Thought to be responsible for more than a hundred crimes, the Golden State Killer has eluded police for decades. In 2020, he was finally captured, launching a forensic technique known as DNA Profiling into the limelight (1). 

This technique operates around one core principle: “Every contact leaves a trace.” Also known as Locard’s Principle, this statement posits that for every contact between two items, there will be an exchange of microscopic material. In crime scenes, when the perpetrator touches a surface, they will leave behind biological material that contains their DNA (2). As every individual’s DNA is distinct, DNA evidence can be analyzed to track down possible suspects, constituting a process called DNA Profiling (3). 

The commonly used methodology for DNA Profiling analyzes Short Tandem Repeats (STR). A STR contains multiple repeats of a short sequence of DNA that appear on different loci, which are specific locations on chromosomes where genes can be found (4). The length of each STR is highly variable between individuals (3). For example, in the below figure, for each person, the STR (repeats of the sequence CTAG) on the same locus had different lengths as the sequence CTAG was repeated a different amount of times. 

Figure 1 Showing how three different people have a different number of CTAG repeats on the same locus, making the length of their STRs different (2). 

The FBI’s Combined DNA Index System (CODIS), the national genetic database, records STR profiles in 20 specified loci from DNA collected from crime scenes, convicted offenders, or missing persons (5). Comparing STRs at 20 different sites, it’s highly unlikely that two individuals will have the same length for all STRs at every site. This uniqueness allows the FBI to match DNA samples from crime scenes to those in the database. 

However, as STR Typing relies on matching within existing databases, it poses a greater societal problem: the DNA for Black individuals have been collected twice-and-a-half as much as those of White individuals, making Black individuals more likely to be arrested using DNA evidence. The overrepresentation of Black individuals in DNA databases thus may further fuel the disproportionate incarceration rates (6). 

STR Typing further encountered limitations in the case of the Golden State Killer. Though the police gathered suspects through decades of work, none had DNA that matched the DNA in existing databases, making it impossible to generate exact matches by STR profiles (1). Investigators instead turned to a new technique: familial DNA profiling. Unlike STR Analysis, Familial DNA Profiling does not require exact matches but matches based on partial profiles. Running DNA from crime scenes into the public genealogy database GEDMatch, investigators narrowed in on a list of individuals who have similar DNA profiles who are believed to be related to the killer. Using the data to construct a family tree, law enforcement was able to finally capture Joseph James DeAngelo (1). 

Familial DNA Profiling accesses public commercial genetic databases like GEDMatch, where users upload their genetic data to identify their genealogy. Most of these commercial databases focus on Single Nucleotide Polymorphisms (SNP) Profiles, recording variations in single nucleotides (7). Given a specific nucleotide in an individual, if more than 1% of the population does not carry the same nucleotide at that location, then the nucleotide variation is classified as a SNP. Each individual can have four to five million SNPs in their genome (8). Compared to STRs which only account for small parts of the genome, SNP Profiles provide more information to help investigators identify common genetic markers of those who are related, allowing investigators to determine familial matches that may not be obvious solely through STR typing. 

Though the broad scope of Familial DNA Profiling may be beneficial to criminal justice, its greater implications for citizens’ privacy is cause for concern. Using commercial genetic databases, researchers were able to identify the identity of a Utah woman solely based on her anonymous genetic profile (9).  This can have further implications even for those who don’t share their genetic profiles: as “every contact leaves a trace,” DNA left behind by our everyday contact with different surfaces can then be traced back to our identities, vastly compromising everyday privacy. Familial DNA Profiling also may affect existing racial disparities in genetic databases. Contrary to FBI’s CODIS where Black individuals are overrepresented, in commercial genetic databases, White individuals are instead overrepresented (10). With the increasing use of commercial databases, law enforcement has also been using cross-referencing techniques that access both CODIS and commercial databases (7). Still, it remains to be seen whether this cross-referencing will be effective in mitigating existing racial biases. 

Thus, while both STR Typing and Familial DNA Matching yield considerable results in law enforcement, they still may perpetuate drastic consequences on racial equality and citizens’ privacy. The appropriate usage of both must be carefully deliberated to ensure that justice is carried out in all aspects of society, not just in catching criminals. 




  1. Zhang, S. (2018, April 28). How a Genealogy Website Led to the Alleged Golden State Killer. The Atlantic.
  2. Udogadi, N. S., Abdullahi, M. K., Bukola, A. T., Imose, O. P., & Esewi, A. D. (2020). Forensic DNA Profiling: Autosomal Short Tandem Repeat as a Prominent Marker in Crime Investigation. Malaysian Journal of Medical Sciences, 27(4), 22–35.
  3. DNA Evidence Basics. (2012). National Institute of Justice.
  4. Forensics, DNA Fingerprinting, and CODIS | Learn Science at Scitable. (2008). Scitable by Nature Education.
  5. CODIS and NDIS Fact Sheet. (2020, June 30). Federal Bureau of Investigation.
  6. Murphy, E. E., & Tong, J. (2019). The Racial Composition of Forensic DNA Databases. SSRN Electronic Journal. Published.
  7. Nature Editorial. (2018). Supercharged crime-scene DNA analysis sparks privacy concerns. Nature.
  8. SNP definition. (n.d.). Scitable by Nature Education.
  9. Erlich, Y., Shor, T., Pe’er, I., & Carmi, S. (2018). Identity inference of genomic data using long-range familial searches. Science, 362(6415), 690–694.
  10. Thompson, H. (2021, March 4). DNA databases are too white, so genetics doesn’t help everyone. How do we fix that? Science News.