STR Typing and Capillary Electrophoresis
STR basics
STR alleles are named by the number of repeat units; microvariants fall between integer bins. Multiplex kits amplify many loci with dye-labeled primers. Amelogenin indicates sex but can be deleted or mutated, so interpret it with Y-STR or other markers. Traps: stutter is a minor peak one repeat shorter or longer than the true allele and is not an allele; incomplete adenylation produces split peaks; pull-up is a high peak in another dye channel; dye blobs are broad and not allele-shaped.
Capillary electrophoresis
Capillary electrophoresis separates by size using an internal lane standard; allelic ladders calibrate bins. Spectral calibration and matrix correction prevent cross-talk. Electrokinetic injection favors smaller fragments, so peak heights are not directly proportional to template. Traps: off-ladder alleles, mobility shifts, and poor spectral calibration can cause miscalls. Analytical thresholds, often 50 to 200 RFU, are validated per laboratory; do not call peaks below them.
Worked rule
A peak at 120 RFU at the stutter position, 8 percent of the parent allele height, is consistent with stutter and should not be called. A peak at 60 percent of the parent height, one repeat shorter, is a candidate allele requiring review. Always check the ladder, internal standard, and injection quality before accepting a call.