CSI - Crime Scene Investigator Biological and Trace Evidence Questions and Answers 1 — Questions and Answers
Question 1: An investigator uses a luminol-based test on a carpet at a homicide scene, which results in a positive reaction indicating the possible presence of blood. Which of the following statements is most accurate regarding this result?
- The test is a presumptive test and only suggests that blood might be present, requiring further confirmatory testing. (Correct answer)
- The luminol reaction is a confirmatory test, and no further analysis is needed to state that human blood is present.
- A positive luminol result definitively proves the substance is human blood and can be used to identify a suspect.
- The test is only considered valid if the resulting chemiluminescence is visible for more than five minutes without a light source.
Correct answer: The test is a presumptive test and only suggests that blood might be present, requiring further confirmatory testing.
Luminol is a classic example of a presumptive (or screening) test. It is highly sensitive to the presence of hemoglobin but can produce false positives with other substances like bleach, certain metals, and plant peroxidases. Therefore, a positive result only indicates the possibility of blood, and a more specific confirmatory test, such as RSID or immunological tests, must be performed to confirm its presence and origin.
Question 2: At a scene, a CSI finds several loose, dark fibers on a light-colored couch near the victim. Which of the following is the most appropriate method for collecting these fibers to prevent loss and contamination?
- Brushing the fibers directly into a large paper evidence bag.
- Using a standard vacuum cleaner with a new bag to collect all debris from the couch.
- Applying adhesive fingerprint tape to lift the fibers from the surface.
- Using clean forceps to pick up individual fibers and placing them into a folded paper bindle, which is then placed in a labeled envelope. (Correct answer)
Correct answer: Using clean forceps to pick up individual fibers and placing them into a folded paper bindle, which is then placed in a labeled envelope.
The preferred method for collecting visible trace evidence like fibers is to use clean forceps or tweezers to handle them individually. This prevents contamination and preserves their integrity. The fibers should then be placed into a paper bindle (also known as a druggist's fold), which keeps the tiny evidence secure. This bindle is then placed into a properly sealed and labeled envelope or other container. Using tape can be destructive, and brushing or vacuuming a large area risks cross-contamination and loss of context.
Question 3: A forensic lab receives a small, degraded bone fragment from a decades-old cold case. Investigators hope to link the remains to a maternal relative of the suspected victim. Which type of DNA analysis would be most suitable and likely to yield a result?
- Short Tandem Repeat (STR) analysis of nuclear DNA.
- Restriction Fragment Length Polymorphism (RFLP) analysis.
- Mitochondrial DNA (mtDNA) analysis. (Correct answer)
- Y-STR analysis.
Correct answer: Mitochondrial DNA (mtDNA) analysis.
Mitochondrial DNA (mtDNA) is ideal for this scenario for two key reasons. First, mtDNA exists in much higher copy numbers per cell compared to nuclear DNA, increasing the chance of obtaining a viable profile from small or degraded samples like old bone. Second, mtDNA is inherited maternally, meaning an individual's mtDNA sequence is the same as their mother, siblings, and other relatives along the maternal line, making it perfect for establishing a link to a maternal relative.
Question 4: An officer is collecting a Gunshot Residue (GSR) kit from the hands of a suspect who was apprehended three hours after a shooting. What is the correct procedure for using the SEM stubs for collection?
- Rub the stub vigorously across both the palm and back of each hand.
- Use a dabbing or pressing motion to sample the web area between the thumb and index finger, as well as the palms. (Correct answer)
- Use one stub for the right hand, one for the left hand, one for the face, and one as a control.
- Moisten the stub with distilled water before applying it to the suspect's skin to improve adhesion.
Correct answer: Use a dabbing or pressing motion to sample the web area between the thumb and index finger, as well as the palms.
The proper technique for collecting GSR with a scanning electron microscope (SEM) stub is to use a dabbing or pressing motion, not rubbing. This ensures the microscopic particles adhere to the stub's surface. Key areas to sample are the back of the hand, especially the web area between the thumb and forefinger, and the palm. Each hand should be sampled with a separate, clearly labeled stub.
Question 5: Which of the following pieces of trace evidence should be packaged in an airtight container to prevent its degradation or loss?
- A single human hair with the root attached.
- A blood-soaked piece of fabric that is still damp.
- A small sample of dry soil from a shoeprint.
- A piece of charred wood flooring suspected of containing an accelerant. (Correct answer)
Correct answer: A piece of charred wood flooring suspected of containing an accelerant.
Evidence from a suspected arson scene, such as charred material that may contain a volatile accelerant (e.g., gasoline), must be packaged in an airtight container like an unlined metal paint can or a glass jar. This prevents the volatile chemical compounds from evaporating before they can be analyzed at the laboratory. Biological evidence like wet bloodstains should be air-dried and packaged in paper to prevent mold and bacterial growth, while other dry trace evidence is typically packaged in paper bindles or vials.
Question 6: A DNA profile is generated from a hair shaft (not the root). Which of the following is true about the evidentiary value of this profile?
- It provides a unique nuclear DNA profile that can identify a single individual.
- It cannot be used for any meaningful comparison as DNA is only found in the hair root.
- It can be used to definitively link a suspect to a crime scene with the same certainty as a blood sample.
- It yields mitochondrial DNA (mtDNA), which can exclude individuals but cannot individualize; it can only associate a person with a maternal lineage. (Correct answer)
Correct answer: It yields mitochondrial DNA (mtDNA), which can exclude individuals but cannot individualize; it can only associate a person with a maternal lineage.
The hair shaft contains mitochondrial DNA (mtDNA), not nuclear DNA (which is found in the root). An mtDNA profile is not unique to an individual. Instead, it is shared among all individuals in the same maternal line (mother, siblings, maternal aunts, etc.). Therefore, it can be powerful for excluding a suspect but cannot be used to identify a single person with the certainty of nuclear DNA; it can only show an association to a maternal family line.
CSI - Crime Scene Investigator Biological and Trace Evidence Questions and Answers 1
An investigator uses a luminol-based test on a carpet at a homicide scene, which results in a positive reaction indicating the possible presence of blood.
Which of the following statements is most accurate regarding this result?