What Does Mixed DNA Mean ?
Mixed DNA means the sample contains DNA from more than one person.
In forensic reporting this is often called a mixed DNA profile.
It is important to understand that DNA can be present on an item a person never touched.
The two critical questions for mixed DNA are who the DNA came from and how the DNA got there.
Mixed DNA in Police Reports
DNA reports often use terms with specific meanings. The meaning of those terms contributes to the value of the evidence.
Caution is advised with police DNA reports, as these are often early results that can change closer to trial.
“Cannot be excluded” means the person might be a contributor to the DNA profile. A DNA profile titled this way cannot be used for evidentiary purposes in court.
Implies there is DNA present but it is too low quantity or quality to provide a match.
A single source DNA profile, meaning DNA from one person can be stated as a frequency, such as: 1 in 100 billion unrelated people would be expected to match this DNA profile.
A frequency cannot be applied to a mixed DNA profile.
If someone touches a surface after shaking hands with multiple people, they might leave some or all of that DNA on the surface. The DNA might be in varying amounts and quality, meaning some peoples DNA might be identifiable, and others may not. It may even be that the person who touched the item did not leave their own DNA.
Scientifically, it can be quite problematic to establish who did and who did not contribute DNA to the mixture on the surface.
Where a report states “unknown contributor”, it can mean there is DNA that can be matched to a person, but that person has not been identified by name.
For example, it is Mr Smith’s DNA, but Mr Smith has not provided his DNA to compare to the crime scene sample. If you had Mr Smith’s DNA reference profile it would match.
** Variation in assessments of suitability and number of contributors for DNA mixtures - 2023 - R. Austin Hicklin et al
