FDC Forensic Laboratory Procedures 2 — Questions and Answers
Question 1: When preparing a 1% agarose gel for DNA electrophoresis, what is the correct order of steps?
- Dissolve agarose in buffer, cool, add ethidium bromide, pour (Correct answer)
- Add ethidium bromide to dry agarose, dissolve in buffer, pour
- Pour buffer into mold first, add agarose, then heat
- Dissolve agarose in water, add buffer after cooling
Correct answer: Dissolve agarose in buffer, cool, add ethidium bromide, pour
Agarose is dissolved in electrophoresis buffer by heating, cooled to ~60°C, then ethidium bromide or other dye is added before pouring into the mold.
Question 2: A forensic analyst receives a blood stain on fabric. What is the recommended first step in processing this evidence?
- Immediately extract DNA using organic extraction
- Document and photograph the stain prior to any manipulation (Correct answer)
- Cut the stain from the fabric and place in a tube
- Swab the stain with a wet swab
Correct answer: Document and photograph the stain prior to any manipulation
Documentation and photography must precede any physical manipulation to preserve the evidentiary record.
Question 3: What is the purpose of including a reagent blank in a DNA extraction batch?
- To calibrate the spectrophotometer
- To detect contamination introduced during the extraction process (Correct answer)
- To serve as a positive control for PCR amplification
- To normalize the DNA concentration across samples
Correct answer: To detect contamination introduced during the extraction process
A reagent blank contains all extraction reagents but no biological material, allowing detection of any contamination introduced during processing.
Question 4: Which labeling approach is required on all forensic evidence containers to maintain chain of custody?
- Case number only
- Analyst initials only
- Case number, item number, collector identity, date, and seal initials (Correct answer)
- Bar code generated by the LIMS system only
Correct answer: Case number, item number, collector identity, date, and seal initials
Chain of custody requires comprehensive labeling including case number, item number, collector identity, collection date, and initials on any seal.
Question 5: During STR capillary electrophoresis, a pull-up artifact appears. What does this indicate?
- Insufficient injection voltage
- Spectral overlap between dye channels causing a false peak in an adjacent channel (Correct answer)
- DNA degradation during electrophoresis
- Incorrect ladder concentration
Correct answer: Spectral overlap between dye channels causing a false peak in an adjacent channel
Pull-up (bleed-through) occurs when fluorescent dye emission bleeds into an adjacent detection channel due to imperfect spectral separation.
Question 6: What is the minimum number of STR loci required by NDIS (CODIS) for a forensic unknown profile to be uploaded?
- 7 loci
- 10 loci (Correct answer)
- 20 loci
- All 20 CODIS core loci
Correct answer: 10 loci
NDIS requires a minimum of 10 core STR loci for uploading a forensic unknown (crime scene) profile, though more loci increase discriminating power.
Question 7: A forensic laboratory discovers a validated method produces results outside acceptable control ranges. What is the immediate required action?
- Continue testing and note the deviation in the report
- Stop testing, investigate the cause, and document corrective action before resuming (Correct answer)
- Rerun samples with double the reagent volume
- Contact the proficiency test provider
Correct answer: Stop testing, investigate the cause, and document corrective action before resuming
Out-of-control results require immediate cessation of testing, root-cause investigation, and documented corrective action per accreditation standards.
FDC Forensic Laboratory Procedures 2
When preparing a 1% agarose gel for DNA electrophoresis, what is the correct order of steps?