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

backgroundPopulation databases are essential for estimating allele frequencies and forensic statistical parameters. However, the sample sizes required to accurately characterize allelic diversity, particularly rare alleles, remain poorly understood. This study evaluated whether allelic diversity, rare-allele representation, and forensic statistical parameters exhibit different responses to increasing sample size using the largest autosomal STR dataset currently available for the Argentine population.

methodsA total of 8,237 unrelated Argentine individuals genotyped for 22 autosomal STR loci were analyzed. Allele frequencies and forensic parameters were estimated for the complete dataset. To assess the influence of sample size, 1,000 random resampling replicates were generated for cohort sizes ranging from 500 to 8,237 individuals. Allele recovery, rare-allele recovery (< 1% frequency), combined match probability (CMP), and combined power of exclusion (CPE) were evaluated.

resultsA total of 344 distinct alleles were observed, of which 169 (49.1%) had frequencies below 1%. Allele recovery increased from 74.9% at 500 individuals to 90.7% at 3,000 and 98.5% at 7,000 individuals, relative to the allelic diversity observed in the full dataset. In contrast, rare allele recovery was considerably slower, reaching only 81.0% at 3,000 individuals and 90.4% at 5,000 individuals. Combined forensic parameters remained highly stable across all sampling scenarios, with minimal variation in CMP and consistently high CPE values. Locus-specific analyses showed that saturation dynamics were strongly associated with the proportion of rare alleles, with loci such as FGA and D21S11 requiring substantially larger sample sizes to achieve complete allele representation.

conclusionsAllelic-diversity recovery and forensic-parameter stability represent distinct properties of a forensic population database and may require substantially different sampling efforts. While combined forensic parameters showed comparatively limited variation across the evaluated sample sizes, comprehensive representation of allelic diversity, particularly rare alleles, requires substantially larger datasets. These findings indicate that population-database adequacy should be evaluated according to its intended forensic application rather than using a single sample-size criterion.