The research team, led by Maryam Sharafi Farzad and colleagues at the University of Copenhagen, set out to validate the Precision ID mtDNA Whole Genome Panel from Thermo Fisher Scientific specifically for telogen hair analysis and to implement it in routine forensic casework. Mitochondrial DNA offers a crucial advantage in this context: cells contain many more copies of mitochondrial DNA than nuclear DNA, and the molecule is far more resistant to degradation. Although mtDNA is maternally inherited and therefore lacks the discriminatory power to identify a single individual, it remains highly valuable for maternal lineage analysis, missing-person investigations, disaster victim identification, and cases where only degraded biological material is available. Traditionally, mtDNA typing relied on Sanger sequencing of the control region alone, a labor-intensive and costly approach with limited discriminatory power. Massively parallel sequencing technologies such as the Precision ID panel now allow entire mitochondrial genomes to be sequenced quickly and affordably, dramatically increasing both sensitivity and information content.

To establish the sensitivity of the workflow, the researchers prepared dilution series from NIST Standard Reference Material DNA and six reference samples, spanning an extraordinary range from 3 to 3,600 mitochondrial DNA copies per microliter. The results were striking. Complete mitochondrial genome sequences were consistently recovered in 99 percent of samples across the dilution series at DNA input levels as low as 47 mtDNA copies, demonstrating remarkable robustness under low-template conditions. Full profiles were obtained in half of the samples containing only 23 copies, and even some samples at 12 copies produced partial results. Statistical modeling using log-log linear regression revealed that library yield scaled nearly proportionally with mtDNA input in the NIST series, with a doubling of input DNA producing an approximately 2.16-fold increase in library output. Read depth increased linearly up to roughly 560 to 1,235 copies per reaction before plateauing due to saturation, underscoring the importance of accurate library quantification and normalization.

The heart of the study involved 100 volunteers who each provided 20 shed hairs collected from their own clothing, with 98 of them also contributing buccal swab samples for comparison. Microscopic examination identified the club-shaped roots characteristic of telogen hairs, and one hair per donor was selected for analysis. Quantification revealed that 98 of the 100 hair extracts met or exceeded the manufacturer-recommended input of 3,000 mtDNA copies, and both small and large mtDNA quantification targets were detected in every single sample. In contrast, 25 hairs contained no detectable nuclear DNA at all, and most of the remainder showed concentrations below 6.8 picograms per microliter with extensive degradation. The researchers also found that reducing the post-extraction volume from 50 to 30 microliters significantly increased both mtDNA copy number and sequencing read depth, a simple procedural change that boosted sensitivity without introducing contamination. Notably, donor variables such as age, gender, hair dye use, and washing habits showed no statistically significant association with mtDNA quantity or quality.

Comparing the mitochondrial genomes sequenced from 91 paired hair and buccal samples revealed fascinating subtleties of mitochondrial biology. Initial sequence concordance between hairs and their corresponding buccal references was 93.4 percent. However, when the team applied the interpretation guidelines of the International Society for Forensic Genetics and excluded low-confidence discrepancies associated with high strand bias, heteroplasmic variants, and sequencing artifacts, concordance rose to a perfect 100 percent. This finding is critically important for forensic practice: apparent mismatches between a hair and a reference sample frequently reflect technical limitations and tissue-specific heteroplasmy rather than true genetic differences. Heteroplasmy, the coexistence of multiple mitochondrial variants within an individual, arises from bottleneck effects and drift during tissue development, and previous research has shown that it can drift notably in hair. According to ISFG guidelines, a single mismatch or heteroplasmic difference should never be interpreted as evidence of exclusion, and this study provides empirical weight for that principle in the context of whole mitochondrial genome sequencing.

Reproducibility testing further strengthened the case for implementation. The researchers amplified and sequenced duplicate libraries from 43 telogen hair samples and 46 buccal samples, finding over 99 percent concordance of the software-classified true variants between replicates. The handful of discrepancies involved shifts between point and length heteroplasmy classifications, insertion-length differences at homopolymeric C-tracts, and low-frequency artifacts at known hotspots, none of which affected haplotype interpretation. Haplogroup assignments remained identical across all replicates and dilution levels, confirming that reduced template input did not compromise phylogenetic classification. The data analysis itself was also highly repeatable, with two independent runs of the Converge analysis software producing identical results across every measured parameter, from variant frequencies and read depths to strand bias metrics and variant state classifications.

