Investigative Genetic Genealogy (IGG)

Investigative Genetic Genealogy (IGG) is a modern forensic technique for identifying criminal suspects or the remains of unknown individuals by using genetic information voluntarily submitted by consumers to public genetic genealogy databases. It works by identifying distant genetic relatives of the unknown individual and then using traditional genealogical research to build out family trees and pinpoint the source of the DNA. Its primary importance lies in its incredible power to generate new leads in cold cases that have exhausted all other investigative avenues, including traditional DNA database searches like CODIS.

The Science Behind IGG

At its core, IGG combines the power of modern DNA analysis with old-fashioned genealogical detective work. Unlike the Short Tandem Repeat (STR) analysis used for CODIS, which requires a very close match (parent, child, or sibling), IGG uses a different type of DNA marker: Single Nucleotide Polymorphisms (SNPs). Forensic laboratories generate a SNP profile from the crime scene DNA, which can contain hundreds of thousands of data points.

This SNP profile is then uploaded to one or more public genetic genealogy databases, such as GEDmatch or FamilyTreeDNA (which have specific policies allowing for law enforcement use). The database compares the uploaded profile against its entire collection of user-submitted profiles to find individuals who share significant blocks of DNA with the unknown source. The amount of shared DNA, measured in units called centimorgans (cM), allows genealogists to estimate the genetic distance between the unknown person and their matches—for example, a third cousin.

Once a list of distant relatives is generated, the real genealogical work begins. Investigators and genetic genealogists analyze the family trees of these matches, looking for common ancestors. By building out these family trees forward in time, they can triangulate and identify a specific couple who are the likely great-great-grandparents of the unknown suspect. From there, they trace all descendants down to the present day, narrowing the pool of potential candidates based on age, location, and other case-specific details until a likely individual is identified. Finally, law enforcement must obtain a direct DNA sample from the suspect for traditional $STR$ comparison to confirm the match.

Forensic Significance

IGG has proven to be one of the most significant breakthroughs in forensic science since the advent of DNA profiling itself. Its value is most profound in two specific areas:

• Solving Violent Crime Cold Cases: IGG has been instrumental in identifying perpetrators in decades-old homicide and sexual assault cases that had no viable leads. The most famous example is the 2018 identification of the Golden State Killer. After 40 years of investigation, law enforcement used IGG to find distant relatives of the killer, build a massive family tree, and ultimately zero in on Joseph James DeAngelo. His identity was confirmed with a DNA sample collected from his discarded trash.
• Identifying Unknown Human Remains: For every “John Doe” or “Jane Doe,” there is a family left without answers. IGG provides a powerful tool for giving these individuals their names back. By creating a SNP profile from the remains and searching genealogy databases, investigators can identify relatives and reconstruct the decedent’s family, finally providing closure to loved ones. A notable case is the identification of the “Buckskin Girl,” Marcia King, who was murdered in 1981 and remained unidentified for 37 years until IGG was used to trace her family.

The process has breathed new life into hundreds of stagnant cases, demonstrating its remarkable ability to provide investigative leads where none previously existed. However, it’s not a magic bullet; it’s a labor-intensive, complex process that raises significant ethical and privacy considerations regarding the use of personal genetic data.