FDC Evidence Collection & Preservation 4 — Questions and Answers
Question 1: When collecting fingernail scrapings from a deceased victim for touch DNA analysis, the collector should use a:
- Metal nail file to abrade the underside of each nail
- Clean wooden or plastic stick to scrape under each nail separately, changing tools between fingers (Correct answer)
- Single cotton swab moistened with water applied to all nails simultaneously
- Sterile metal scalpel to clip the distal nail edge from all ten fingers into one container
Correct answer: Clean wooden or plastic stick to scrape under each nail separately, changing tools between fingers
Each finger must be scraped with a separate tool to prevent cross-contamination between fingers, and non-metallic tools are preferred to avoid static that disperses trace material.
Question 2: Which condition MOST accelerates hydrolytic degradation of DNA in a biological sample stored at room temperature?
- Low relative humidity below 20%
- High relative humidity above 60% (Correct answer)
- Exposure to visible light wavelengths
- Neutral pH environment (pH 7.0)
Correct answer: High relative humidity above 60%
High moisture content promotes hydrolysis of phosphodiester bonds and accelerates microbial growth, both of which fragment DNA rapidly at room temperature.
Question 3: In a sexual assault case where the victim has bathed prior to examination, which body location MOST likely retains spermatozoa suitable for DNA analysis?
- External skin of the perineum
- Cervical os and cervical canal (Correct answer)
- Vaginal introitus external surface
- Inner thigh skin
Correct answer: Cervical os and cervical canal
Spermatozoa can persist in the cervical canal for up to 5–7 days because the cervical mucus protects them from the mechanical removal of bathing.
Question 4: According to FBI Quality Assurance Standards, a forensic DNA laboratory must conduct reagent blank testing to:
- Confirm that reagents amplify the control DNA at the expected RFU threshold
- Detect DNA contamination introduced by laboratory reagents or consumables (Correct answer)
- Verify that the DNA quantification standard curve meets linearity requirements
- Confirm that the electrophoresis matrix separates alleles at the correct base-pair resolution
Correct answer: Detect DNA contamination introduced by laboratory reagents or consumables
Reagent blanks (extraction and amplification blanks) detect any exogenous DNA introduced via laboratory reagents, plasticware, or environmental contamination during processing.
Question 5: When packaging multiple items from the same crime scene into a single evidence bag for submission, the PRIMARY risk is:
- Increased risk of cross-contamination between items (Correct answer)
- Violation of chain of custody due to mingled exhibits
- Excess packaging bulk that may delay laboratory processing
- Reduced documentation accuracy on the outer label
Correct answer: Increased risk of cross-contamination between items
Packaging multiple items together risks DNA cross-transfer between items during transport, potentially creating mixed profiles that obscure the true source of evidence.
Question 6: A criminalist collecting a blood sample from a vehicle interior in subfreezing temperatures should:
- Apply warm water to the stain to rehydrate it before swabbing outdoors
- Collect the stain using the double-swab technique, then allow swabs to air-dry at room temperature before packaging (Correct answer)
- Freeze the swabs immediately after collection to preserve the sample as collected
- Use a portable heat lamp to soften the frozen substrate before scraping
Correct answer: Collect the stain using the double-swab technique, then allow swabs to air-dry at room temperature before packaging
Swabs must be allowed to air-dry before packaging to prevent condensation from trapped moisture causing mold growth and DNA degradation during transport.
Question 7: The 'substrate control' collected alongside biological evidence from a surface serves what purpose in DNA analysis?
- It provides a DNA standard against which the evidence profile is compared
- It identifies any DNA already present in the surface material that might contribute to the evidence profile (Correct answer)
- It confirms the biological nature of the stain by detecting hemoglobin
- It verifies that the collection swab brand meets SWGDAM specifications
Correct answer: It identifies any DNA already present in the surface material that might contribute to the evidence profile
A substrate control is collected from an unstained area adjacent to the stain to reveal background DNA from the surface itself, preventing misinterpretation of the evidence profile.
FDC Evidence Collection & Preservation 4
When collecting fingernail scrapings from a deceased victim for touch DNA analysis, the collector should use a: