CFC DNA Analysis & Interpretation 4 — Questions and Answers
Question 1: Which quality assurance standard governs accredited forensic DNA laboratories in the United States?
- ISO 17025 only
- FBI Quality Assurance Standards (QAS) for Forensic DNA Testing Laboratories (Correct answer)
- ASTM E1459 standard
- NIST SP 800-series guidelines
Correct answer: FBI Quality Assurance Standards (QAS) for Forensic DNA Testing Laboratories
The FBI's Quality Assurance Standards for Forensic DNA Testing Laboratories set mandatory requirements for all laboratories submitting profiles to CODIS.
Question 2: What is the purpose of a 'negative control' in a forensic DNA amplification run?
- To confirm the extracted DNA is of sufficient quantity
- To detect contamination by showing no DNA amplification in the absence of a sample (Correct answer)
- To verify the PCR reagents are functioning
- To calibrate the capillary electrophoresis instrument
Correct answer: To detect contamination by showing no DNA amplification in the absence of a sample
A negative control contains all reagents except a DNA template; any amplification signals contamination of the reagents or process.
Question 3: In probabilistic genotyping, what does a likelihood ratio (LR) greater than 1 indicate?
- The evidence is more likely if the suspect contributed than if an unknown person did (Correct answer)
- The evidence is more likely if an unknown person contributed than the suspect
- The DNA profile is a definitive match to the suspect
- The mixture has more than two contributors
Correct answer: The evidence is more likely if the suspect contributed than if an unknown person did
An LR > 1 means the observed DNA evidence is more probable under the prosecution hypothesis (suspect contributed) than the defense hypothesis (unknown contributor).
Question 4: A forensic DNA laboratory receives a reference swab and a crime scene sample. Before comparison, the analyst confirms the samples were collected and stored separately. Why is this critical?
- To satisfy chain of custody requirements only
- To prevent contamination and ensure the reference did not inadvertently transfer DNA to the evidence sample (Correct answer)
- To comply with privacy regulations
- To determine which sample has higher DNA concentration
Correct answer: To prevent contamination and ensure the reference did not inadvertently transfer DNA to the evidence sample
Separate handling and storage of reference and evidence samples prevents cross-contamination that could create a false association.
Question 5: Which forensic scenario is best addressed by familial DNA searching in a CODIS database?
- Confirming a known suspect is the source of evidence DNA
- Identifying an unknown suspect through a close relative whose profile is in CODIS (Correct answer)
- Determining the ethnicity of an unknown donor
- Comparing two evidence samples to determine if they share a common source
Correct answer: Identifying an unknown suspect through a close relative whose profile is in CODIS
Familial searching looks for partial matches in CODIS that may indicate a close biological relative of the unknown contributor, potentially leading to an unidentified suspect.
Question 6: What does 'peak height ratio' (PHR) indicate in STR analysis, and what threshold commonly triggers concern?
- The ratio of signal to noise; concern below 10%
- The relative height of two alleles at a heterozygous locus; imbalance below 60-70% may indicate dropout or mixture (Correct answer)
- The ratio of stutter to true allele; concern above 50%
- The ratio of ladder peaks to sample peaks; concern above 80%
Correct answer: The relative height of two alleles at a heterozygous locus; imbalance below 60-70% may indicate dropout or mixture
PHR compares allele peak heights at heterozygous loci; ratios below laboratory thresholds (typically 60-70%) may indicate allelic dropout or suggest a mixture.
Question 7: A defense attorney challenges a DNA match by arguing that databases used for statistical calculations don't represent the defendant's ethnic group. What is the most scientifically appropriate response?
- The challenge is invalid because all human populations share the same allele frequencies
- Use the population database most representative of the defendant's ethnic group and apply the NRC II recommendations, including the ceiling principle if needed (Correct answer)
- Refuse to recalculate and rely on the original statistics
- Calculate statistics for all major population groups and use the highest (least favorable to prosecution) figure
Correct answer: Use the population database most representative of the defendant's ethnic group and apply the NRC II recommendations, including the ceiling principle if needed
NRC II guidelines recommend using the most relevant population database and applying conservative corrections such as the ceiling principle to address database limitations.
CFC DNA Analysis & Interpretation 4
Which quality assurance standard governs accredited forensic DNA laboratories in the United States?