In forensic science, DNA is used to connect people to crime scenes, identify unknown remains, link separate cases to the same offender, and clear people who were wrongly convicted. The process moves through four stages: careful collection of biological material at the scene, laboratory analysis that converts that material into a numerical profile, comparison of the profile against a database or a known reference sample, and courtroom presentation subject to admissibility rules. Since forensic DNA profiling became standard in the 1990s, the FBI’s national database has produced over 781,000 matches and assisted in more than 758,000 investigations.1FBI Law Enforcement. CODIS-NDIS Statistics Each stage has its own rules, and each has its own failure points.
What Gets Collected at the Scene
Every forensic DNA case begins with biological material. Blood, saliva, semen, hair roots, skin cells, and bone fragments all carry DNA, and even touching an object can deposit enough skin cells to analyze. Technicians photograph, label, and package each sample separately. Wet samples are air-dried first, and everything goes into containers that allow airflow. Sealed plastic traps moisture and breaks the DNA down.
From the moment a sample is collected, every person who handles it has to document the transfer. That paper trail is the chain of custody. Each handler signs a log recording when they received the evidence, what they did with it, and when they passed it along.2National Institute of Justice. Law 101 Legal Guide for the Forensic Expert – A Chain of Custody The Typical Checklist A gap in the chain gives the defense an opening to argue the sample was contaminated, mislabeled, or tampered with, and a judge can exclude the evidence entirely on that basis.3National Center for Biotechnology Information. StatPearls – Chain of Custody
How a Lab Turns a Sample Into a DNA Profile
Once a sample reaches a forensic laboratory, several steps convert the raw biological material into a numerical profile.
First, technicians chemically separate DNA from proteins, cell membranes, and other debris. They measure how much usable DNA they recovered. If the quantity is sufficient, they amplify it with polymerase chain reaction (PCR), which makes millions to billions of copies of specific DNA regions.4US Department of Justice Office of the Inspector General. The FBI DNA Laboratory A Review of Protocol and Practice Vulnerabilities – Section: Overview of the DNA Testing Process
The regions being copied are called Short Tandem Repeats (STRs), stretches of DNA where a short pattern repeats a varying number of times from person to person. By measuring the repeat count across 20 designated locations, called the CODIS core loci and expanded from 13 in 2017, analysts build a profile that is effectively unique.5FBI Law Enforcement. Combined DNA Index System The copied fragments are then separated by size and displayed as an electropherogram, a chart of peaks that translates into a string of numbers representing that individual’s profile.
When Nuclear DNA Isn’t Enough
Standard STR profiling assumes the lab recovered a reasonable amount of intact nuclear DNA. When a sample is old, badly degraded, or comes from a source with little nuclear DNA, such as a rootless hair shaft, analysts can turn to mitochondrial DNA. Each cell carries hundreds of mitochondria, so mtDNA survives conditions that destroy nuclear DNA. The trade-off is that mtDNA is inherited from the mother and shared by all maternal relatives, so it narrows the field rather than singling out one person.
Y-chromosome analysis takes a similar narrowing approach for male-lineage questions. Because the Y chromosome passes from father to son with minimal change, it helps separate male DNA from female DNA in mixed samples, which is useful in sexual assault cases involving material from both a victim and an assailant.
Newer Methods
Traditional STR profiling relies on amplicon sizes that can be too large to survive severe degradation. Single Nucleotide Polymorphism (SNP) profiling, performed through next-generation sequencing, targets much smaller DNA fragments. A 2025 study on 83-year-old skeletal remains found that SNP profiling recovered substantially more genetic information than traditional STR analysis, especially from the most degraded samples.
