DNA Transfer, Persistence and Contamination

Independent review of how DNA may have been deposited

DNA can transfer in ways that are not intuitive.

The presence of a person’s DNA on an item may be entirely consistent with the prosecution narrative – but it may also arise through indirect transfer, prior innocent contact, environmental factors, or handling and packaging.

Roebuck Forensics provides independent review of DNA transfer, persistence and contamination issues, with a focus on what can (and cannot) be concluded from the DNA results in the real-world context of the case.

This service is designed for matters where the key question is not simply:

“Whose DNA is it?”

…but rather:

“How did it get there – and what does it mean?”

In forensic science, those questions often sit at activity level and require careful consideration of transfer mechanisms, persistence timeframes, handling pathways, and the limitations of what the testing can resolve.

 

DNA can transfer in ways that are not intuitive.

The presence of a person’s DNA on an item may be entirely consistent with the prosecution narrative – but it may also arise through indirect transfer, prior innocent contact, environmental factors, or handling and packaging.

Roebuck Forensics provides independent review of DNA transfer, persistence and contamination issues, with a focus on what can (and cannot) be concluded from the DNA results in the real-world context of the case.

This service is designed for matters where the key question is not simply:

“Whose DNA is it?”

…but rather:

“How did it get there – and what does it mean?”

In forensic science, those questions often sit at activity level and require careful consideration of transfer mechanisms, persistence timeframes, handling pathways, and the limitations of what the testing can resolve.

 

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.

GET A QUOTE

Direct versus indirect DNA transfer

In forensic reporting, direct transfer is typically used to describe primary transfer — where biological material is deposited onto an item through immediate contact with the original source.

Indirect transfer refers to secondary transfer and beyond, including tertiary, quaternary (and further) transfer, where biological material is moved via one or more intermediate surfaces or contacts before it is detected.

Understanding this distinction is often critical where the presence of DNA is being relied upon as evidence of contact or involvement.

Helen Roebuck DNA expert giving evidence

Experience that translates to courtroom reliability

Transfer and contamination issues are often where DNA evidence is most easily overstated — not because the science is unsound, but because the context is uncertain and the mechanisms of deposition are rarely observable. Roebuck Forensics’ approach is grounded in extensive experience across crime scene, laboratory and courtroom contexts, including serious criminal matters where activity-level propositions are central to the dispute.

The focus remains consistent: to identify what the DNA evidence supports, what it does not support, and where transfer pathways, persistence limitations, handling factors, or alternative explanations must be considered before the evidence is relied upon in court.

Beyond the statistic: what we review

Transfer and contamination assessment is case-specific. Depending on the material available, I may assist by:

• reviewing reported DNA findings and associated documentation
• evaluating whether transfer and persistence factors have been properly considered
• identifying assumptions that may be embedded in the interpretation
• considering whether alternative transfer pathways remain plausible
• assessing contamination risk factors (collection, handling, packaging and laboratory issues)
• preparing clear advice that is usable for strategy, negotiations and court

Where appropriate, I can also assist with drafting cross-examination themes focused on what the evidence does not establish.

DNA contamination riskS

Contamination is an umbrella term describing the introduction of DNA into a sample or onto an exhibit through pathways other than the alleged event.

It may arise through:

  • scene collection and handling
  • packaging and transport
  • storage conditions
  • examination workflows
  • or laboratory processes

The presence of robust laboratory procedures reduces contamination risk, but does not eliminate the need to consider whether contamination remains a realistic possibility in the circumstances of the case.

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

Why Roebuck Forensics

I have worked across every part of the forensic process - from crime scenes, to laboratory interpretation, to expert witness testimony.

My focus is not simply the presence of DNA or a statistic.

It is what the evidence can actually support — in the context of the case.

Next Step

If you’d like to discuss whether the forensic evidence in your matter warrants further review:

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.

QCertain 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 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). In forensic cases, DNA transfer means that the presence of DNA does not necessarily indicate direct contact or involvement in an alleged offence.

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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