Familial DNA Searching (FDS) is a forensic investigative technique that seeks to identify a potential suspect in a crime by searching a DNA database for a close biological relative of the individual who left a DNA sample at a crime scene. When a direct match to a suspect’s profile is not found, FDS uses the shared genetic material between relatives to generate a lead that investigators can pursue. This method is a powerful extension of traditional DNA matching, allowing law enforcement to solve long-dormant cold cases by leveraging the genetic information of family members already present in forensic or consumer databases.
How Familial DNA Searching Works
FDS relies on the principle that close biological relatives share a greater amount of genetic information than unrelated individuals. Standard forensic DNA analysis, such as that used in the Combined DNA Index System (CODIS), typically compares 13 to 20 specific locations, or loci, on the genome known as Short Tandem Repeats (STRs). A full match at all these loci is considered a unique identifier, establishing a one-to-one match between a crime scene sample and a known individual in the database.
Familial searching, however, is a deliberate search for a partial match rather than an exact one. Specialized software compares the crime scene DNA profile against database profiles, looking for a higher-than-expected number of shared alleles. This indicates a potential familial relationship, such as a parent, child, or full sibling, who would share a significant portion of these genetic markers.
Analysts use statistical calculations, often involving a Likelihood Ratio (LR) or Identity-by-State (IBS) method, to determine the genetic distance between the unknown crime scene profile and the known profiles. The result is a ranked list of potential candidates who are most likely to be a close relative of the individual who left the evidence. This statistical ranking provides a quantifiable estimate of the biological relationship used to generate an investigative lead.
Using FDS to Identify Suspects and Victims
Familial DNA Searching serves as a last resort, typically employed only after all traditional investigative leads have been exhausted and no direct match is found in existing databases. The technique is applied to high-profile, violent, and long-unsolved cold cases, such as homicides, rapes, and felony sexual offenses. By generating a list of potential relatives, FDS transforms a stagnant investigation into an active one.
Once a candidate relative is identified through the partial match, law enforcement develops an investigative lead. This involves traditional police work like surveillance, interviews, and genealogical research to construct a family tree and pinpoint the potential perpetrator. This process is often narrowed by using secondary genetic testing, such as Y-STR comparison for males (tracing the paternal line) or mitochondrial DNA analysis (tracing the maternal line). The familial match itself is not used as evidence in court.
The final step is obtaining a confirmation DNA sample directly from the newly identified suspect. This sample must be collected legally, such as through consent or a search warrant, and is then compared to the original crime scene evidence to establish an exact, one-to-one match. FDS is also used in humanitarian efforts to identify unidentified human remains by comparing the DNA to profiles in databases to find family members who may have reported a missing person.
Governing the Use of FDS
The regulatory landscape for Familial DNA Searching is highly variable and lacks a uniform federal standard. The FBI’s national CODIS database (NDIS) prohibits searches specifically intended to uncover a familial match. This means FDS is generally limited to searches of state (SDIS) and local (LDIS) DNA index systems, leaving implementation up to individual state or local laboratory policies.
States that permit FDS impose strict procedural requirements to balance investigative needs with privacy concerns. These often include limiting FDS only to serious, violent crimes, requiring that all other reasonable investigative leads have failed, and mandating a formal, written request for the search. Some jurisdictions require the establishment of an external review board to oversee and approve FDS requests before the search can be conducted.
A distinct form of FDS involves the use of consumer genetic databases, such as those maintained by direct-to-consumer (DTC) genealogy companies. This practice, often referred to as Forensic Genetic Genealogy (FGG), uses Single Nucleotide Polymorphisms (SNPs) to construct comprehensive family trees. Law enforcement access to these databases is governed by the companies’ specific terms of service and, increasingly, by state laws that require a judicial warrant before any genetic data can be disclosed.
The Privacy Debate Surrounding Genetic Kinship
The practice of FDS has generated debate regarding the genetic privacy of individuals who have not consented to a search or are not suspected of a crime. Familial searching inherently involves the investigation of innocent people whose only connection to a crime is a shared biological relationship with the perpetrator. This raises concerns about “genetic net widening,” where the scope of a criminal investigation extends beyond the individual in the database to encompass their entire family network.
FDS disproportionately affects certain demographic groups who are overrepresented in forensic databases due to historical disparities in arrest and conviction rates. Because the database is not a statistically random cross-section of the population, the relative of an individual in the database is more likely to be identified as a potential lead. This leads to a form of genetic surveillance that impacts entire communities, compromising the privacy rights of innocent relatives whose genetic information is indirectly searched.
The potential for a search to uncover unexpected or sensitive family information, such as unknown paternity or adoption, also adds complexity to the privacy debate. Policy discussions center on whether the public safety benefit of solving violent cold cases outweighs the intrusion on the genetic privacy of non-suspect citizens. This tension drives the effort to establish clear, consistent, and constitutionally sound legal frameworks for the use of kinship-based DNA searching.