Forensic Science: DNA and Fingerprint Analysis
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1
Scenario: A DNA mixture contains three apparent contributors, but one contributor's alleles overlap extensively with another's. What analytical concern must be addressed?
Contributor overlap and uncertainty in mixture interpretation must be evaluated using validated methods and appropriate statistical models.
2
Scenario: A DNA sample produces low-level peaks near the laboratory's analytical threshold. What should the analyst consider?
Stochastic effects, possible allele dropout or drop-in, contamination, and the method's validated interpretation limits.
3
Scenario: A suspect's DNA is detected on a knife recovered from a shared household. What important question remains unresolved?
How and when the DNA was deposited, since source attribution alone does not establish use of the knife during the offense.
4
Scenario: A forensic DNA profile contains a locus with three alleles. What explanations should be evaluated?
A DNA mixture, a biological variant such as a tri-allelic pattern, or an analytical artifact, depending on the full profile.
5
Scenario: A DNA extraction blank produces an unexpected human DNA profile. What is the appropriate response?
Investigate contamination and assess the effect on associated samples before interpreting or reporting affected results.
6
Scenario: A latent fingerprint shows several corresponding ridge features but significant unexplained differences from a suspect's print. What should the examiner do?
Evaluate distortion and other explanations, and avoid identification when significant differences cannot be satisfactorily resolved.
7
Scenario: Two latent fingerprint examiners disagree on the sufficiency of ridge detail. What procedure is appropriate?
Apply the laboratory's documented conflict-resolution or independent review procedure rather than forcing agreement.
8
Scenario: A fingerprint recovered from curved glass shows stretched ridge spacing. What factor may explain the distortion?
Contact pressure, movement, and the geometry of the curved surface can distort transferred ridge impressions.
9
Scenario: Automated fingerprint software ranks a candidate first. Does this establish identification?
No. A ranked candidate is an investigative lead requiring appropriate qualified examination and verification.
10
Scenario: A latent print is developed using powder before possible DNA collection. What should be considered?
The sequence of examinations, potential contamination, and whether fingerprint processing may affect subsequent biological testing.
11
Scenario: A bullet has matching class characteristics with a test-fired bullet but insufficient individual detail. What conclusion is appropriate?
The evidence may support class consistency, but a specific firearm identification is not justified without sufficient discriminating detail.
12
Scenario: A cartridge case has firing-pin and breechface marks, but a firearm's parts were replaced after the incident. What concern arises?
Replacement parts may alter the marks produced, complicating comparison with evidence from an earlier firing.
13
Scenario: A shooting victim has no detectable gunshot residue on the hands. Does this establish that the person did not fire a weapon?
No. Residue may be absent or lost because of ammunition type, environmental conditions, time, activity, or sampling limitations.
14
Scenario: An apparent bullet entrance wound lacks soot and stippling. Can the examiner determine that the shot was fired from a long distance?
No. The absence of these features alone does not establish distance; ammunition, barriers, clothing, and other conditions must be considered.
15
Scenario: A bullet passes through glass before striking a victim. What effect may this have on trajectory reconstruction?
The barrier may deflect, deform, or destabilize the bullet, limiting straight-line trajectory assumptions.
16
Scenario: A blood sample tests positive on a presumptive blood test but negative on a more specific examination. What conclusion is appropriate?
The presumptive reaction alone does not confirm the presence of blood; the results and limitations of both tests must be evaluated.
17
Scenario: A suspected bloodstain contains a mixture of human and animal biological material. What analytical concern arises?
Species-specific testing and interpretation of mixed biological components may be necessary.
18
Scenario: A DNA swab from a bloodstain contains a profile inconsistent with the victim. Does this prove that the blood came from another person?
No. DNA may originate from other biological material or contributors, so the source of the blood itself requires separate evaluation.
19
Scenario: A bloodstain pattern appears elongated on an inclined surface. What should be considered before estimating impact angle?
Surface texture, slope, stain formation, distortion, and the limitations of geometric assumptions.
20
Scenario: A scene contains a large blood pool but relatively few impact stains. Can the examiner conclude that no violent impact occurred?
No. Blood pooling and impact staining reflect different mechanisms, and the absence of one pattern does not exclude another event.
21
Scenario: A deceased person has elevated blood alcohol measured from a decomposed body. What complication must be evaluated?
Postmortem ethanol production and specimen quality, using appropriate corroborating samples and findings.
22
Scenario: A urine drug screen is positive for an amphetamine-class substance. What is needed before identifying a specific drug conclusively?
An appropriately validated confirmatory method capable of distinguishing the relevant substances.
23
Scenario: A drug is detected in postmortem blood at a concentration associated with toxicity in some cases. Does this alone establish fatal poisoning?
No. Interpretation requires consideration of tolerance, interactions, postmortem redistribution, pathology, and the circumstances of death.
24
Scenario: A toxicology report shows a drug concentration below a therapeutic reference range. Can the drug be excluded as a contributor to death?
No. Individual susceptibility, interactions, specimen type, and other circumstances may affect interpretation.
25
Scenario: A suspected poisoning sample contains an unknown compound that produces a library match in mass spectrometry. What is the next analytical concern?
