BSFS Forensic Biology and DNA Analysis 2 — Questions and Answers
Question 1: A mixed DNA profile occurs when a forensic sample contains genetic material from:
- Two or more contributors (Correct answer)
- A degraded single-source sample
- Amplified DNA that has cycled too many times
- Only male contributors to the sample
Correct answer: Two or more contributors
Mixed profiles contain alleles at one or more loci from multiple individuals, which complicates statistical interpretation and contributor deduction.
Question 2: Touch DNA refers to DNA recovered from:
- Skin cells deposited when a person handles or touches an object (Correct answer)
- Blood left at a scene after cleaning
- Saliva on cigarette butts only
- Hair with attached follicles
Correct answer: Skin cells deposited when a person handles or touches an object
When a person touches a surface, they deposit epithelial (skin) cells containing nuclear DNA, which can be collected and profiled even without visible biological fluid.
Question 3: Y-STR analysis is specifically used in sexual assault cases because it can:
- Distinguish the male contributor's profile from a predominantly female victim sample (Correct answer)
- Determine the exact number of sperm cells present
- Identify the blood type of the male contributor
- Date when sexual contact occurred
Correct answer: Distinguish the male contributor's profile from a predominantly female victim sample
Y-STRs target the male-specific Y chromosome, allowing the male contributor's profile to be recovered even from mixtures dominated by female DNA.
Question 4: The differential extraction technique is used in sexual assault evidence to:
- Separately isolate sperm cell DNA from non-sperm (epithelial) cell DNA in a mixed sample (Correct answer)
- Extract nuclear DNA from mitochondrial DNA
- Remove blood contamination from semen samples
- Isolate nuclear DNA from hair shaft samples
Correct answer: Separately isolate sperm cell DNA from non-sperm (epithelial) cell DNA in a mixed sample
Differential extraction exploits the different cell wall structure of sperm cells to separate them from epithelial cells before DNA extraction, reducing mixture complexity.
Question 5: In STR analysis, 'allelic dropout' refers to:
- Failure to detect one or both alleles at a locus due to low or degraded DNA (Correct answer)
- The presence of extra alleles from contamination
- Deletion of an STR locus from the CODIS database
- An artifact produced during PCR amplification
Correct answer: Failure to detect one or both alleles at a locus due to low or degraded DNA
Allelic dropout occurs when a true allele is not amplified or detected, typically due to degraded template DNA or inhibitors, potentially causing a false exclusion.
Question 6: Phenolphthalein (Kastle-Meyer) test is a presumptive test used to detect:
- Blood (Correct answer)
- Semen
- Saliva
- Urine
Correct answer: Blood
The phenolphthalein test detects the peroxidase-like activity of hemoglobin in blood, producing a pink color as a rapid presumptive screening result.
BSFS Forensic Biology and DNA Analysis 2
A mixed DNA profile occurs when a forensic sample contains genetic material from: