DNA Transfer in Criminal Cases

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Understanding DNA Transfer, Persistence and Contamination

DNA transfer means your DNA can be found on something you never touched, or from contact long ago.
In forensic science, DNA transfer refers to the movement of DNA from one person, object, or surface to another. It may occur through direct contact (primary transfer) or indirectly via an intermediate surface or person (secondary transfer).

In criminal cases, DNA transfer is critical because the detection of a DNA profile does not, of itself, explain how the DNA was deposited, when it was deposited, or whether it resulted from the alleged activity.

Modern forensic science recognises that DNA can be transferred through intermediary surfaces, other individuals, shared environments, or routine handling. As a result, the presence of DNA does not necessarily indicate direct involvement in an offence, nor does it reliably establish timing or manner of contact.

The sensitivity of contemporary DNA testing means that profiles may be generated from very small amounts of biological material. Without careful evaluation at the activity level, DNA evidence may be given greater probative weight than the science can properly support.

DNA transfer must also be considered alongside persistence and contamination. DNA may persist for variable periods, and handling or environmental factors may influence where DNA is ultimately detected. These factors further complicate assumptions about the significance of DNA findings.

Forensic laboratory reports typically address DNA at the source level only. Questions of transfer mechanism and evidentiary significance therefore require independent scientific evaluation.

'Her task was to determine whether the execution of the search and the handling of exhibits indicated a mass transference event that rendered the DNA evidence unreliable. At hearing, Helen’s opinion was preferred to that of the prosecution expert and proved pivotal to our successful application to have the DNA evidence excluded.'
Michael Parella Parella Legal
'Professional and concise report on the prosecution evidence. The result was that the DNA evidence was not led.'
John Gooley Stacks Collins Thompson
'Disclosure to the prosecution of Helen’s comprehensive report resulted in the prosecution determining not to adduce the DNA evidence at all.'
Andrew Culshaw Barrister, Len King Chambers
'Helen’s assistance in two matters now, both where there was a real risk that the Crown’s proposed DNA evidence may have been given undue weight by the jury.'
James Leaver Samuel Griffith Chambers
'The DNA evidence purporting to implicate my client, seemed on the face of it seemed to have strength. Following service of the report, prosecution withdrew the DNA evidence.'
Amelia Ramsay Senior Associate, Doogue and George
'The Prosecution case relied on DNA evidence, raising the difficult issue of how to challenge insufficiently reliable forensic science evidence. Although Helen was prepared to be called by the Defence to give expert evidence, this was not necessary as the Magistrate dismissed all charges at the close of the Prosecution case, agreeing that it was dangerous to convict on circumstantial and unreliable evidence.'
Maeve Curry Barrister, Sir Owen Dixon Chambers
Helen Roebuck CV
2025: Contamination case – DNA excluded – no trial ⟶
2019: DNA Transfer case – DNA excluded - pre-trial ⟶

How DNA transfer occurs

Secondary DNA transfer and criminal evidence

We assist criminal lawyers by:
  • – Evaluating how the DNA may have been deposited, and what alternative pathways remain plausible
  • – Reviewing DNA findings alongside collection, handling and packaging information
  • – Assessing transfer and persistence considerations at activity level- Identifying assumptions that may be driving the interpretation
  • – Providing clear, courtroom-focused advice to support case strategy
  • – Assisting with cross-examination preparation and expert reporting where required
Issues we frequently encounter during reviews of DNA evidence:
  • – Low-level DNA and increased uncertainty
  • – Source-level results overstated as “contact” or involvement
  • – Unknown biological source
  • – Handling and packaging pathways overlooked
  • – Investigator-mediated contamination risks
  • – Unsupported “wearer” or “user” assumptions
  • – Statistics relied upon without context
Consider a review of the DNA evidence when:
  • – The DNA finding is central to the case
  • – The profile is low-level, mixed, or complex
  • – How the DNA got there is disputed (transfer is in issue)
  • – DNA presence is being treated as proof of contact/use
  • – Handling, packaging or contamination risks are realistic
  • – Indirect transfer is a plausible alternative explanation
Depending on your needs we can provide:
  • – Written advice / conference notes
  • – Preliminary guidance
  • – Expert witness report
  • – Cross-examination support materials
  • – Case consultations via video conference

If you are unsure whether the forensic evidence warrants expert involvement, I can provide an initial view and a quote for the appropriate level of review — from targeted advice through to full reporting and trial support.

