Biological and DNA Evidence Handling
What this domain tests
Biological evidence questions test contamination avoidance, appropriate swabbing and drying, packaging, storage, and the difference between presumptive and confirmatory testing.
Core rules and concepts
- DNA sources include blood, semen, saliva, hair root, bone, teeth, and touch DNA; epithelial cells transfer readily.
- Wear gloves, change them often, use single-use tools, and avoid talking or coughing over evidence.
- Air-dry wet biological evidence; package in paper; store cool and dry; freeze only per laboratory policy.
- Collect substrate controls and elimination samples from victims and known donors.
- Presumptive tests such as Kastle-Meyer or luminol indicate possible blood; confirmatory tests are needed before a definitive statement.
- STR analysis generates a profile; CODIS eligibility depends on jurisdiction and profile quality.
Worked example
A steering wheel may hold touch DNA. You moisten a sterile swab with sterile water, swab the area, air-dry the swab, place it in a paper envelope, and collect the owner's elimination sample. Reasoning: moisture improves cell collection; air drying and paper reduce degradation; the elimination sample explains the owner's expected profile.
Traps
Using plastic for a damp swab; reusing a swab; touching the swab tip; drying with heat; skipping controls; presumptive testing the entire sample intended for DNA.