Probabilistic genotyping software uses computer simulations to interpret complicated samples, particularly mixtures containing DNA from more than one person. It calculates the probability that a person of interest contributed to the sample, expressed as a likelihood ratio. Different software packages can produce different results from the same data, and courts have raised questions about transparency when developers refuse to release their source code for independent review.6U.S. Government Accountability Office. Probabilistic Genotyping Software
How CODIS Matches Profiles
The Combined DNA Index System (CODIS) is the national DNA database maintained by the FBI. Federal law authorizes it to index DNA profiles from people convicted of crimes, people charged with crimes, crime scene evidence, unidentified human remains, and voluntary contributions from relatives of missing persons.7Office of the Law Revision Counsel. 34 U.S. Code 12592 – Index to Facilitate Law Enforcement Exchange of DNA Identification Information As of late 2025, the system held over 19.2 million offender profiles, 6.1 million arrestee profiles, and 1.4 million forensic profiles.1FBI Law Enforcement. CODIS-NDIS Statistics
When a crime lab develops a DNA profile from evidence, it uploads that profile and searches for matches. A match to an offender profile gives investigators a name. A match between two crime scene profiles from different cases suggests a serial offender. CODIS itself does not store names or addresses; it stores only the DNA profile and a lab identifier. When a match occurs, the labs involved contact each other through law enforcement channels to exchange details.5FBI Law Enforcement. Combined DNA Index System
Conventional analysis takes weeks or months, but Rapid DNA instruments compress the timeline to one or two hours. The device accepts a cheek swab and runs the entire extraction, amplification, and profiling process automatically inside a sealed cartridge, with no human intervention. The Rapid DNA Act of 2017 authorized the FBI to develop standards for using these instruments at police booking stations, so an arrestee’s profile can be generated and searched against CODIS within 24 hours. As of July 2025, quality assurance standards also cover the use of rapid DNA in forensic casework, not just booking.8FBI Law Enforcement. Rapid DNA
What Forensic DNA Is Actually Used For
Identifying Suspects and Linking Cases
The most direct use is matching a crime scene profile to a known individual. If someone’s profile is already in CODIS from a prior conviction or arrest, a new crime scene match identifies them. Without a database hit, investigators can compare crime scene DNA to a specific suspect’s sample obtained through a warrant or voluntary submission. When the same unknown profile turns up at multiple scenes, it points toward one perpetrator across several incidents.
Identifying Victims and Missing Persons
DNA plays a critical role when remains are too degraded for visual identification. After mass disasters, military conflicts, or cold cases involving skeletal remains, profiles from bone or teeth can be compared to profiles from family members or to reference samples the person provided during their lifetime, such as military records or medical biopsies. CODIS includes a Missing Persons Index for these comparisons.5FBI Law Enforcement. Combined DNA Index System
Familial and Genealogical Searching
When a crime scene profile does not match anyone in CODIS, some jurisdictions permit a familial search that looks for partial matches suggesting the unknown person is a close biological relative of someone already in the database. This generates leads rather than direct identifications and effectively subjects family members who were never suspected to genetic scrutiny.
Investigative genetic genealogy (IGG) goes further. Investigators convert crime scene DNA into a SNP profile and upload it to a public genealogy database, then reconstruct family trees from distant relative matches until the suspect pool narrows to one person. The technique produced the arrest of the Golden State Killer in 2018 after investigators uploaded a profile to GEDmatch and eventually confirmed a match to DNA the suspect had discarded in public.9National Library of Medicine. Forensic Genealogy, Bioethics and the Golden State Killer Case
The Department of Justice’s interim policy restricts federal use of IGG to violent crimes (murder, attempted murder, and sexual assaults) and unidentified human remains, requires that traditional methods and CODIS searches be exhausted first, and requires prosecutor concurrence before the search proceeds. The policy binds federal agencies and any agency receiving federal funding for IGG work. State and local agencies operating without that funding are not bound by it.
Where DNA Evidence Can Go Wrong
DNA evidence carries enormous weight with juries, which makes its limits important. Finding someone’s DNA at a scene proves their biological material was there. It does not prove they committed a crime, or even that they were present.
Transfer
Modern techniques can detect DNA from just a few skin cells. A handshake can transfer your DNA onto someone else’s hand, and whatever they touch next may carry your DNA as a secondary transfer. In one documented case, investigators found a university staff member’s DNA on a knife the person had never touched or been in the same room with. Current forensic methods cannot distinguish DNA deposited by direct contact from DNA that arrived through secondary or tertiary transfer. When an analyst reports that your DNA was found on an object, the evidence alone cannot show how it got there.
