What is a mixed DNA profile?

A mixed DNA profile is a DNA result that contains genetic material from more than one person.

This occurs when biological material from multiple individuals is present in the same sample. Mixed profiles are common in forensic work and may arise from direct contact between people, indirect transfer, shared surfaces, or the accumulation of DNA over time.

Interpreting a mixed DNA profile is more complex than interpreting a single-source sample. The DNA from different contributors may overlap, and it may not be possible to determine exactly how many people contributed or in what proportions.

Laboratory analysis may identify potential contributors or indicate that a person’s DNA is consistent with the mixture. However, this does not establish when the DNA was deposited, how it was transferred, or what activity led to its presence.

Statistical methods are often used to evaluate mixed DNA profiles. These rely on modelling and assumptions about contributors, population data, and the amount of DNA present. The outcome of this analysis can influence how strongly the findings are expressed.

In Australia and New Zealand, mixed DNA profiles are commonly evaluated using probabilistic genotyping software known as STRmix. This system uses statistical modelling to assess the likelihood that a person’s DNA contributed to a mixture. In other jurisdictions, different probabilistic software systems may be used, each applying its own modelling approaches and assumptions.

While these tools are scientifically sophisticated, their outputs depend on the quality of the DNA data and the assumptions applied during analysis. As a result, interpretation of mixed DNA profiles remains a specialised area requiring careful evaluation in the context of the case.