CFC DNA Analysis & Interpretation 3 — Questions and Answers
Question 1: Mitochondrial DNA (mtDNA) is particularly useful in forensic cases involving which type of evidence?
- Fresh bloodstains with high DNA yield
- Hair shafts without roots, bones, and teeth (Correct answer)
- Semen samples from sexual assault kits
- Touch DNA from hard surfaces
Correct answer: Hair shafts without roots, bones, and teeth
mtDNA is found in high copy numbers per cell, making it ideal for analyzing highly degraded or low-template samples like rootless hairs and ancient bones.
Question 2: The Combined DNA Index System (CODIS) database uses which core set of STR loci for forensic DNA profiling in the United States?
- 13 core loci established in 1997, expanded to 20 core loci in 2017 (Correct answer)
- 15 core loci established in 2000, unchanged since
- 20 core loci established in 1997
- 10 core loci selected by the FBI in 2010
Correct answer: 13 core loci established in 1997, expanded to 20 core loci in 2017
CODIS originally used 13 core STR loci; the FBI expanded the core to 20 loci effective January 2017 to reduce adventitious hits and increase discriminating power.
Question 3: In interpreting a DNA mixture, a forensic analyst determines that a suspect cannot be excluded. What additional information is essential to include in the report?
- The suspect's criminal history
- A statistical weight such as combined probability of inclusion or likelihood ratio (Correct answer)
- The number of other suspects tested
- Whether the victim consented to DNA collection
Correct answer: A statistical weight such as combined probability of inclusion or likelihood ratio
A cannot-exclude conclusion alone is insufficient; a statistical weight must accompany it to convey the significance of the non-exclusion.
Question 4: Which type of DNA testing is most appropriate for determining the maternal lineage of skeletal remains when nuclear DNA is too degraded?
- Y-chromosome STR analysis
- Mitochondrial DNA HVI/HVII sequencing (Correct answer)
- Autosomal SNP profiling
- X-chromosome STR analysis
Correct answer: Mitochondrial DNA HVI/HVII sequencing
Mitochondrial DNA is maternally inherited and present in high copy numbers, making mtDNA sequencing ideal for degraded samples and maternal lineage tracing.
Question 5: A DNA profile from evidence shows alleles at 18 of 20 CODIS loci matching a suspect. The two missing loci show no data. How should the examiner interpret this?
- The suspect is definitively the source of the DNA
- The profile is a match with a partial profile; statistical weight must reflect the tested loci only (Correct answer)
- The two missing loci are presumed to match and the full profile statistics apply
- The evidence is inconclusive due to incomplete data and no comparison should be made
Correct answer: The profile is a match with a partial profile; statistical weight must reflect the tested loci only
Comparisons are valid for tested loci, but statistics must be calculated only on loci with results, and the partial nature must be disclosed.
Question 6: What is 'touch DNA' and what challenge does it present in forensic casework?
- DNA from epithelial cells deposited by skin contact; challenges include very low template amounts and transfer/secondary transfer issues (Correct answer)
- DNA extracted from blood; challenges include degradation during collection
- DNA from saliva; challenges include bacterial contamination
- DNA from hair roots; challenges include limited copy number
Correct answer: DNA from epithelial cells deposited by skin contact; challenges include very low template amounts and transfer/secondary transfer issues
Touch DNA consists of skin cells left by handling objects; the very low quantities and risk of secondary transfer make interpretation and attribution challenging.
Question 7: A forensic examiner is asked to interpret a DNA result where the evidence profile has more than two alleles at multiple loci. What does this indicate?
- Sample contamination only
- The presence of a DNA mixture from multiple contributors (Correct answer)
- PCR over-amplification artifact
- Off-ladder allele designation errors
Correct answer: The presence of a DNA mixture from multiple contributors
More than two alleles at multiple loci is the defining indicator that the sample contains DNA from more than one individual.
CFC DNA Analysis & Interpretation 3
Mitochondrial DNA (mtDNA) is particularly useful in forensic cases involving which type of evidence?