genetic genealogy

Genetic genealogy is an investigative method in which an unknown DNA trace is compared with profiles in DNA-based genealogy databases in order to find familial connections and thereby narrow down or identify the person the DNA trace originates from. The method differs from traditional forensics by not searching for a direct match in the police's own registers, but instead mapping kinship relations to known individuals in publicly accessible genealogical databases — and working from there toward a possible perpetrator or an unidentified victim. In Danish legal practice the method is officially referred to as genetic genealogy research (genetisk slægtsforskning). In 2025 it was made a distinct legal option in the Danish Administration of Justice Act (retsplejeloven) and may only be used under specific conditions in cases of sufficiently serious character. The legislation thus provides an express legal basis and sets limits on when the police may lawfully make use of the method.

Internationally, genetic genealogy has become known first and foremost through American cases, where the method has broken decades-long investigative deadlocks. In cases such as the Fox Hollow Farm murders and investigations into unidentified victims, the method has made it possible to link biological traces to named family lines, even where no direct DNA match existed in police registers. Similar principles were applied in the attempt to establish the identity of Benjaman Kyle, who for years was without a verifiable identity. The method has also been used in cases that are not about finding a perpetrator, but about confirming or refuting biological relations of legal significance — for example in the case of Kathleen Folbigg, where genetic analyses formed part of the evidentiary basis for reassessing the conviction.

The use of genetic genealogy raises questions of principle concerning privacy, since the method in effect draws on genetic information from people who have never themselves provided DNA to the police, but have merely taken part in commercial genealogy services. It is precisely for this reason that legislators in several countries — including Denmark with retsplejeloven § 754 f — have chosen to regulate the method explicitly rather than let it fall under the general rules on DNA investigation.