Sibling assessment based on likelihood ratio and total number of shared alleles using 21 short tandem repeat loci included in the GlobalFiler™ kit

Abstract

Sibling assessment using the 15 autosomal short tandem repeat (STR) loci included in the Identifiler® kit can be difficult when comparing an unidentified party to an alleged sibling. Therefore, we investigated the likelihood ratio (LR) and the total number of shared alleles (TNSA) for sibship determination using the 21 autosomal STR loci included in the GlobalFiler™ kit. We computationally generated the genotypes of 10,000 sibling pairs and 10,000 unrelated pairs based on previously reported allele frequencies of the 15 Identifiler loci and the remaining 6 GlobalFiler loci. The LR and the TNSA were then calculated in each pair using the 15 and 21 loci. Next, these calculations were applied to 22 actual sibling pairs. LR values ⩾ 10,000 were observed in 48% of the sibling pairs using the 15 loci and in 80% of the sibling pairs using the 21 loci. The TNSA distribution between siblings and unrelated pairs was more divergent in GlobalFiler than in Identifiler. TNSA values ⩾ 20 were found only in true siblings in Identifiler, while TNSA values ⩾24 in GlobalFiler. In Identifiler, all pairs with TNSA ⩾ 24 had LR values ⩾ 10,000 and the same was true in GlobalFiler for TNSA ⩾29. Therefore, increasing the number of loci is very efficient for sibship determination. The LR is most reliable for determining sibship. However, TNSA values may be useful for the preliminary method of LR values because LR value demonstrated a significantly positive correlation with TNSA value in both Identifiler and GlobalFiler.

Keywords: Forensic science; GlobalFiler kit; Identifiler kit; Likelihood ratio (LR); Sibship determination; Total number of shared alleles (TNSA).