DNA profiling is a method of identifying a person by reading short, repeating sequences at specific locations in the non-coding portion of their DNA and comparing the resulting numerical pattern against another sample or a database. It has been in forensic use since the mid-1980s and now supports criminal investigations, paternity cases, immigration verification, and post-conviction review.1National Institute of Justice. Crime Scene and DNA Basics for Forensic Analysts – DNA Typing by RFLP Analysis Federal law sets rules for when the government can take your DNA, what it can do with the profile, and when that profile has to come out of the database.
What a DNA Profile Actually Contains
Most human DNA is identical from person to person. Forensic labs ignore the genes that code for proteins or physical traits and look instead at non-coding regions that carry no known medical information. Within those regions are Short Tandem Repeats (STRs): short sequences of DNA that repeat back-to-back, with the number of repeats varying widely between people. A profile is simply the repeat count at each of a set of chromosomal locations, written as a string of numbers. The FBI’s national standard currently requires analysis at 20 core locations, which makes the chance of two unrelated people sharing a full profile vanishingly small.2Federal Bureau of Investigation. CODIS and NDIS Fact Sheet
Because the analysis targets only identity markers, a forensic profile reveals nothing about disease risk, appearance, or other medical traits. That limit matters legally: it is one of the reasons courts have allowed the government to take and store these profiles at all.
How a Lab Builds the Profile
The process starts with a biological sample. Saliva, blood, skin cells left on a surface, or hair follicles can all work. Technicians break open the cells and separate out the DNA. They then run Polymerase Chain Reaction (PCR), which makes millions of copies of just the STR regions the lab needs to read. PCR is what makes it possible to build a profile from tiny or partially degraded samples.
After amplification, the lab separates the copied fragments by size using capillary electrophoresis. An electric current pulls the DNA through a narrow tube, shorter fragments travel faster, and a laser reads fluorescent tags as each fragment passes. The output is a graph called an electropherogram, and each peak on that graph corresponds to a specific STR marker. Technicians translate the peaks into the numerical profile that any accredited lab can compare against another.
How Profiles Get Compared Across Cases
The Combined DNA Index System, or CODIS, is the FBI-managed database that lets law enforcement compare profiles across jurisdictions. It operates at local, state, and national levels, with the National DNA Index System (NDIS) at the top. When a crime-scene profile is uploaded, CODIS searches for matches against profiles already stored, which include convicted offenders and, in many jurisdictions, people arrested for certain serious crimes.2Federal Bureau of Investigation. CODIS and NDIS Fact Sheet
A CODIS hit does not by itself prove guilt. It gives investigators probable cause to seek a warrant for a fresh reference sample, which is then analyzed independently to confirm or rule out the match.
Rapid DNA at Booking
The Rapid DNA Act of 2017 authorized the use of fully automated instruments at police booking stations. A cheek swab goes in, and a profile comes out in roughly 90 minutes, which can then be compared against NDIS while a person is still in custody.3Congress.gov. H Rept 115-117 – Rapid DNA Act of 2017 The FBI Director must approve the specific instruments, and results generated this way can be uploaded directly into CODIS.4Office of the Law Revision Counsel. 34 USC 40702 – Collection and Use of DNA Identification Information From Certain Federal Offenders Rapid DNA is designed for clean single-source reference samples, not for the mixture analysis that crime-scene evidence often requires.
Investigative Genetic Genealogy
Genetic genealogy works differently from a CODIS search. Investigators upload a more detailed genomic profile to a consumer genealogy platform, look for partial matches indicating relatives, and build a family tree to narrow down a suspect. Department of Justice interim policy restricts federal use of this technique to unsolved violent crimes such as homicides and sexual assaults, or to identifying human remains. Before pursuing it, the forensic profile must already have been searched through CODIS without a confirmed match, and other reasonable leads must be exhausted.5United States Department of Justice. Interim Policy – Forensic Genetic Genealogical DNA Analysis and Searching
The policy adds several guardrails. Investigators may only use platforms that explicitly notify users law enforcement may search the database. A suspect cannot be arrested based on a genealogical association alone; standard STR typing must independently confirm the match. Samples and genealogy profiles cannot be uploaded to CODIS or used to assess medical traits, and if no arrest results, third-party reference samples and associated data must be destroyed.5United States Department of Justice. Interim Policy – Forensic Genetic Genealogical DNA Analysis and Searching Platforms have set their own limits too. GEDmatch, one of the databases most used in these investigations, now defaults to excluding users from law enforcement searches for violent crimes; users have to opt in.6GEDmatch. Join the Genetic Witness Program
Where DNA Evidence Goes Wrong
Profiling is powerful, but it is not infallible. The cleanest results come from a single person’s sample in good condition. Real evidence rarely cooperates.
Mixture Samples
When a sample contains DNA from more than one person, interpretation gets much harder. Overlapping markers, unequal contributions, and amplification artifacts all muddy the picture. Research funded by the National Institute of Justice found significant variation in how different examiners and labs interpreted the same mixture, and that three-person mixtures were “generally beyond the scope of protocol limits for most participating examiners.”7National Institute of Justice. When DNA Samples Are Complicated: Calculating Variation in Mixed Samples Interpretation A separate NIJ-funded review of wrongful convictions found that mixture samples were the most common source of interpretation error among the DNA cases studied.8National Institute of Justice. The Impact of False or Misleading Forensic Evidence on Wrongful Convictions
Touch DNA
Touch DNA is the trace material people shed through casual contact with surfaces. These samples contain very little DNA, and amplifying them exaggerates random effects: an allele may drop out, stutter peaks may mimic real markers, or background contamination may register as a contributor. Running the same touch sample twice can produce different profiles. That does not automatically disqualify the evidence, but it should shape how much weight it deserves.