What we do
We assist criminal lawyers by:
  • – Evaluating how the DNA may have been deposited, and what alternative pathways remain plausible
  • – Reviewing DNA findings alongside collection, handling and packaging information
  • – Assessing transfer and persistence considerations at activity level- Identifying assumptions that may be driving the interpretation
  • – Providing clear, courtroom-focused advice to support case strategy
  • – Assisting with cross-examination preparation and expert reporting where required
Common issues
Issues we frequently encounter during reviews of DNA evidence:
  • – Low-level DNA and increased uncertainty
  • – Source-level results overstated as “contact” or involvement
  • – Unknown biological source
  • – Handling and packaging pathways overlooked
  • – Investigator-mediated contamination risks
  • – Unsupported “wearer” or “user” assumptions
  • – Statistics relied upon without context
When to use us
Consider a review of the DNA evidence when:
  • – The DNA finding is central to the case
  • – The profile is low-level, mixed, or complex
  • – How the DNA got there is disputed (transfer is in issue)
  • – DNA presence is being treated as proof of contact/use
  • – Handling, packaging or contamination risks are realistic
  • – Indirect transfer is a plausible alternative explanation
What you get
Depending on your needs we can provide:
  • – Written advice / conference notes
  • – Preliminary guidance
  • – Expert witness report
  • – Cross-examination support materials
  • – Case consultations via video conference
Quotation

If you are unsure whether the forensic evidence warrants expert involvement, I can provide an initial view and a quote for the appropriate level of review — from targeted advice through to full reporting and trial support.

DISCUSS DNA TRANSFER IN YOUR CASE

R v Kabbout [2020] NSWDC 707

Judicial comment:

“Ms Roebuck opined - If the accused had ridden in the vehicle his DNA could have been deposited on surfaces in it and be available for indirect transfer to the gun. She records, the DNA present on the gun may have been redistributed during handling of it when it was rendered safe by the police. In these circumstances, she opined that it was not possible to determine whether the DNA detected on the handle or grip of the gun was deposited through direct contact or indirect transfer.”

— Judge Buscombe, R v Kabbout [2020] NSWDC 707

State of Western Australia v Piccioni [2025] WADC 69

Judicial comment:

“Ms Roebuck, in providing her opinion, is both more explicit, as well as more accurately aligned with the evidentiary onus being upon the prosecution -The extensive flaws in the crime scene procedure demonstrate numerous pathways for [the accused's] available DNA to have been transferred - Ms Roebuck’s opinion more accurately reflects the significance of secondary transfer … I prefer and adopt her formulation.”

— Judge Astill, State of Western Australia v Piccioni [2025] WADC 69

R v Ke [2019] NSWDC 349

Judicial comment:

“I heard evidence on the application from Helen Roebuck - It is her opinion trace DNA can be transferred directly and indirectly. I refuse to admit the evidence sought to be adduced by the prosecution because if it does have probative value, that value is outweighed by the danger of unfair prejudice to the accused by reason of the possibility of misuse of the evidence by the jury”

— Judge Grant, R v KE [2019] NSWDC 349

Helen Roebuck DNA expert giving evidence

DNA Persistence: Why Timing Assumptions Are Unreliable

DNA COntamination AND handling considerations

  • - DNA may transfer between samples
  • - Shared equipment or surfaces may introduce DNA
  • - Low-level DNA detection increases sensitivity to background DNA

Why DNA TRANSFER REQUIRES EXPERT INTERPRETATION

The presence of a person’s DNA on an item does not establish that they directly handled it. DNA can be deposited through indirect pathways, including secondary or tertiary transfer. Lawyers must guard against assumptions that equate DNA presence with physical interaction, particularly where objects are commonly handled or have moved between multiple people or environments.

Secondary DNA transfer occurs easily through everyday interactions such as handshakes or shared surfaces. This mechanism is well documented in forensic studies but is frequently underexplained in court. Lawyers must consider whether an alternative, innocent transfer pathway could reasonably account for the DNA findings, especially in the absence of corroborating evidence.

Highly sensitive DNA testing can detect extremely small quantities of genetic material. While powerful, low-level DNA results are more vulnerable to contamination, transfer, and stochastic effects. Lawyers must assess whether trace DNA evidence has been given disproportionate evidential weight relative to its scientific limitations and uncertainty.

Forensic DNA analysis generally addresses source-level questions, not how or when DNA was deposited. Activity-level propositions require careful scientific consideration beyond profile comparison. Lawyers must ensure that DNA evidence is not used to infer specific actions unless transfer, persistence, and context have been properly evaluated and explained.

DNA interpretation involves assumptions about contributors, transfer mechanisms, and context. Different assumptions can lead to different conclusions from the same data. Lawyers must be alert to whether these assumptions are justified, disclosed, and balanced, as unsupported assumptions can significantly overstate the apparent strength of DNA evidence.

Direct Contact

The presence of a person’s DNA on an item does not establish that they directly handled it. DNA can be deposited through indirect pathways, including secondary or tertiary transfer. Lawyers must guard against assumptions that equate DNA presence with physical interaction, particularly where objects are commonly handled or have moved between multiple people or environments.

