Forensic DNA evidence plays a central role in many criminal cases across Australia. It is often perceived as definitive, objective, and conclusive. However, DNA evidence is only as strong as the way it is collected, analysed, interpreted, and presented.
For lawyers, a working understanding of forensic DNA analysis is essential – not to become scientists, but to ask the right questions, recognise limitations, and ensure DNA evidence is evaluated fairly and accurately. This article outlines the key concepts every lawyer should understand when dealing with forensic DNA evidence in Australian courts.
WhAT IS FORENSIC DNA ANALYSIS?
Forensic DNA analysis involves examining biological material (such as blood, saliva, skin cells, hair or semen) to generate a DNA profile that can be compared with known individuals or databases.
The typical process includes:
- Collection of biological material
- Extraction of DNA
- Quantification (measuring how much DNA is present)
- Amplification using PCR
- Profile generation
- Interpretation and reporting
Each stage introduces potential limitations, assumptions, and sources of uncertainty that can affect the final result.
DNA Profiling Does Not Equal Identification
A common misconception is that a DNA profile “identifies” a person. In reality, forensic DNA results are statistical in nature.
DNA analysis evaluates:
- 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
This likelihood is expressed using statistical measures, not absolute certainty. Understanding this distinction is critical when assessing the strength of DNA evidence.
Types of DNA Evidence Commonly Encountered in Court
Lawyers may encounter several types of DNA evidence, including:
Single-Source DNA
DNA that appears to come from one individual. These profiles are typically easier to interpret but can still raise issues around contamination or transfer.
Mixed DNA Profiles
DNA from two or more contributors. Mixed profiles are common and often significantly more complex to interpret, particularly when contributors are present at different levels.
Low-Level DNA PROFILE
Very small quantities of DNA which may be attributable to skin cells or biological fluids. Low- Level DNA complicates and provided greater ambiguity around transfer considerations.
TRACE DNA
DNA which cannot be attributed to a specific biological fluid, commonly from skin cells. Trace DNA is often aligned to what the lay person refers to as touch DNA.
DNA Transfer and Activity-Level Questions
One of the most important concepts for lawyers is that DNA presence does not explain activity.
Key questions include:
- How did the DNA get there?
- When was it deposited?
- Was it transferred directly or indirectly?
DNA analysis generally cannot answer these questions on its own. Expert interpretation is required to assess whether the DNA findings are consistent with the alleged activity or whether alternative explanations are plausible.
Secondary Transfer and Its Legal Significance
Secondary DNA transfer – where DNA is transferred via an intermediary – is now well established in forensic science literature.
This means:
- 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
Courts increasingly expect experts to address transfer mechanisms rather than relying solely on profile comparisons.
Probabilistic Interpretation and STRmix™
Many Australian laboratories use probabilistic genotyping software, such as STRmix™, to interpret complex DNA mixtures.
These systems:
- 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.
Lawyers should understand that software output is not infallible and that expert oversight is essential when reviewing probabilistic results.
Common Pitfalls in DNA Evidence Interpretation
Some recurring issues seen in forensic DNA cases include:
- 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
An independent DNA expert can assist in identifying whether conclusions and opinions are appropriately supported by the data.
The Role of the Independent DNA Expert Witness
An independent forensic DNA expert provides:
- 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
Independence is particularly important where DNA evidence is central to the prosecution case or where interpretational assumptions may be contested.
When Should Lawyers Seek a DNA Case Review?
A DNA case review should be considered when:
- 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
Early expert involvement allows legal strategy to be informed by scientific reality rather than assumptions.
Conclusion
Forensic DNA analysis is a powerful tool, but it is not infallible. Understanding its limitations, assumptions, and interpretational challenges is essential for lawyers navigating DNA evidence in Australian courts.
With proper expert guidance, DNA evidence can be evaluated accurately, fairly, and in context — ensuring it assists the court rather than misleading it.
Helen Roebuck is a strongly credentialed and highly regarded forensic DNA expert, trusted within both the legal and scientific communities. She is the most prolific and judgment-published independent DNA expert witness in Australia, with her evidence extensively referenced across various jurisdiction
Need clarity on forensic DNA evidence in your case?
If DNA analysis is central to the matter, an independent review can help identify limitations, interpret complex results, and assess whether the conclusions drawn are scientifically justified.
Contact Roebuck Forensics to discuss independent DNA case review or expert witness support