WhAT IS FORENSIC DNA ANALYSIS?
- Collection of biological material
- Extraction of DNA
- Quantification (measuring how much DNA is present)
- Amplification using PCR
- Profile generation
- Interpretation and reporting
DNA Profiling Does Not Equal Identification
- How likely the observed DNA profile is if it originated from a particular individual
- Compared to how likely it is if it originated from an unknown, unrelated person
Types of DNA Evidence Commonly Encountered in Court
Single-Source DNA
Mixed DNA Profiles
Low-Level DNA PROFILE
TRACE DNA
DNA Transfer and Activity-Level Questions
- How did the DNA get there?
- When was it deposited?
- Was it transferred directly or indirectly?
Secondary Transfer and Its Legal Significance
- A person’s DNA can appear on an object they never touched
- DNA presence does not necessarily imply involvement in a crime
- Context is critical when interpreting results
Probabilistic Interpretation and STRmix™
- Apply mathematical models to evaluate competing propositions
- Produce likelihood ratios rather than categorical conclusions
- Are highly powerful but depend on correct assumptions and input parameters, including appropriate ethnicity base selection and familial associations.
Common Pitfalls in DNA Evidence Interpretation
- Overstating the strength of the evidence
- Failing to address transfer and contamination
- Treating statistical results as definitive proof
- Inadequate explanation of limitations to the court
The Role of the Independent DNA Expert Witness
- Objective interpretation of laboratory results
- Review of DNA casework and reporting
- Clear explanation of complex science for courts
- Assistance with cross-examination and expert conferences
When Should Lawyers Seek a DNA Case Review?
- DNA is the primary evidence
- Mixed or low-level DNA is involved
- Secondary transfer is a realistic possibility
- Conclusions appear overstated or simplified
- The case hinges on activity-level interpretation