In criminal proceedings involving DNA evidence, the question of transfer is almost always present. Even experienced practitioners recognise that the presence of DNA does not, of itself, establish how it was deposited. The familiar proposition — that science cannot say how DNA got there — is now widely understood within the profession.

In practice, DNA transfer issues arise in almost every trial where DNA evidence is relied upon — whether the issue is direct contact, indirect transfer, persistence, or contamination risk.

This understanding often shapes trial strategy.

Some matters proceed on the basis that transfer is inherently uncertain, and that maintaining simplicity in cross-examination avoids confusing the jury or inadvertently strengthening the prosecution narrative. In other cases, the issue is engaged more directly, with attempts to challenge the forensic basis for any inference about how DNA may have been deposited.

Both approaches are understandable. Neither fully reflects how transfer issues typically unfold in the courtroom.

How DNA Transfer Is Framed in Oral Evidence

At trial, a prosecution expert will rarely assert that science can determine precisely how DNA was deposited. At its highest, the evidence will acknowledge that transfer mechanisms cannot be resolved conclusively. But this does not mean the issue remains simple.

In forensic terms, DNA transfer refers to the movement of DNA to a surface through direct contact or via indirect pathways such as secondary transfer.

Oral evidence frequently introduces layers of qualification. Experts may refer to factors such as:

• the amount of DNA recovered
• the location of the sample
• the nature and duration of contact
• friction or handling
• background DNA levels
• persistence
• findings in published studies or validation work

Individually, these points appear measured and reasonable. Collectively, they can shift the narrative.

Without stating that transfer occurred directly, the expert may convey an impression that certain mechanisms are more likely than others. References to “data,” “research,” or “validation” can carry significant persuasive weight, particularly where they are not examined in detail.

In practice, this can place defence practitioners in a difficult position.

One approach is to step back from the complexity and return to the central proposition: that science cannot determine how DNA was deposited. The difficulty is that the jury has already heard a series of apparently scientific explanations suggesting why one pathway may be more plausible than another.

The alternative is to engage each point directly. This often results in deeper technical discussion, where the expert is operating within their own domain and the forensic detail becomes increasingly difficult to control.

Neither outcome is ideal.

What is often overlooked is that the scope of the oral discussion around DNA transfer is rarely determined for the first time in the witness box.

It is shaped well before trial.

Where transfer is addressed through structured forensic reporting, the boundaries of what can and cannot properly be said may already be established. Questions such as:

• whether available data can be applied to the specific circumstances
• whether likelihood can be expressed about transfer mechanisms
• whether research findings are relevant to the conditions of the case

can be addressed in advance.

If those parameters are not challenged in responsive reporting, they may materially influence how the issue is approached in oral evidence.

In practical terms, this means that the apparent simplicity of DNA transfer at trial can be misleading. The issue rarely turns on a single proposition that DNA could have arrived through multiple pathways. It turns on how far an expert is able to move beyond that proposition in their oral evidence.

Managing that movement is difficult once the discussion is underway.

For practitioners considering how to approach DNA transfer in a particular matter, the question is often not whether transfer exists as a concept, but how it is likely to be framed and developed in the forensic evidence before the court.

Where DNA forms a central part of the prosecution case, early consideration through a preliminary DNA case review may assist in clarifying how transfer may be addressed, and can materially influence how the issue ultimately unfolds at trial.

Independent forensic input does not replace legal strategy. It assists in defining the scientific ground on which that strategy operates — including issues explored further in the Lawyers Portal, where what can properly be asserted, what remains uncertain, and where the evidentiary limits lie are examined in a forensic context.

Concerned about how DNA transfer will be presented at trial?
What appears simple in theory can quickly become complex in oral evidence, particularly where experts introduce research, persistence, and likelihood considerations

An independent forensic review can assist in clarifying how DNA transfer may be framed, and whether the evidence supports the inferences being drawn.