Explainer
CODIS vs. Forensic Genetic Genealogy: How They Actually Find People
Both use DNA. They answer completely different questions — explained plainly.
When a cold-case headline says investigators “used DNA,” readers often picture one giant database quietly searching every sample ever collected. That is not how it works.
The two systems most often confused are CODIS and forensic genetic genealogy, or FGG. They may appear in the same investigation, but they use different kinds of DNA profiles, search different databases, and produce different kinds of results. The simplest way to hold them apart:
CODIS asks: Does this crime-scene DNA match a profile already in the criminal-justice system?
FGG asks: Does this DNA share enough inherited material with genealogy users to help investigators build a family tree?
Neither system determines guilt by itself.
First, the Rule That Governs All of This
Before any of the mechanics, the part that matters most: a lead is not a person. Both methods can point investigators toward someone, but neither one, on its own, identifies a guilty party. Forensic genetic genealogy in particular works by surfacing relatives — and a family tree can hold many people who share DNA with an unknown sample and had nothing to do with the event under investigation. Innocent relatives can appear in that tree for one reason only: they are related.
That is why the work demands restraint, and why the stakes here are human rather than technological. Behind the science is a family waiting to learn what happened to someone who disappeared, or a victim finally known by name instead of a case number. We keep that in front — not buried under the jargon that follows.
What CODIS Is
CODIS stands for the Combined DNA Index System. The FBI uses the term for both its program supporting criminal-justice DNA databases and the software that lets participating laboratories compare DNA profiles. The national level of that system is the National DNA Index System, or NDIS. New to the terms? See the glossary for STR, SNP, CODIS, and NDIS.
CODIS holds several legally defined categories of profiles — among them DNA from forensic casework, convicted offenders, and, where state law authorizes it, arrestees, along with profiles used in missing-person and unidentified-remains work.
In a criminal case, a laboratory may develop an STR profile from evidence believed to come from an unknown perpetrator. STRs are selected locations in DNA that are highly useful for human identification. That forensic profile can then be searched against eligible profiles in CODIS.
A CODIS search can produce two especially important kinds of investigative information:
- A candidate match between crime-scene DNA and a profile associated with a known person.
- A match between DNA from two crime scenes — linking cases even when the person who left the DNA is still unknown.
So “match-to-known” is useful shorthand for CODIS, but not the whole story: CODIS can also connect one unknown forensic profile to another. And a CODIS hit is not the final courtroom test. Laboratories verify the match and coordinate with investigators; a known reference sample may then be collected and analyzed so the comparison can be presented as evidence in court.
What Forensic Genetic Genealogy Is
Forensic genetic genealogy begins where a standard CODIS search may end: with no probative match.
FGG combines DNA analysis with traditional genealogical research. Instead of the smaller set of STR markers used in CODIS, it generally uses a much denser SNP profile containing thousands — often hundreds of thousands — of genetic markers spread across the genome. Those markers can reveal inherited segments shared with biological relatives. Under applicable policies and database rules, a case profile may be compared with profiles made available for law-enforcement matching in participating genealogy databases.
The result is usually not a suspect’s name on a screen. It is more often a list of possible relatives. Genealogists then use those genetic relationships alongside public records to build family trees, working backward from the matches and forward through descendants — narrowing branches by age, location, sex, family structure, and facts already known from the case. That process may point to one person, or to a small group who fit the evidence.
For the full step-by-step process, see How Forensic Genetic Genealogy Actually Works. This page answers the narrower question: why FGG is not simply “CODIS with a bigger database.”
The Key Difference: Direct Identification vs. Kinship Reconstruction
CODIS compares identification profiles inside a regulated criminal-justice system. FGG searches for shared inherited DNA in genealogy data and uses the resulting relationships to reconstruct a family line.
| CODIS | Forensic genetic genealogy |
|---|---|
| Primarily uses STR profiles | Uses dense SNP profiles |
| Searches criminal-justice DNA indexes | Searches genealogy databases that permit the relevant use |
| Looks for a direct candidate match or a forensic-to-forensic link | Looks for biological relatives, including distant relatives |
| Produces a database hit or case linkage | Produces genealogical leads and family-tree hypotheses |
| Requires verification and follow-up | Requires genealogical vetting and confirmatory DNA testing |
The databases are not interchangeable. A person can have distant relatives in a genealogy database even when that person has never submitted DNA and has no profile in CODIS. Conversely, a person whose qualifying profile is already in CODIS may be identified through a direct candidate match without any family-tree work. That is the real investigative advantage of FGG: it can build a path to an unknown person through relatives several branches away, even when the unknown person is absent from the searched database.
FGG Produces Leads, Not Proof
The National Institute of Justice describes FGG as a method for generating investigative leads. Its output is a list of potential relatives and possible individuals connected to the case sample — not a finding of guilt.
Once genealogy points investigators toward a person, a separate DNA sample must ordinarily be obtained through lawful investigative means. A forensic laboratory can then run confirmatory STR testing and compare that known sample with the original evidence. That step — not the family-tree work — is what actually tests whether the biology fits, and it protects both the integrity of the case and the people whose names surface during genealogical research.
A lead can be wrong. A family tree can contain errors. Records can be incomplete, and biological relationships may differ from documented ones. Investigators still have to test the hypothesis, corroborate the surrounding evidence, and follow the law. For unidentified remains the goal is different but the discipline is the same: genealogy may help restore a name, but the proposed identity must be confirmed.
Why Both Tools May Appear in the Same Case
CODIS is commonly an early step because it can deliver a direct match quickly when the relevant profile is already present. Under the Department of Justice framework summarized by NIJ, qualifying FGG work generally follows an unsuccessful CODIS search in covered cases. So the sequence may look like this:
- A forensic laboratory develops a DNA profile from evidence.
- The eligible STR profile is searched in CODIS.
- CODIS produces no probative match.
- Investigators assess whether the case and sample are suitable for FGG.
- A SNP profile is developed and compared in permitted genealogy databases.
- Genealogists build and test family-tree hypotheses.
- Investigators identify a lead.
- A new reference sample is obtained and tested against the forensic evidence.
- The DNA result is weighed with all the other evidence.
The headline may compress those steps into “genealogy solved the case.” The accurate version is more careful: genealogy may have shown investigators where to look; confirmatory testing and the broader investigation determine what the evidence can support.
The case file
For related terms, see the True-Crime and Courtroom Glossary. For a real identification effort where advanced DNA work became part of a decades-long search for answers, read the Baby Boy Doe case file — Mansfield, 1985.
The Rule Worth Remembering
CODIS searches for a matching forensic identity profile already inside the system. Forensic genetic genealogy searches for relatives and builds a route toward an identity. Both can move an investigation forward. Neither substitutes for confirmation, corroboration, due process, or the presumption of innocence.
Sources: FBI, Frequently Asked Questions on CODIS and NDIS; National Institute of Justice / Forensic Technology Center of Excellence, An Introduction to Forensic Genetic Genealogy Technology (2022). This is a plain-English explainer, not legal advice; state rules, agency policies, database terms, and judicial requirements may differ.