In solving cold cases, investigators are turning to a method that relies not on a perpetrator’s DNA, but on that of their relatives. This approach uses genetic material from family members who have submitted their DNA to public databases to indirectly identify a person of interest. Known as investigative genetic genealogy, this technique provides leads when traditional methods fail. It leverages the expanding network of individuals exploring their ancestry for forensic science.
What Is Collateral DNA?
The term “collateral DNA” in criminal investigations refers to the genetic material of a suspect’s relatives. This is distinct from direct DNA evidence, such as blood or skin cells, left at a crime scene by a perpetrator. While direct evidence provides a genetic fingerprint of the individual, collateral DNA offers an indirect path to identification by pointing to a shared family lineage. The power of this method lies in its ability to generate leads when the crime scene DNA does not match any profiles in law enforcement databases.
This approach should not be confused with “touch DNA,” which is a form of direct evidence characterized by the small number of skin cells left behind when a person touches an object. The distinction is whose DNA is being analyzed, not how it was deposited. Collateral DNA analysis focuses on the genetic information of relatives found in public genealogy databases to build a family tree that, through careful research, can lead investigators to a potential suspect.
The genetic information used is not from criminal databases like the FBI’s Combined DNA Index System (CODIS). Instead, it comes from direct-to-consumer genetic testing companies where individuals have willingly shared their data to discover more about their heritage. These consumer databases provide a different pool of genetic information, one that can reveal distant familial relationships, such as third or fourth cousins, creating a vast web of potential connections for investigators to explore.
The Investigative Process Using Genetic Genealogy
The process begins when investigators have an unknown DNA profile from crime scene evidence but no matches in law enforcement databases. This profile is then converted into a format compatible with public genealogy websites and uploaded to one or more of these services, such as GEDmatch or FamilyTreeDNA, which permit law enforcement searches. These databases compare the unknown profile to their user-submitted data to find individuals who share significant portions of DNA, indicating a familial relationship.
Once the database identifies potential relatives, who can be as distant as third or fourth cousins, the work of genetic genealogists begins. They analyze the amount of shared DNA to estimate the closeness of the relationship and start constructing a family tree. Using public records like birth certificates, census data, and obituaries, they trace the family lines of the identified relatives both backward in time to find a common ancestor and then forward to the present day. This meticulous work narrows down the possibilities to a specific branch of the family tree.
This genealogical research, combined with other investigative information like the location of the crime and the likely age of the suspect, helps to pinpoint a specific person of interest. The case of the Golden State Killer was famously solved using this method; investigators uploaded crime scene DNA to a public database and found distant relatives of Joseph James DeAngelo. Genealogical research over several months led them to identify DeAngelo as the suspect, and his DNA, obtained from a discarded item, was a direct match to the evidence.
Privacy and Ethical Considerations
The use of investigative genetic genealogy introduces complex ethical questions, primarily centered on consent and privacy. An individual who uploads their DNA to a genealogy website to explore their ancestry may not be aware that they are also making their relatives, including distant cousins, potential subjects of a criminal investigation without their knowledge or consent. This creates a situation where the actions of one person can have significant privacy implications for their entire extended family.
This practice has led to the concept of a “genetic lineup,” where entire families can be placed under suspicion based on a partial DNA match. Unlike a traditional police lineup, the individuals involved are often unaware that their genetic information is being scrutinized by law enforcement. The debate centers on balancing the significant power of this tool to solve violent crimes against the fundamental right to genetic privacy and autonomy.
The societal discussion reflects a conflict between two valid interests. On one hand, law enforcement gains an invaluable tool for solving cold cases and bringing offenders to justice. On the other hand, it raises concerns about the erosion of privacy and the potential for genetic information to be used in ways that were never intended by the person who submitted it. As this technology becomes more common, it fuels an ongoing debate about where the line should be drawn between public safety and individual rights.