The proposed identity should be evaluated using validated identification criteria, appropriate standards, and possible alternative compounds.
26
Scenario: A body has extensive decomposition and an apparent wound with irregular margins. What should be considered?
Postmortem tissue changes, insect activity, and environmental damage may mimic or obscure injuries.
27
Scenario: A skull fracture has sharp margins but no visible healing. Can its timing automatically be classified as perimortem?
No. Fracture morphology, bone condition, associated damage, and the broader context must be evaluated.
28
Scenario: A fire victim has substantial thermal injury but no detectable carbon monoxide in blood. Does this prove death occurred before the fire?
No. Carbon monoxide exposure depends on fire conditions and circumstances; the finding must be interpreted with other evidence.
29
Scenario: A submerged body shows advanced decomposition. Why may conventional postmortem interval estimates be unreliable?
Water temperature, currents, depth, scavenging, and environmental conditions can substantially alter decomposition.
30
Scenario: Insect larvae recovered from a corpse are older than the documented period since the body was placed at the discovery site. What possibilities should be investigated?
Colonization before relocation, inaccurate scene timelines, species or age-estimation errors, and environmental assumptions.
31
Scenario: A questioned signature differs from known specimens in slant but matches several complex movement patterns. What should be evaluated?
Natural variation, writing conditions, disguise, and the combined significance of similarities and differences.
32
Scenario: Two entries on a contract have visually identical ink colors but different infrared responses. What is the appropriate inference?
The inks may differ in composition or behavior, warranting further examination without automatically proving alteration.
33
Scenario: An ESDA examination reveals an indented date different from the visible date on a document. What can be concluded?
An indentation corresponding to different writing is present, but its origin, timing, and relationship to the visible entry require investigation.
34
Scenario: A photocopied signature exactly overlays a known signature. Does this establish that the questioned document was genuinely signed?
No. The signature may have been reproduced or transferred digitally or mechanically, so original execution has not been established.
35
Scenario: A purportedly old document contains a printing material introduced more recently. What forensic question arises?
Whether the material is inconsistent with the claimed production date, considering possible later additions or replacements.
36
Scenario: Glass fragments from a suspect's shoes and a broken window share color and refractive index but differ in elemental composition. What conclusion is justified?
The fragments are distinguishable despite their shared physical characteristics.
37
Scenario: Fibers on a victim's clothing match a suspect's jacket in color and polymer type, but that fiber is widely used. How should the association be described?
As a limited association whose strength depends on the prevalence and discrimination of the observed characteristics.
38
Scenario: A paint fragment physically fits a damaged vehicle panel and its layer sequence is continuous. What is the evidential significance?
The complementary physical features provide strong support for an association between the fragment and the panel.
39
Scenario: Soil recovered from footwear resembles soil at a crime scene, but the scene has widespread imported fill. What must be evaluated?
The background distribution and discriminating value of the soil characteristics, including alternative sources.
40
Scenario: A recovered hair has no root tissue but contains usable mitochondrial DNA. What limitation applies?
A matching mitochondrial DNA profile may be shared by maternal relatives and other individuals and does not uniquely identify one person.
41
Scenario: A laboratory instrument fails calibration after several case samples were analyzed. What should the laboratory investigate?
The period and extent of the failure, its effect on prior results, and whether reanalysis or corrected reporting is necessary.
42
Scenario: A positive control fails during a batch of forensic tests. Can the batch results automatically be accepted?
No. The failed control must be investigated, and affected results should not be accepted without an appropriate documented justification or corrective action.
43
Scenario: A forensic analyst receives information that the suspect confessed before examining ambiguous evidence. What risk arises?
Contextual bias may influence interpretation, supporting the use of task-relevant information controls.
44
Scenario: An examiner changes an interpretation after technical review. What documentation should be retained?
The original observations, review findings, reasons for the change, and the final authorized interpretation.
45
Scenario: A laboratory reports a measurement of 0.98 with expanded uncertainty of 0.06 near a legal threshold of 1.00. What should be considered?
The measurement uncertainty and applicable decision rule before classifying the result relative to the threshold.
46
Scenario: An expert says that two samples are indistinguishable and therefore must share a source. What is the scientific error?
Failure to distinguish an inability to differentiate samples from proof that they originated from one unique source.
47
Scenario: An expert calculates a very large likelihood ratio but does not identify the competing propositions. What is missing?
The explicit propositions, relevant data, assumptions, and model necessary to interpret the likelihood ratio.
48
Scenario: A forensic report contains a conclusion beyond the method's validated capabilities. What should the reviewer require?
Revision to a conclusion supported by the validated method, documented observations, and stated limitations.
49
Scenario: An expert discovers during cross-examination that a reported result was based on an incorrect calculation. What is the proper professional response?
Acknowledge the error, explain its potential significance, and initiate appropriate correction and notification procedures.
50
Scenario: DNA, fingerprint, and trace evidence independently support contact between a suspect and an object. Does this automatically prove the suspect committed the charged offense?
No. The evidence must be evaluated with transfer mechanisms, timing, alternative explanations, and all other relevant case facts.
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