FDC Forensic Laboratory Procedures 4 — Questions and Answers
Question 1: A forensic laboratory uses Chelex-100 extraction. What is the primary advantage of this method over organic extraction?
- It produces higher-purity DNA free of all inhibitors
- It is faster and simpler with fewer toxic reagents (Correct answer)
- It consistently yields more DNA from degraded samples
- It is required by CODIS for all convicted offender samples
Correct answer: It is faster and simpler with fewer toxic reagents
Chelex-100 extraction is faster, involves fewer steps, and does not require hazardous organic solvents like phenol-chloroform.
Question 2: In forensic DNA analysis, what does the term 'stochastic effects' refer to?
- Random variation in peak heights resulting from low template DNA amounts (Correct answer)
- Systematic bias introduced by analyst error
- Instrument noise during capillary electrophoresis
- Contamination from environmental DNA sources
Correct answer: Random variation in peak heights resulting from low template DNA amounts
Stochastic effects arise when low template DNA is amplified, causing random allele dropout, drop-in, or extreme peak height imbalance.
Question 3: Which of the following is a correct statement about laboratory accreditation under ISO/IEC 17025?
- Accreditation certifies that individual analysts are competent
- Accreditation confirms that the laboratory's management system and technical competence meet international standards (Correct answer)
- Accreditation guarantees all laboratory results are correct
- Accreditation is required only for toxicology laboratories, not DNA labs
Correct answer: Accreditation confirms that the laboratory's management system and technical competence meet international standards
ISO/IEC 17025 accreditation assesses both the management system and technical competence of the testing laboratory, not individual analysts.
Question 4: What is the purpose of a negative PCR control in a forensic STR amplification batch?
- To verify that the PCR machine reached the correct temperature
- To confirm that no contaminating DNA was introduced during PCR setup (Correct answer)
- To establish the allele ladder for sizing reference
- To measure the efficiency of the fluorescent dye labeling
Correct answer: To confirm that no contaminating DNA was introduced during PCR setup
A negative PCR control contains all amplification reagents but no DNA template; any profile it yields indicates contamination during PCR setup.
Question 5: A forensic analyst observes a 'minus A' artifact on an electropherogram. What causes this?
- Polymerase failing to add the non-templated adenylate nucleotide to PCR products (Correct answer)
- A deletion mutation at the STR locus being typed
- Degradation of the PCR product during electrophoresis
- Excessive injection voltage during capillary electrophoresis
Correct answer: Polymerase failing to add the non-templated adenylate nucleotide to PCR products
Taq polymerase typically adds a non-templated adenosine to PCR products; incomplete adenylation (-A) produces a peak one base pair smaller than the fully adenylated product.
Question 6: Which type of DNA is most appropriate to use when calculating a random match probability for a CODIS profile upload?
- mtDNA haplogroup frequencies
- Population-specific autosomal STR allele frequencies from the FBI's population database (Correct answer)
- Y-STR haplotype frequencies from YHRD
- Single nucleotide polymorphism (SNP) frequencies from 1000 Genomes
Correct answer: Population-specific autosomal STR allele frequencies from the FBI's population database
CODIS profile statistics use autosomal STR allele frequencies from the FBI's population database, applying the product rule across typed loci.
Question 7: When validating a new DNA quantification assay, which parameter assesses the assay's ability to give correct results across multiple concentration levels?
- Precision
- Sensitivity
- Linearity (Correct answer)
- Specificity
Correct answer: Linearity
Linearity validation demonstrates that the assay produces results proportional to the true concentration across the specified analytical range.
FDC Forensic Laboratory Procedures 4
A forensic laboratory uses Chelex-100 extraction.
What is the primary advantage of this method over organic extraction?