A case of "false tri-allelic pattern" on D7S820, caused by invasion of a short SE33 allele into the bins of D7S820.

Autosomal short tandem repeats (asSTR) serve as genetic markers for discriminating individuals and have been extensively used for criminal investigations as well as the establishment of genetic relationships. Tri-allelic pattern usually occurs due to chromosomal duplication, trisomy, and chimerism during mitotic division, but a false tri-allelic pattern at the D7S820 locus was encountered in our laboratory during the analysis of a case exhibit. DNA isolation from exhibit for profiling was done as per manufacturer's protocol. This is the first report which observed false tri-allelic pattern (10, 11, 14.1 allele) on D7S820 locus by analysis with GlobalFiler™ PCR Amplification Kit in Indian population. Findings were re-confirmed using other available asSTR kits in the laboratory, viz., AmpFLSTR™ Identifiler™ Plus PCR Amplification Kit and PowerPlex® Fusion 6C System. Two alleles (10, 11) found at D7S820, apart from SE33 marker, showed homozygous condition, but one Off Marker (OMR) peak was observed before start of SE33 marker region with the analysis using PowerPlex® Fusion 6C System. As it has been confirmed that the OMR allele belongs to the SE33 locus, this could be possible because of the adjacent locations of the D7S820 and the SE33 in the GlobalFiler® PCR amplification kit. 14.1 allele appeared within the allelic window of D7S820. The false tri-allelic pattern was due to the overlapping of SE33 marker allele (1.2 repeat) with bin window of D7S820 Marker. This finding might create confusion for the establishment of genetic relationships. We, therefore, conclude that such uncommon observations with rare events should be carefully investigated and interpreted.