FDC Forensic Laboratory Procedures 5 ā Questions and Answers
Question 1: A forensic DNA laboratory must retain case records after a case is closed. Under most accreditation standards, what is the minimum record retention period?
- 2 years
- 5 years
- At least as long as the statute of limitations or as required by law, whichever is longer (Correct answer)
- Indefinitely for all criminal cases
Correct answer: At least as long as the statute of limitations or as required by law, whichever is longer
Accreditation standards and legal requirements dictate that records must be retained at least as long as the applicable statute of limitations or the legally mandated period, whichever is longer.
Question 2: Which instrument parameter is routinely monitored during CE to ensure consistent DNA fragment sizing?
- The concentration of the polymer matrix
- The capillary temperature and voltage ramp profile
- The laser power and fluorescence detector gain
- All of the above (Correct answer)
Correct answer: All of the above
Consistent CE sizing requires monitoring polymer concentration, capillary temperature, voltage ramp, laser power, and detector gain to ensure reproducible electrophoretic conditions.
Question 3: What distinguishes a 'mixture interpretation threshold' from an 'analytical threshold' in forensic DNA analysis?
- They are synonymous terms used interchangeably in laboratory SOPs
- Analytical threshold defines the minimum peak height to be considered a true signal; mixture interpretation threshold sets the minimum height to include a peak as a potential contributor allele (Correct answer)
- Mixture interpretation threshold is set lower than the analytical threshold
- Analytical threshold applies only to single-source samples
Correct answer: Analytical threshold defines the minimum peak height to be considered a true signal; mixture interpretation threshold sets the minimum height to include a peak as a potential contributor allele
The analytical threshold (AT) is the minimum peak height above instrument noise, while the mixture interpretation threshold (MIT) is a higher, laboratory-set threshold above which peaks are considered for mixture interpretation.
Question 4: A forensic lab analyst swabs the inside of a buccal (cheek) swab collection device from a reference sample. The swab is then stored in a plastic bag and sealed. What critical error was made?
- The buccal swab should be swabbed on the outside of the device
- The swab should be air-dried before sealing in the plastic bag to prevent mold and DNA degradation (Correct answer)
- Buccal swabs should only be stored in paper envelopes
- Reference samples do not require chain of custody documentation
Correct answer: The swab should be air-dried before sealing in the plastic bag to prevent mold and DNA degradation
Wet biological samples sealed in plastic bags create a moist anaerobic environment that promotes mold growth and bacterial degradation, destroying DNA.
Question 5: In the context of forensic DNA laboratory quality assurance, what is a 'technical review'?
- An annual laboratory-wide audit performed by an external accreditation body
- A review of case documentation, data, and conclusions by a qualified analyst other than the one who conducted the analysis (Correct answer)
- A review of instrument calibration records by the laboratory director
- A statistical review of population genetics data used for random match probabilities
Correct answer: A review of case documentation, data, and conclusions by a qualified analyst other than the one who conducted the analysis
A technical review is an independent examination of a case file, raw data, and conclusions by a second qualified analyst to identify errors before a report is issued.
Question 6: Which PCR inhibitor is most commonly encountered in forensic soil samples, and which extraction modification helps overcome it?
- Hemoglobin; add BSA to the PCR reaction
- Humic acid; use an additional wash step or dilute the template (Correct answer)
- Melanin; perform organic extraction instead of Chelex
- Indigo dye; use solid phase extraction
Correct answer: Humic acid; use an additional wash step or dilute the template
Humic acid from soil is a potent PCR inhibitor; additional wash steps during purification or template dilution are common strategies to reduce its effect.
Question 7: What is the significance of the 'Hardy-Weinberg equilibrium' assumption in forensic DNA statistics?
- It proves that allele frequencies are identical across all human populations
- It allows the calculation of genotype frequencies from allele frequencies under the assumption of random mating within a population (Correct answer)
- It eliminates the need for a match probability calculation when profiles are rare
- It establishes that forensic STR loci are always unlinked
Correct answer: It allows the calculation of genotype frequencies from allele frequencies under the assumption of random mating within a population
Under Hardy-Weinberg equilibrium, genotype frequencies can be predicted from allele frequencies using the product of allele probabilities, which is the basis for random match probability calculations.
FDC Forensic Laboratory Procedures 5
A forensic DNA laboratory must retain case records after a case is closed.
Under most accreditation standards, what is the minimum record retention period?