FDC DNA Analysis Techniques 4 — Questions and Answers
Question 1: What is the purpose of a positive amplification control in PCR-based forensic analysis?
- To confirm the absence of contamination in reagents
- To verify that the PCR reagents and conditions are functioning correctly (Correct answer)
- To establish the stochastic threshold for the batch
- To determine the number of contributors in a mixture
Correct answer: To verify that the PCR reagents and conditions are functioning correctly
A positive amplification control contains known DNA and confirms that PCR reagents, conditions, and the thermocycler are performing correctly.
Question 2: Which phenomenon occurs when only one allele is detected at a heterozygous locus due to preferential amplification of one allele?
- Locus dropout
- Allelic dropout (Correct answer)
- Pull-up artifact
- Off-ladder allele
Correct answer: Allelic dropout
Allelic dropout occurs when only one of two heterozygous alleles amplifies and is detected, often due to low template DNA or degradation affecting one allele's primer binding site.
Question 3: In next-generation sequencing (NGS) applied to forensics, what is a key advantage over capillary electrophoresis for STR typing?
- Lower cost per sample
- Ability to resolve isoalleles and detect sequence variation within repeat units (Correct answer)
- Faster turnaround time
- Simpler data analysis requirements
Correct answer: Ability to resolve isoalleles and detect sequence variation within repeat units
NGS can distinguish isoalleles (alleles of the same length but different sequence) and detect sequence variation within STR repeat units, providing greater discrimination power.
Question 4: What is a 'negative reagent blank' used to detect in forensic DNA analysis?
- Allelic dropout caused by low template
- Laboratory contamination introduced during sample processing (Correct answer)
- Incorrect allele binning by analysis software
- Stutter artifacts from highly degraded samples
Correct answer: Laboratory contamination introduced during sample processing
A negative reagent blank contains all reagents but no DNA; any DNA profile detected indicates contamination was introduced during sample processing.
Question 5: Which database does CODIS use as its core set of STR loci for national DNA profile comparison?
- SWGDAM-13 core loci only
- The expanded CODIS 20 core STR loci (Correct answer)
- The Y-STR haplotype reference database
- The mtDNA population database
Correct answer: The expanded CODIS 20 core STR loci
CODIS expanded from 13 to 20 core STR loci in 2017, and all profiles uploaded to the national database must include these 20 loci for searching.
Question 6: What is the term for DNA from an unrelated individual found in a forensic sample due to improper handling or collection?
- Mixture
- Contamination (Correct answer)
- Low copy number
- Secondary transfer
Correct answer: Contamination
Contamination refers to the introduction of extraneous DNA from an unrelated source, such as a technician or improper collection tools, into a forensic sample.
Question 7: Which statistical framework uses the likelihood ratio to compare the probability of evidence under two competing hypotheses?
- Frequentist exclusion statistics
- Bayesian/likelihood ratio approach (Correct answer)
- Hardy-Weinberg population genetics
- The product rule alone
Correct answer: Bayesian/likelihood ratio approach
The likelihood ratio (LR) framework compares the probability of the DNA evidence under the prosecution hypothesis versus the defense hypothesis to quantify the strength of evidence.
FDC DNA Analysis Techniques 4
What is the purpose of a positive amplification control in PCR-based forensic analysis?