CFC DNA Analysis & Interpretation 5 — Questions and Answers
Question 1: During autopsy, bone marrow samples are collected for DNA identification of a decomposed victim. Which extraction method is most commonly used for this sample type?
- Chelex extraction (Correct answer)
- Organic (phenol-chloroform) extraction or differential extraction
- FTA card punch extraction
- Solid-phase extraction only
Correct answer: Chelex extraction
Chelex extraction is commonly used for bone marrow and other forensic samples because it is simple, fast, and effective for low-template or degraded samples.
Question 2: What is the significance of Hardy-Weinberg equilibrium (HWE) in forensic DNA statistics?
- It proves that DNA profiles are unique to each individual
- It provides the mathematical basis for calculating genotype frequencies from allele frequencies in a population (Correct answer)
- It guarantees that population databases contain no errors
- It establishes that STR mutations occur at a negligible rate
Correct answer: It provides the mathematical basis for calculating genotype frequencies from allele frequencies in a population
HWE allows forensic analysts to calculate the expected frequency of a genotype (diploid pair) by multiplying individual allele frequencies, foundational to the product rule.
Question 3: A forensic consultant reviews a case where the original analyst used only inclusion/exclusion rather than probabilistic genotyping for a three-person mixture. What concern should the consultant raise?
- Inclusion/exclusion is always preferred over probabilistic methods
- Binary methods may over-include or under-include contributors in complex mixtures, whereas probabilistic methods provide more accurate statistical weight (Correct answer)
- Probabilistic genotyping is not accepted in US courts
- The mixture type determines which method is mandated by CODIS
Correct answer: Binary methods may over-include or under-include contributors in complex mixtures, whereas probabilistic methods provide more accurate statistical weight
Binary inclusion/exclusion methods for complex mixtures can produce misleading results; probabilistic genotyping provides a more scientifically rigorous evaluation.
Question 4: In sexual assault casework, 'differential extraction' is used to separate which two cell populations?
- Epithelial cells and white blood cells
- Sperm cells and epithelial cells (Correct answer)
- Red blood cells and sperm cells
- Skin cells and bacteria
Correct answer: Sperm cells and epithelial cells
Differential extraction uses a controlled lysis procedure to separate sperm cells (male contributor) from epithelial cells (female victim), yielding separate DNA fractions.
Question 5: A forensic examiner is asked whether a DNA profile developed from a crime scene can be used to infer the donor's physical appearance. What is the current state of forensic DNA phenotyping (FDP)?
- FDP can accurately predict all physical traits including height and weight
- FDP can predict some traits (eye color, hair color, ancestry) with varying accuracy, but has limitations and is not routinely used in court as definitive identification (Correct answer)
- FDP is completely unreliable and has no forensic utility
- FDP is only valid for predicting ancestry, not any physical features
Correct answer: FDP can predict some traits (eye color, hair color, ancestry) with varying accuracy, but has limitations and is not routinely used in court as definitive identification
Forensic DNA phenotyping can predict some visible characteristics from DNA, but accuracy varies by trait and it is used investigatively rather than for definitive identification.
Question 6: A DNA analyst notices that a specific locus in a suspect's reference sample shows three alleles instead of the expected maximum of two. What is the most likely explanation?
- PCR contamination with a third person's DNA
- Trisomy of the chromosome carrying that locus, or a rare triallelic pattern (Correct answer)
- Laboratory error requiring re-extraction
- The locus is non-informative and should be excluded
Correct answer: Trisomy of the chromosome carrying that locus, or a rare triallelic pattern
Three alleles at a single autosomal locus can result from chromosomal trisomy, gene duplication, or a rare triallelic variant — not necessarily contamination.
Question 7: When DNA evidence from a crime scene yields a profile matching the victim only, what forensic conclusion is most appropriate?
- The perpetrator left no biological evidence at the scene
- The DNA evidence is consistent with the victim being present; no foreign DNA was detected in this sample (Correct answer)
- The case lacks any DNA evidence and should be dismissed
- The perpetrator and victim share the same DNA profile
Correct answer: The DNA evidence is consistent with the victim being present; no foreign DNA was detected in this sample
A victim-only profile indicates the tested sample did not yield a foreign contributor profile, but does not rule out the presence of undetected biological evidence elsewhere.
CFC DNA Analysis & Interpretation 5
During autopsy, bone marrow samples are collected for DNA identification of a decomposed victim.
Which extraction method is most commonly used for this sample type?