what Is DNA contamination in forensic science?
- Improper handling at the crime scene
- Inadequate use of personal protective equipment (PPE)
- Laboratory contamination during analysis
- DNA already present on collection tools or packaging
Understanding DNA Transfer: Primary vs Secondary Transfer
Primary DNA Transfer
- Handling a weapon
- Wearing clothing
- Touching a door handle
Secondary DNA Transfer
- Person A shakes hands with Person B
- Person B later handles an object
- Person A’s DNA is detected on the object
What is direct and Indirect DNA transfer?
Why DNA Transfer Challenges Traditional Assumptions
- Type of surface (porous vs non-porous)
- Amount of handling
- Shedding propensity of individuals
- Environmental conditions
- Time elapsed
Low-Level DNA and Increased Risk of Misinterpretation
- More prone to contamination
- More susceptible to transfer effects
- Often complex mixtures from multiple contributors
DNA Mixtures and Transfer Complexity
- Whether contributors deposited DNA at the same time
- Whether contributors were present at the scene
- Whether DNA arrived via indirect transfer
Laboratory Controls and Their Limitations
- Elimination databases
- Negative controls
- Dedicated clean areas
DNA Evidence in Court: Why Context Matters
- Activity-level propositions
- Transfer and persistence mechanisms
- Alternative explanations for DNA presence
- What the DNA results do and do not show
- Whether secondary transfer is plausible
- The limitations of low-level DNA evidence
When Should a DNA Case Review Be Considered?
- DNA is the primary or sole evidence
- Low-level or trace DNA is involved
- DNA mixtures are complex
- Secondary transfer is a plausible explanation
- Conclusions appear overstated or definitive