FDC Evidence Collection & Preservation 3 — Questions and Answers
Question 1: Touch DNA evidence, which involves trace epithelial cells, is MOST susceptible to loss during collection due to:
- High humidity causing cell lysis before collection
- Secondary transfer from an intermediate surface to the collector's hands (Correct answer)
- Ultraviolet light degrading cell membranes on outdoor surfaces
- Contact with non-porous surfaces inhibiting cell adhesion
Correct answer: Secondary transfer from an intermediate surface to the collector's hands
Touch DNA can undergo secondary transfer, where cells move from one surface to another (including the collector's hands), resulting in a mixed or misleading profile.
Question 2: Which biological stain on a garment is MOST likely to remain typeable after repeated machine washing cycles?
- Saliva stains from bite marks
- Semen stains due to sperm head resilience (Correct answer)
- Blood stains from superficial wounds
- Epithelial cells from skin contact
Correct answer: Semen stains due to sperm head resilience
Spermatozoa heads contain tightly packaged protamine-stabilized DNA that resists mechanical and chemical degradation better than most other biological materials.
Question 3: At an outdoor crime scene, environmental factor most damaging to biological evidence DNA integrity over a 72-hour period is:
- Rainfall diluting the sample concentration
- UV radiation causing pyrimidine dimers and strand breaks (Correct answer)
- Wind dispersal of dried material
- Temperature fluctuation between day and night
Correct answer: UV radiation causing pyrimidine dimers and strand breaks
UV radiation induces pyrimidine dimer formation and direct strand breaks in DNA, causing irreversible chemical damage that prevents complete PCR amplification.
Question 4: Under SWGDAM guidelines, a forensic DNA laboratory must retain case evidence for a minimum period AFTER which of the following events?
- Case submission date
- Completion of DNA analysis
- Final legal disposition of the case (Correct answer)
- Issuance of the laboratory report
Correct answer: Final legal disposition of the case
Evidence retention timelines under SWGDAM and FBI QAS guidelines are tied to final legal disposition so evidence remains available for appeals and post-conviction review.
Question 5: When swabbing a firearm for touch DNA evidence, the collector should prioritize which areas of the weapon?
- The barrel interior where propellant residue may trap cells
- High-contact areas such as the trigger, grip, and slide (Correct answer)
- The external sight rail where printing oils accumulate
- The magazine well where DNA concentration is highest
Correct answer: High-contact areas such as the trigger, grip, and slide
The trigger, grip, and slide are the surfaces most frequently and firmly contacted during handling, maximizing the deposit of epithelial cells available for touch DNA analysis.
Question 6: The FTA card technology used for reference sample collection preserves DNA by:
- Cryogenically freezing cells at the point of collection
- Lysing cells and binding DNA to a cellulose matrix with antimicrobial agents (Correct answer)
- Encapsulating whole blood in a protective polymer shell
- Dehydrating samples under vacuum immediately after spotting
Correct answer: Lysing cells and binding DNA to a cellulose matrix with antimicrobial agents
FTA cards contain chaotropic salts and antimicrobial compounds that lyse cells, denature proteins, and immobilize DNA on the cellulose matrix for stable room-temperature storage.
Question 7: A best practice for documenting evidence collection at a crime scene, in addition to written notes, includes:
- Conducting a preliminary DNA screening test on all items before packaging
- Photographing each item in place before and after collection with a scale reference (Correct answer)
- Recording the weight of each item to detect future tampering
- Scanning items with a handheld spectrometer to identify biological fluids
Correct answer: Photographing each item in place before and after collection with a scale reference
Photographs taken before and after collection, with a scale, create a permanent visual record of the item's location, condition, and context that supports chain of custody.
FDC Evidence Collection & Preservation 3
Touch DNA evidence, which involves trace epithelial cells, is MOST susceptible to loss during collection due to: