Identification of a large off-ladder allele of the D21S2055 locus during population genetics analysis

[Abstract] Short tandem repeat (STR) loci within the human genome are extensively utilized in forensic DNA analysis because of their high level of polymorphism. At present, the combined DNA index system (CODIS) STRs are the principal ones adopted in the majority of general-purpose forensic kits. Moreover, non-CODIS STRs can serve as valuable supplementary tools for resolving complex paternity cases, conducting evolutionary studies, and performing population genetics analyses. Along with non-CODIS STRs, unusual DNA profiles have been consistently identified, as observed with CODIS STRs. In this present study, we investigated the allelic distribution and population genetics parameters of the D21S2055 locus in a cohort of 258 unrelated individuals from Fujian Province, China. The locus exhibited significant genetic diversity, with twenty-one distinct alleles, mostly spanning a size range from 16.1 to 38. Notably, a large off-ladder allele, 57.2, which was also included in the findings. The core repeat sequence of this allele was found to be [CTAT]₂CTAA[CTAT]₁₃[TATC]₃TAT[TATC]₁₄. TACTATCTCTATAT[TATC]₃TAT[TATC]₁₄[TATC]₂. The population genetic parameters, including the power of discrimination (PD), probability of paternity exclusion (PE) and polymorphic information content (PIC), were calculated as 0.9687, 0.7388, and 0.8576, respectively. In conclusion, it is essential to conduct population genetic analysis before applying non-CODIS STRs in forensic applications. Special emphasis should be placed on large off-ladder alleles that may extend beyond the designated locus region or overlap with an adjacent locus.