Contamination control, a perennial concern in low-copy-number DNA work, was rigorously assessed. Of 16 negative controls processed during the study, none produced a complete mitochondrial DNA sequence. Six controls showed low-level mtDNA signal consisting of only two to eight variants, predominantly common polymorphisms, and none of these partial profiles corresponded to any of the analyzed samples, the NIST control, or laboratory personnel. Several variants in the controls were traced to primer binding regions, indicating primer variants rather than true biological signal. All work was conducted in a dedicated laboratory following ISFG and SWGDAM guidelines under the ISO 17025 validation framework, and every extraction, quantification, and sequencing batch included negative controls, providing a template for laboratories considering adoption of the method.

Perhaps most compelling for investigators is the performance of the validated workflow on real evidence. The team sequenced 11 archived DNA extracts from hairs collected in 2010 from concluded criminal cases, samples that had previously been deemed unsuitable for STR profiling because they contained no detectable or less than 7 picograms per microliter of nuclear DNA. Remarkably, these aged forensic samples yielded complete mitochondrial genome sequences with an average read depth of 1,037-fold, an exceptionally high coverage figure. Across the three fully reported casework samples, 102 variants were identified, 98 of which were confirmed as true variants with high confidence scores, and no nuclear mitochondrial DNA segments, or NUMTs, were detected to confound the analysis. The success demonstrates that even DNA extracts stored for over a decade can return informative whole mitochondrial genomes, offering hope for cold case reviews.

The study is not without caveats that laboratories must respect. A few amplicons, particularly those covering mitochondrial regions 8622 to 8796, 1317 to 1491, and 1948 to 2111, consistently underperformed across sample types, mirroring poor performance reported in earlier evaluations of the panel. Length heteroplasmy artifacts at homopolymeric stretches such as positions 309 and 11038 appeared with variant strand bias exceeding 0.7, requiring careful filtering. At very low template levels, below roughly 47 copies, stochastic effects increased the frequency of unlikely and unclear variant calls. The authors emphasize that reliable results from low-copy samples demand increased read allocation, replicate sequencing, manufacturer-recommended run thresholds, and stringent filtering of negative controls and NUMTs. The laboratory will continue using an input of 3,000 mtDNA copies for casework despite evidence that lower inputs can suffice, retaining a conservative margin for evidentiary work.

Taken together, the findings position whole mitochondrial genome sequencing of telogen hairs as a validated, sensitive, and contamination-controlled method ready for routine forensic implementation. With more than 85 percent of hair evidence in some laboratories currently unanalyzable by STR profiling, the ability to recover complete mitochondrial genomes from shed hairs with high read depth and near-perfect concordance when properly interpreted represents a meaningful expansion of the forensic toolkit. As massively parallel sequencing continues its migration from research laboratories into accredited casework units, hairs that once sat silently in evidence storage may soon speak, narrowing gaps in investigations, strengthening maternal lineage identifications, and giving cold cases a second chance at resolution.

Subject of Research: Validation of whole mitochondrial genome sequencing of shed telogen hairs for forensic casework

Article Title: Precision ID mtDNA Whole Genome Panel and sequencing of telogen hairs – perspectives for validation and implementation in casework

Article References: Sharafi Farzad, M., Claessens, F., Petersen, C. B., Pereira, V., Morling, N., & Mogensen, H. S. (2026). Precision ID mtDNA Whole Genome Panel and sequencing of telogen hairs – perspectives for validation and implementation in casework. International Journal of Legal Medicine. https://doi.org/10.1007/s00414-026-03992-x

Image Credits: AI Generated

DOI: 10.1007/s00414-026-03992-x

Keywords: mitochondrial DNA, telogen hairs, massively parallel sequencing, Precision ID mtDNA Whole Genome Panel, forensic casework, mtDNA heteroplasmy, low-template DNA, STR profiling, profile concordance, contamination control, ISFG guidelines, hair shaft evidence