The effect of FBI CODIS Core STR Loci expansion on familial DNA database searching

1000 profiles chosen randomly from an in-house database of 6000 profiles were searched against the database for matches with at least one shared allele per locus. The database contains profiles that have been analyzed with Identifiler Plus (15 markers) for biological relationship and DNA identification purposes and both true and false matches are expected to be obtained. 100 pairs of at least one true paternity match and one false match were selected and were initially supplemented with 5 additional STRs representing the new Core CODIS set. Study of the LR value showed that when false matches were treated as paternity matches, the expansion of the marker set severely diminished the LR values obtained compared to true matches and the false positive ratio of familial database searching. When false matches were treated as full-sibling matches, the expansion to 20 STRs also diminished the number of false matches and the corresponding LR values compared to true full-sibling cases, but the effect was less dramatic. Addition of the SE33 marker, further promoted distinction between true and false matches both in paternity and full-sibling cases. Counting the number of shared alleles presented improved distinction efficiency between true and false matches after STR expansion to20 and 21 STRs but remains a less valuable method of familial DNA database searching compared to LR.