Detect Mixed DNA Samples

DNA samples are often a mixture from multiple unknown people.

To detect mixed DNA samples accurately, requires careful consideration of numerous factors.

Mixed DNA samples can be inherently problematic to detect who has, and who has not, contributed DNA to the mixture.

The lab scientist must estimate the number of contributors, and this is not a black and white decision.

The number of contributor estimation is a well-known source of scientific error.

Major 2023 study - 67 forensic laboratories: number of DNA contributors was factually wrong in 21% of samples. **

Many mixed DNA samples contain DNA from people who are reported as “unknown”. This means the person is identifiable, but it is unknown who the person is. It is important to establish investigatively who unknown DNA contributors are, wherever possible.

Number of DNA Contributors

The estimation of number of contributors to a mixed DNA sample is highly subjective.

Most labs in Australia cannot assess DNA mixtures with more than four contributors.

If the number of contributors is assessed incorrectly, interpretation of the profile will be affected.

Some contributors may have only a small amount of DNA present.

Minor contributors can be difficult to detect, distinguish, or interpret reliably, particularly in samples with large numbers of contributors.

Mixed DNA profiles always require assumptions about contributor number. The DNA information along with the number of contributors is uploaded into the computer software known as STRmix.

STRmix will then evaluate the likelihood of the DNA profile in consideration of the ethnicity database.

Mixed DNA samples are inherently more complex than single source DNA samples. Reliability depends on sample quality, number of contributors, quantity of DNA, possible transfer, and interpretation

A mixed DNA result does not automatically prove involvement in the alleged offence.

DNA may be present through innocent handling, transfer, prior/social contact, or unrelated background DNA.

Mixed DNA evidence warrants serious consideration for challenge where there are issues with the number of contributors, unknown contributors, transfer risk, ethnicity database issues, familial association, or when overstated reporting language has been applied

** Variation in assessments of suitability and number of contributors for DNA mixtures - 2023 - R. Austin Hicklin et al

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