Secondary Transfer

Secondary DNA transfer occurs easily through everyday interactions such as handshakes or shared surfaces. This mechanism is well documented in forensic studies but is frequently underexplained in court. Lawyers must consider whether an alternative, innocent transfer pathway could reasonably account for the DNA findings, especially in the absence of corroborating evidence.

Low Level DNA

Highly sensitive DNA testing can detect extremely small quantities of genetic material. While powerful, low-level DNA results are more vulnerable to contamination, transfer, and stochastic effects. Lawyers must assess whether trace DNA evidence has been given disproportionate evidential weight relative to its scientific limitations and uncertainty.

Activity Level

Forensic DNA analysis generally addresses source-level questions, not how or when DNA was deposited. Activity-level propositions require careful scientific consideration beyond profile comparison. Lawyers must ensure that DNA evidence is not used to infer specific actions unless transfer, persistence, and context have been properly evaluated and explained.

Interpretation v Data

DNA interpretation involves assumptions about contributors, transfer mechanisms, and context. Different assumptions can lead to different conclusions from the same data. Lawyers must be alert to whether these assumptions are justified, disclosed, and balanced, as unsupported assumptions can significantly overstate the apparent strength of DNA evidence.

Helen Roebuck DNA expert Sydney

Next Step

Contact Roebuck Forensics

A preliminary DNA expert review can be a useful step in assessing the broad strengths and weaknesses of a particular matter. This process can also identify documentary and any further evidence requirements.

Rigorous interrogation of the evidence will uncover underlying issues and determine the most appropriate pathway towards reviewing and reporting the matter.

Certain complex matters benefit from a draft DNA expert report, which can open areas for discussion with Counsel, and potentially allow for defence to approach the prosecution.

Following a thorough evaluation of the evidence, a DNA expert report will be issued in accordance with the expert witness code of conduct. The report will be suitable for submission in evidence.

Preparations with Counsel are often conducted, such that the probative value of the evidence is weighed effectively and persuasively at Voir Dire, should such a hearing be required.

Extensive preparations are generally conducted in anticipation of substantive hearing. Which may include cross examination and evidence in chief scenarios specific to the matter .

DNA TRANSFER QUESTIONS

What is DNA transfer in forensic cases?

DNA transfer refers to the movement of DNA from a person to an object or surface. This can occur through direct contact (primary transfer) or indirectly via another person or object (secondary transfer).

Can DNA be transferred without a person touching an object?

Yes. Secondary DNA transfer is well established in forensic science. DNA can be transferred through intermediaries such as other people, clothing, or surfaces. This means DNA may appear on an object even if the person never directly handled it, which is a critical consideration in criminal cases.

Does the presence of DNA prove someone handled an item?

No. DNA presence alone cannot establish how or when the DNA was deposited. DNA analysis typically cannot distinguish between direct contact, indirect transfer, or innocent prior contact. Courts must consider DNA findings alongside transfer mechanisms and case context rather than assuming handling occurred.

How common is secondary DNA transfer?

Secondary DNA transfer is common and can occur during routine everyday interactions. Studies have shown that brief contact, such as a handshake followed by handling an object, can result in detectable DNA transfer. Its likelihood depends on factors such as shedding propensity, surface type, and handling conditions.

Why is DNA transfer a risk in touch DNA cases?

Touch DNA samples often contain very small amounts of DNA. These low-level samples are particularly vulnerable to transfer, contamination, and interpretational uncertainty. As a result, touch DNA evidence carries a higher risk of misleading conclusions if transfer mechanisms are not carefully considered.

Can forensic DNA analysis determine when DNA was deposited?

Generally, no. Forensic DNA analysis cannot reliably determine when DNA was deposited or how long it has been present. Without additional contextual evidence, DNA results cannot establish whether DNA was deposited during the alleged offence or at an earlier, innocent time.

How does DNA transfer affect mixed DNA profiles?

DNA transfer can contribute to complex DNA mixtures involving multiple contributors. Transfer mechanisms may introduce DNA from individuals who were not present at the alleged activity. Interpreting such mixtures requires careful assessment of transfer possibilities and contributor assumptions.

Is DNA transfer routinely considered in forensic reports?

DNA transfer is not always fully addressed in standard forensic reports, particularly where conclusions focus on profile comparisons or statistics. An independent DNA expert can assess whether transfer mechanisms were adequately considered and whether alternative explanations were properly evaluated.

Can DNA transfer lead to wrongful conclusions in court?

Yes. If DNA transfer is not properly considered, courts may incorrectly infer direct involvement based solely on DNA presence. Overlooking transfer risks can lead to overstated conclusions and misinterpretation of the evidence, particularly in cases relying heavily on DNA results.

When should a lawyer seek expert advice on DNA transfer?

Expert advice should be sought when DNA evidence is central to the case, involves low-level or mixed profiles, or where indirect transfer is a plausible explanation. Early expert input helps ensure DNA evidence is  interpreted accurately and prevents assumptions from shaping legal strategy.

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