Forensic Biology and DNA Analysis Practice Exam 2026 – Free Blood, Semen, and Skeletal Remains Practice Questions and Study Guide

The main idea is that the readout comes from the sizes of fluorescently labeled STR fragments. In capillary electrophoresis STR typing, each STR locus is amplified with primers tagged with fluorescent dyes. The resulting PCR fragments vary in length according to the number of repeat units at that locus. The capillary instrument separates these fragments by size, and a laser detects the fluorescence as each fragment passes the detector. The resulting signal, an electropherogram, shows peaks whose positions (migration time) correspond to fragment lengths and whose colors identify the locus. By comparing these lengths to known allele size ladders, you determine the number of repeats at each locus, i.e., the genotype. This is different from sequencing data, methylation patterns, or simply measuring total DNA quantity, which are not the primary readouts of this method.