Contamination and Handling Errors
Contamination during collection, mislabeling, and errors during amplification are all documented failure points. The science is sound when applied properly. “Applied properly” is doing a lot of work in that sentence.
Getting DNA Evidence Into Court
A profile is only useful in court if a judge admits it. Federal courts and most state courts apply the Daubert standard, under which the trial judge evaluates whether an expert technique has been tested, peer reviewed, has a known error rate, has controlling standards, and is generally accepted in the relevant scientific community. A smaller number of states still use the older Frye standard, which asks only whether the technique has gained general acceptance. Standard STR profiling clears both bars for single-source samples. Mixture and touch-DNA interpretations face more scrutiny, and most real courtroom fights are about how a specific sample was handled and interpreted, not whether STR analysis works in principle.
Chain of Custody
Flawless lab work does not help if the evidence trail is broken. Chain of custody is the documented record of who collected a sample, when, where, how it was packaged, everyone who handled it, and when it reached the lab. The National Institute of Justice identifies the essentials: detailed scene notes with recovery location, time and date, item description and condition, unique identifiers, and signed transfer receipts at every handoff. Every person taking custody signs off, or completes a secure electronic transfer.9National Institute of Justice. Collecting DNA Evidence at Property Crime Scenes – Chain of Custody A gap in that chain does not automatically make evidence inadmissible, but it gives a defense attorney solid ground to argue contamination or tampering.
DNA in Family, Inheritance, and Immigration Cases
Outside criminal court, genetic testing resolves disputes about biological relationships. Paternity is the most common. A child support order cannot be entered against someone until parentage is legally established, and a cheek swab of the man, mother, and child can create a legal presumption of paternity. Courts can compel a putative father to submit to testing, and refusal can lead to a default judgment establishing paternity.10Office of Child Support Enforcement. Child Support Handbook – Chapter 3 – Establishing Fatherhood Accredited labs typically return results within about a week after receiving all samples. Laboratory fees for court-ordered testing generally run from $200 to $1,500 depending on complexity and the number of people tested, and courts have discretion over who pays.
Inheritance disputes are a growing area. When someone dies without a will, biological heirs may need to prove their relationship to claim assets under state intestacy laws. Consumer DNA kits have made this more contentious by surfacing previously unknown relatives who may qualify as heirs, particularly at more distant degrees where intestacy distribution rules are harder to apply.
Federal immigration agencies also use genetic testing to verify claimed family relationships. The State Department may suggest DNA testing when a visa applicant cannot provide sufficient documentary proof of a biological relationship, and treats it as a last resort because of cost and delay.11U.S. Department of State. DNA Relationship Testing Procedures USCIS accepts DNA results to establish sibling relationships when primary evidence is unavailable or unreliable.12U.S. Citizenship and Immigration Services. USCIS Updates Policy on DNA Evidence in Support of Sibling Relationships
Your Legal Rights Over Your Genetic Information
Genetic Discrimination: What GINA Covers and What It Does Not
The Genetic Information Nondiscrimination Act bars two categories of discrimination. Employers with 15 or more employees cannot use genetic information in hiring, firing, promotion, pay, or job assignment decisions.13Equal Employment Opportunity Commission. Fact Sheet – Genetic Information Nondiscrimination Act Health insurers cannot use genetic information to determine eligibility or set premium rates.14National Human Genome Research Institute. Genetic Discrimination
GINA does not reach life insurance, long-term care insurance, or disability insurance.14National Human Genome Research Institute. Genetic Discrimination Insurers in those markets can legally ask about genetic test results and factor them into coverage decisions. Some states have filled the gap, many have not. Anyone considering testing should weigh the possible effect on those types of coverage.
When the Government Can Take Your DNA
The Fourth Amendment constrains warrantless collection. In Maryland v. King, the Supreme Court held that taking a cheek swab from someone arrested for a serious offense is a reasonable booking procedure, comparable to fingerprinting or photographing.15Legal Information Institute. Maryland v King, 569 US 435 (2013) The Court emphasized that the analysis was limited to non-coding identification markers, and it noted that the Maryland statute restricted database entries to identification records and made it a crime to test a sample for anything else. Those statutory guardrails were part of why the search was reasonable, and the ruling did not authorize blanket collection from everyone the government encounters.
Getting a Profile Removed From the Federal Database
If your DNA profile is in the national index and the legal basis disappears, federal law requires that it come out. Under 34 U.S.C. ยง 12592, the FBI Director must promptly expunge a profile from the national index when the underlying conviction is overturned, the charge is dismissed, the case ends in acquittal, or no charge is filed within the applicable time period. States participating in CODIS must follow the same rules.16Office of the Law Revision Counsel. 34 USC 12592 – Index to Facilitate Law Enforcement Exchange of DNA Identification Information
Removal is not automatic. You or your attorney must send a written request to the FBI’s Federal DNA Database Unit in Quantico, Virginia, with a certified copy of the final court order showing the conviction was overturned or the charge resolved in your favor. The order must be signed by a judge, dated, and carry enough identifying information to link it to you and the specific case. An order is not “final” for this purpose while time to appeal remains.17Federal Bureau of Investigation. DNA Fingerprint Act of 2005 Expungement Policy Without the certified court order, the FBI will not process the request. People who assume the profile disappears on its own when charges are dropped are the ones who lose track of it.