What DNA evidence can establish
Forensic DNA analysis compares genetic information from questioned and reference specimens. Depending on the case and validated method, it can support or weaken propositions about source, biological relationship or contribution to a mixed sample. DNA evidence does not by itself establish when, why or how biological material was deposited.
Key principles for interpreting DNA evidence
Inclusion and exclusion
An exclusion means the observed genetic findings are incompatible with the tested proposition under the laboratory’s validated criteria. An inclusion or non-exclusion means the profile is compatible; the strength of that compatibility must then be assessed statistically.
Statistical weight
A match is not appropriately communicated as simply ‘the same DNA’. The evidential weight may be expressed using a random match probability, likelihood ratio, paternity/kinship index or another validated statistic appropriate to the question.
Mixtures
A DNA mixture contains genetic material from more than one contributor. Interpretation may require contributor-number assessment, consideration of allele sharing, dropout, drop-in, stutter and validated probabilistic or semi-continuous methods.
Transfer and persistence
A strong source-level DNA result does not necessarily answer activity-level questions. DNA can be transferred directly or indirectly and may persist. Case circumstances and other evidence remain essential.
Questions for the expert
Ask what proposition was evaluated, which population data and statistical model were used, whether the sample was mixed or low-level, what quality limitations existed, and whether alternative explanations were considered.
Forensic/legal use note: This material is educational. Evidentiary admissibility, collection authority, consent and procedural requirements are jurisdiction- and case-specific. Follow applicable law, court directions and the requesting authority’s requirements.
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