Forensic Identity Testing with a Degraded Sample
A forensic laboratory receives a bone sample from a 20-year-old cold case. The sample is highly degraded, and the DNA yield is low (0.1 ng/µL). The laboratory uses a short tandem repeat (STR) typing kit that amplifies 15 loci plus amelogenin. The initial STR profile shows peak heights below the laboratory's analytical threshold (50 RFU) at several loci, and some loci show allele drop-out. The laboratory's policy is to report only profiles with at least 10 loci with peak heights above 50 RFU. The sample is from a missing person, and a reference sample from a sibling is available. The laboratory director must decide whether to report the partial profile or request re-extraction and re-amplification.
1. What are the challenges with degraded DNA in STR typing, and how should the laboratory proceed?
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Degraded DNA is fragmented, which can lead to allele drop-out and peak height imbalance because larger STR amplicons are less efficiently amplified. Low template DNA can also cause stochastic effects, such as allelic drop-in and increased stutter. The laboratory should first attempt to improve the sample by re-extracting DNA using a method optimized for degraded samples (e.g., silica-based extraction) and re-amplifying with a kit that uses smaller amplicons (mini-STRs) to increase sensitivity. If the profile still does not meet the laboratory's reporting threshold, the laboratory may consider reporting a partial profile with a clear caveat about the limitations, but only if it can be used for comparison with the sibling reference. The laboratory should follow SWGDAM guidelines for mixture interpretation and statistical analysis. In this case, re-extraction and re-amplification are warranted before reporting a partial profile.
2. What are the quality assurance and legal considerations for reporting a partial STR profile?
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Quality assurance considerations include documenting all steps of the analysis, including extraction, quantification, amplification, and interpretation. The laboratory should use appropriate controls and ensure that the analytical threshold is validated. The interpretation of partial profiles should be conservative, and the laboratory should avoid overstating the strength of the evidence. Legal considerations include the admissibility of the DNA evidence in court, which may be challenged under the Daubert standard. The laboratory should be prepared to explain the limitations of the partial profile and the statistical weight of the comparison. The report should clearly state the loci that were successfully typed and the loci that were not, and should include a statement about the potential for allele drop-out. The laboratory should also consider the possibility of contamination and document negative controls.