Mixed Samples
Crime scene evidence frequently contains DNA from more than one person. Two-person mixtures are generally manageable, but research has found that three-person mixtures push beyond the reliable interpretation limits for most forensic examiners.10National Institute of Justice. When DNA Samples Are Complicated – Calculating Variation in Mixed Samples Interpretation Different analysts working from the same data can reach different conclusions, and variation exists both within and between laboratories. Probabilistic genotyping has helped, but the lack of standardization across software platforms means two programs can produce different likelihood ratios from the same sample.6U.S. Government Accountability Office. Probabilistic Genotyping Software
Lab Quality
Forensic DNA laboratories that contribute profiles to CODIS must meet the FBI’s Quality Assurance Standards and maintain accreditation that requires documented procedures, proficiency testing, and periodic external audits. These safeguards reduce errors but do not eliminate them. Contamination during collection, handling, or analysis remains a risk, and defense attorneys routinely scrutinize lab procedures, analyst qualifications, and internal quality reviews when challenging DNA evidence.
How DNA Evidence Gets Into Court
Before results reach a jury, a judge decides admissibility. Federal courts and many states apply the standard set by the Supreme Court in Daubert v. Merrell Dow Pharmaceuticals (1993), which requires the judge to evaluate whether the scientific methods used are testable, have known error rates, have been subjected to peer review, and are generally accepted in the relevant scientific community.11National Institute of Justice. Law 101 Legal Guide for the Forensic Expert – Daubert and Kumho Decisions Some states still use the older Frye standard, which asks only whether the technique is generally accepted by the relevant scientific community.12Legal Information Institute. Frye Standard
Standard STR profiling clears both standards easily. Legal battles now tend to focus on newer techniques like probabilistic genotyping, on the quality of the specific sample, and on whether collection or handling errors compromised the results. A forensic scientist typically testifies as an expert witness, explaining the analysis, the statistical significance of a match, and any limitations. A profile match tells the jury the person’s biological material was present. What that presence means depends on the rest of the case.
Post-Conviction DNA Testing
DNA testing has freed more than 200 wrongly convicted people in the United States, some of whom spent decades in prison. Those exonerations exposed problems with other forms of evidence that had supported the original convictions, including eyewitness misidentification, false confessions, and flawed forensic disciplines.
Federal law gives people convicted of federal crimes the right to request post-conviction DNA testing under 18 U.S.C. ยง 3600. The applicant must assert actual innocence under penalty of perjury, identify specific evidence that was either never tested or that can now be tested with substantially more probative technology, and show that results could raise a reasonable probability they did not commit the offense.13Office of the Law Revision Counsel. 18 U.S. Code 3600 – DNA Testing The evidence must still be in government possession with an intact chain of custody, and the proposed testing must use scientifically sound methods. At the state level, all but a handful of states have enacted their own post-conviction DNA testing statutes, though eligibility requirements and procedural hurdles vary considerably.14National Institute of Justice. Defendants Access to Postconviction DNA Testing
A persistent barrier is evidence preservation. Many jurisdictions destroyed or lost biological evidence before post-conviction testing was available, leaving inmates with no material to test. Where evidence survives, SNP profiling and more sensitive STR kits can now extract information from samples that yielded nothing with older methods, so cases once considered untestable may be worth revisiting.
Privacy Limits on DNA Collection
The Supreme Court addressed the constitutional threshold for taking DNA from arrestees in Maryland v. King (2013). The Court held that a cheek swab during booking is a legitimate police procedure, comparable to fingerprinting, and reasonable under the Fourth Amendment when the arrest is supported by probable cause for a serious offense.15Legal Information Institute. Maryland v King The decision described the intrusion as minimal and noted that the genetic markers used in CODIS are not designed to reveal medical information.
Critics argue that this reasoning underestimates the privacy implications, because DNA inherently contains far more information than a fingerprint and the technology to extract that information keeps advancing. Investigative genetic genealogy raises a distinct concern because it reaches beyond the suspect to identify and scrutinize relatives who have no connection to any crime. The DOJ’s interim policy is administrative guidance rather than binding law.
For anyone whose profile enters CODIS, the profile remains searchable indefinitely unless the underlying legal authority for its collection is invalidated. State laws on removing an arrestee’s profile after charges are dropped or an acquittal is entered vary widely. In practice, expungement usually requires the individual to petition for removal rather than relying on the system to purge it automatically.