, which suggests the problem was not classic PCR inhibition at the quantification stage but optical distortion—the instrument was effectively being blinded by the plastic.

Quantification errors alone would be serious, because an analyst who believes a sample contains almost no DNA may choose an aggressive low-template strategy or may even decline to test it at all. But the study found that polystyrene also attacks the amplification stage directly. When DNA spiked with 25 micrograms per milliliter of polystyrene was run through STR profiling, the markers D10S1248, TH01, and D12S391—all part of the expanded European and international standard sets—amplified poorly or dropped out. At 100 micrograms per milliliter, the quality-sensing markers within the quantification kit showed a heterozygote peak height balance of just 0.44, far below the balanced ratios expected in a clean reaction, which the authors interpret as significant PCR inhibition. The mechanism is likely multifaceted: polystyrene can sequester DNA strands and primers through the same binding interactions predicted in silico, compete with the polymerase, and interfere with the fluorescent chemistry that reports amplification progress.

The downstream effect on DNA profiles was consistent across every concentration the team tested. Short tandem repeat profiling, the backbone of human identification, depends on every locus amplifying reliably and symmetrically. When some markers fail, drop out, or produce imbalanced peaks, the resulting profile becomes harder to interpret, harder to match against a suspect or a database, and more vulnerable to challenge in court. The authors emphasize that such compromised profiles are especially dangerous for low-template DNA samples—the faint traces from a single touched surface, a fingerprint residue, or a hair—and for mixture samples containing DNA from multiple contributors, where every lost allele complicates the already delicate task of deconvoluting who contributed what. In other words, microplastic contamination hits hardest precisely where forensic science is already operating at its limits.

What makes the study timely is the growing recognition that microplastics are inside us. Recent reviews have documented polystyrene and other polymers in human blood, sputum, skeletal tissues, and a widening list of organs, and research on laboratory animals has linked polystyrene exposure to mitochondrial disruption and genotoxic effects. For forensic practitioners, this reframes contamination risk. It is no longer sufficient to think of sample contamination purely in terms of handling errors, environmental bacteria, or chemical inhibitors like humic acids and indigo dye. A victim’s or perpetrator’s own body may carry polymer particles into a bloodstain or tissue sample, and clothing, packaging, and plastic evidence bags may shed additional fragments onto exhibits during storage. The study’s authors, who have previously examined how metal contaminants interfere with STR analysis, position microplastics as an emerging contaminant class that forensic workflows have not yet been designed to detect or counter.

The researchers argue that the problem warrants immediate action and call for suitable mitigation strategies to strengthen the routine forensic DNA workflow. Practical responses could take several forms. Laboratories might incorporate purification steps that separate plastic particles from DNA before quantification, adopt quantification chemistries less susceptible to fluorescence interference, or add validation studies that characterize how common polymers—polystyrene, polyethylene, polypropylene, polyethylene terephthalate—affect each step from extraction to interpretation. Evidence-handling protocols could also be revisited, since plastic packaging is ubiquitous in forensic storage. The study stops short of prescribing specific remedies, but its message is clear: until mitigation exists, microplastic contamination is an unmeasured variable in casework, capable of silently shrinking DNA estimates by an order of magnitude and erasing genetic markers without any obvious warning sign to the analyst.

There is also a broader scientific payoff in the paper’s mechanistic approach. By combining docking predictions, infrared spectroscopy, quantitative PCR, and full STR profiling, the team built a coherent causal chain from molecular binding to profile degradation—a model that can now be applied to other polymers and other contaminants. The same framework previously illuminated how metals sabotage PCR, and it could guide the design of inhibitors-resistant enzyme formulations or buffer additives that shield DNA from polymer surfaces. For a field whose credibility rests on reproducibility, understanding why and how a contaminant distorts results is the first step toward neutralizing it. As microplastics continue to accumulate in bodies, waterways, and dust, the intersection of environmental pollution and forensic genetics is no longer hypothetical. This study provides the first detailed mechanistic account of how one of the world’s most common plastics collides with DNA evidence, and it suggests that crime laboratories worldwide may need to add a new item to their contamination checklist—one measured in microns.

Subject of Research: Mechanistic interaction between polystyrene microplastics and DNA and its impact on forensic DNA profiling

Article Title: Understanding the mechanistic interaction between DNA and polystyrene microplastic and the effect of microplastic on forensic DNA analysis

Article References: Yadav, N., Tehsin, S., Tanpure, D., Sahoo, S., Dash, A. A., Priyadarshini, K., Priyadarshini, P., & Dash, H. R. (2026). Understanding the mechanistic interaction between DNA and polystyrene microplastic and the effect of microplastic on forensic DNA analysis. International Journal of Legal Medicine. https://doi.org/10.1007/s00414-026-04017-3

Image Credits: AI Generated

DOI: 10.1007/s00414-026-04017-3

Keywords: microplastics, polystyrene, forensic DNA analysis, DNA profiling, STR markers, PCR inhibition, Taq DNA polymerase, ATR-FTIR, DNA quantification, low-template DNA, contamination, International Journal of Legal Medicine