How Big Must a DNA Database Be? Massive Argentine Study Redefines Forensic Sampling Rules

Every courtroom conviction built on DNA evidence rests on a quiet statistical foundation: a population database that tells investigators how likely it is that a random person shares a particular genetic profile. For decades, forensic laboratories have assembled these databases with relatively modest sample sizes, often a few hundred individuals, assuming that such numbers adequately capture the genetic diversity of the populations they serve. A new study from Argentina, based on the largest autosomal STR dataset ever compiled for that country, now shows that this assumption deserves far more scrutiny than it has traditionally received.

The research, published in the International Journal of Legal Medicine, analyzed 8,237 unrelated Argentine individuals genotyped at 22 autosomal short tandem repeat loci, the repetitive DNA sequences that form the backbone of forensic identification worldwide. Rather than simply reporting allele frequencies for this enormous cohort, the team led by Antonella Belén Penacino and José Alonso Aguilar-Velázquez asked a more fundamental question: how does the size of the sample shape what the database appears to contain? To answer it, they generated 1,000 random resampling replicates for cohort sizes ranging from 500 individuals up to the full 8,237, effectively simulating thousands of alternative databases of different sizes drawn from the same population.

The results reveal a striking asymmetry between two properties of forensic databases that are often conflated. Across the 22 loci, the full dataset contained 344 distinct alleles, and nearly half of them, 169 alleles or 49.1 percent, occurred at frequencies below 1 percent. These rare alleles are the hidden tail of human genetic diversity, and it turns out they are extraordinarily expensive to capture. Overall allele recovery climbed quickly with sample size: a 500-person database captured 74.9 percent of the allelic diversity observed in the full cohort, a 3,000-person database reached 90.7 percent, and by 7,000 individuals the figure stood at 98.5 percent. Rare-allele recovery followed a much shallower trajectory, reaching only 81.0 percent at 3,000 individuals and still just 90.4 percent at 5,000.

This divergence matters because the two quantities serve different forensic purposes. The combined forensic parameters that courts care most about, the combined match probability and the combined power of exclusion, remained remarkably stable across all sampling scenarios in the study. The combined match probability showed minimal variation regardless of how many individuals were sampled, and the combined power of exclusion stayed consistently high throughout. In practical terms, even a 500-person database produced paternity and identification statistics that looked very similar to those derived from the full cohort of more than 8,000. For many routine applications, the headline numbers of forensic genetics appear almost indifferent to sample size.

Yet the stability of those combined parameters conceals what is happening at the level of individual alleles. When a DNA profile from a crime scene contains an allele that has never been observed in the reference database, laboratories must apply a minimum allele frequency, a floor value that prevents the match probability from being calculated as zero. The accuracy of that floor, and of the frequency estimates for genuinely rare alleles, depends directly on whether the database has sampled enough people to encounter them. A database that has recovered only 81 percent of the rare alleles present in its population is systematically underestimating the diversity that real casework will encounter.

The locus-specific analyses added another layer of nuance. Saturation dynamics, the point at which additional sampling stops revealing new alleles, were strongly associated with the proportion of rare alleles at each locus. Highly polymorphic markers such as FGA and D21S11, which harbor many low-frequency variants, required substantially larger sample sizes to achieve complete allele representation than less variable loci. This means that a single sample-size criterion applied uniformly across a multiplex panel is inherently misleading: some loci saturate quickly while others continue yielding new alleles thousands of samples later. The authors’ finding suggests that adequacy assessments should be conducted locus by locus, weighted by the intended application of the database.

The Argentine context makes the study particularly significant. Argentina’s population reflects complex admixture among Indigenous American, European, and other ancestral contributions, and earlier work by some of the same research community, including studies of urban Argentine populations and regional databases from Patagonia and the central provinces, has documented meaningful genetic structure across the country. Building a reference database at this scale, with informed consent from all participants, provides the forensic community with a resource whose allele frequency estimates carry far narrower confidence intervals than the small regional datasets that have historically been used. It also offers a benchmark against which the sufficiency of smaller national databases elsewhere can be judged.

The methodological approach draws on concepts borrowed from ecology, where rarefaction and species-accumulation curves have long been used to estimate how much of a community’s biodiversity a survey has captured. The study’s citation of foundational work on individual-based rarefaction by Robert Colwell and colleagues signals this intellectual lineage: alleles at a forensic locus are treated much like species in an ecosystem, and the resampling replicates function as accumulation curves revealing how discovery slows as sampling proceeds. The team’s analytical toolkit included standard population genetics software such as Arlequin and the STRAF online platform for forensic STR evaluation, alongside the R statistical environment for the resampling analyses.

The implications for forensic practice are direct. International guidelines, including the revised recommendations for publishing genetic population data issued by leading forensic geneticists in 2017, have grappled with how large a population sample must be, and recent work by other groups has begun questioning conventional sampling guidance for highly polymorphic STR loci. The Argentine study provides the strongest empirical answer yet: the answer depends on the question being asked. If the goal is stable combined forensic parameters for routine match probability and paternity calculations, moderate sample sizes perform adequately. If the goal is comprehensive representation of allelic diversity, particularly the rare alleles that populate nearly half of the allelic spectrum, then even several thousand individuals may not suffice, and databases aiming at full allele recovery should plan for substantially larger sampling efforts.

Perhaps the most enduring contribution of the study is conceptual. By demonstrating that allelic-diversity recovery and forensic-parameter stability are distinct properties that respond differently to sample size, the researchers have given the forensic community a framework for evaluating population databases according to their intended use rather than a one-size-fits-all threshold. As DNA phenotyping advances, as new multiplex kits expand the number of loci typed, and as courts increasingly scrutinize the statistical foundations of DNA evidence, that distinction will only grow in importance. A database that looks statistically adequate on paper may still be missing half the rare genetic variants its population carries, and knowing exactly which questions a database can and cannot answer is now an empirical matter that studies of this scale are finally equipped to resolve.

Subject of Research: Sample size effects on allele diversity and forensic parameters of autosomal STR loci in the Argentine population

Article Title: Sample size effects on allele diversity, rare-allele recovery, and forensic parameters of 22 autosomal STRs in a cohort of 8,237 Argentines

Article References: Penacino, A. B., Carvajal-Pérez, C. E., Rangel-Villalobos, H., Elsztein, L. D., Puentes, P. A., Zapata, F. A., Becerra-Loaiza, D. S., Moreno-Ortiz, J. M., Penacino, G. A., & Aguilar-Velázquez, J. A. (2026). Sample size effects on allele diversity, rare-allele recovery, and forensic parameters of 22 autosomal STRs in a cohort of 8,237 Argentines. International Journal of Legal Medicine. https://doi.org/10.1007/s00414-026-04032-4

Image Credits: AI Generated

DOI: 10.1007/s00414-026-04032-4

Keywords: forensic genetics, STR loci, allele frequencies, rare alleles, sample size, population database, combined match probability, power of exclusion, Argentine population, genetic diversity, DNA profiling, International Journal of Legal Medicine