The Fisher King: Part 1 - S1-E22
Corrected entry: We are told that an analysis of the DNA from the hair sample held in the evidence bag shows it came from Rebecca Bryant, the first victim of the serial killer they are hunting. Later we see a clear shot of the hair - it is a lock of hair, cleanly cut off, and tied with a ribbon. There are no roots, which are the only parts of the hair that contain DNA - hair shafts do not. They would not be able to harvest DNA from this hair sample.
Correction: That's true, but they probably would have cut off that part and used to to get the DNA, the roots are small and they probably would have had to use it all, but they would still keep the hair it came from as the evidence.
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