Almost all of your DNA is identical to everyone else’s, so a DNA profile skips nearly all of it and reads only a few places where people differ. The result is a short set of numbers, not a readout of your whole genetic code. That pattern is rare enough that a full match from a single person can point to one individual among millions.1National Institute of Justice. Population Genetics and Statistics for Forensic Analysts How much a match proves, though, depends as much on how the sample got there and how it was handled as on the lab test itself.2PubMed. Secondary and subsequent DNA transfer during criminal investigation
The Part of Your DNA a Profile Reads
Much of your DNA doesn’t carry instructions for making proteins. Some of that stretch contains short letter patterns, such as TATT, repeated back to back.3National Institute of Justice. What Is STR Analysis? These are called short tandem repeats, or STRs, the term you’ll see in news reports and court papers.4National Institute of Justice. What Is STR Analysis? Each repeat unit is only 2 to 6 letters long, and the whole stretch is usually under about 1,000 letters.5PubMed Central. Forensic DNA Profiling: Autosomal Short Tandem Repeat as a Prominent Marker in Crime Investigation Because the number of repeats at each spot varies between people, it works like a measurable personal tag.6National Institute of Justice. What Is STR Analysis? Because these counts sit in non-coding DNA, a profile reveals almost nothing about your health or traits.
For example, one person might have 9 and 12 copies of TATT at a spot, and another 7 and 9. The lab never reads the letters themselves, only how long each stretch is. That’s also why identical twins, who share every count, can’t be told apart by a standard profile.
You inherit one copy of each spot from each parent, so you carry two counts at every spot, and they may match or differ. A profile lists those pairs of counts across many spots. Some spots have simple patterns while others mix several patterns, which labs account for when reading them.7PubMed Central. Forensic DNA Profiling: Autosomal Short Tandem Repeat as a Prominent Marker in Crime Investigation
A single shared count proves little, since a good fraction of strangers might match you at one spot. The share who match at every spot at once shrinks with each spot added, so the identifying power comes from stacking many independent spots.
From Swab to Profile in the Lab
Every profiling method extracts DNA from the sample, measures how much there is, separates the pieces by size, and detects them.8PubMed Central. DNA Profiling in Human Identification: From Past to Present To copy a repeat region, the lab targets the unchanging DNA on either side of it.9PubMed Central. DNA Fingerprinting: Use of Autosomal Short Tandem Repeats in Forensic DNA Typing That makes millions of copies of just the repeat regions, enough to measure even from small samples.10PubMed Central. Forensic DNA Profiling: Autosomal Short Tandem Repeat as a Prominent Marker in Crime Investigation
This copying step, called PCR, repeatedly doubles the target regions, so a few cells’ worth of DNA becomes a measurable amount. Because it aims at specific spots, it ignores the rest of the DNA and most other material in the sample. Older methods had no copying step. They needed a stain large and fresh enough to hold plenty of intact DNA and took weeks, which limited early casework. Kits now test many spots together, saving both sample and time.
Next, the copies are pulled through a thin gel-filled tube by electric current, and shorter pieces travel faster. Each piece’s length is read against a ruler of known-size DNA, which gives the repeat count.11PubMed Central. Capillary electrophoresis applied to DNA: determining and harnessing sequence and structure to advance bioanalyses Some spots produce pieces of similar length, so four different fluorescent colors tell them apart.12PubMed Central. Capillary electrophoresis applied to DNA: determining and harnessing sequence and structure to advance bioanalyses A single run can size at least 16 spots at once.13PubMed Central. Capillary electrophoresis applied to DNA: determining and harnessing sequence and structure to advance bioanalyses
The result is a graph called an electropherogram: a row of colored peaks lined up along a horizontal axis by fragment length, with the color showing which dye tags them. The analyst or software compares each peak’s position to the ruler and converts it into a repeat number such as 9 or 12. A person with two different counts at a spot shows two peaks, while identical counts show one taller peak. A single person shows at most two peaks per spot, so more than that hints at a mixture. Peak height roughly reflects how much DNA was present, so faint, short peaks warn the analyst that the sample was small or degraded.
Early kits looked at 9 spots; the current US standard uses 20 core spots.14Behind the Bench. Capillary Electrophoresis (CE) or Next-Generation Sequencing (NGS) for STR Analysis? These spots feed CODIS, the FBI’s national DNA database, which originally used 13.15PubMed Central. Capillary electrophoresis applied to DNA: determining and harnessing sequence and structure to advance bioanalyses Adding spots makes chance matches rarer and lets countries share profiles.16Behind the Bench. Capillary Electrophoresis (CE) or Next-Generation Sequencing (NGS) for STR Analysis?
How a Match Is Judged
If the counts differ at any clearly read spot, the samples came from different people. If all spots agree, the lab estimates how common each count is using population data.17National Institute of Justice. Population Genetics and Statistics for Forensic Analysts Because the standard spots are inherited independently, the per-spot chances are multiplied together.18National Institute of Justice. Population Genetics and Statistics for Forensic Analysts
The final number is the random match probability, the chance an unrelated stranger would share the profile.19MetricGate. DNA Random Match Probability Explained Calculator Relatives and closely related communities share more DNA, so labs can add a correction for that.20MetricGate. DNA Random Match Probability Explained Calculator When only one count shows at a spot, labs use a cautious formula that slightly raises the chance figure.21District of Columbia Department of Forensic Sciences. FBS22 – STR Statistical Calculations Guidelines
A figure like one in a billion is easy to misread. It describes how rare the profile is among unrelated people, not the probability that the suspect is innocent or guilty. It also says nothing about how or when the DNA got there, since that depends on the circumstances rather than the lab math.
Relatives complicate things. Siblings inherit from the same two parents, so they share many counts and can match at more spots than strangers do. A brother is far likelier to match than a stranger, so labs ask whether a close relative could be the source when that is a realistic possibility.
Many crime samples also contain DNA from two or more people. Software now weighs all the possible combinations of contributors, and such tools have handled mixtures of up to five people, including faint contributors.
When DNA Evidence Can Mislead
DNA left by touching an object is usually scant and often damaged.22PubMed Central. Touch DNA Sampling Methods: Efficacy Evaluation and Systematic Review It comes from a few skin cells, so the sample may hold only a handful of cells’ worth of DNA. That can fall below what modern kits can read, and parts of a profile may drop out by chance.23PubMed Central. Touch DNA Sampling Methods: Efficacy Evaluation and Systematic Review The copying step can miss one of a person’s two counts, making a single-peak spot look like a different genotype. Chance may also add a spurious peak, so repeated tests of the same tiny sample can disagree.
DNA also travels. Sensitive tests can pick up a person’s DNA on an object they never touched, carried there by others.24PubMed. Secondary and subsequent DNA transfer during criminal investigation If you shake hands with someone who then handles an item, your DNA can end up on it. Investigators’ gloves can move DNA from one item to another, too.25PubMed. Secondary and subsequent DNA transfer during criminal investigation How much DNA gets left, and the type of surface, change how readily it transfers.26PubMed. Secondary and subsequent DNA transfer during criminal investigation A finding that someone’s DNA is on an object says it is there, but not how or when it arrived.
The most common lab failures are contamination and human error.27PubMed. Error rates in forensic DNA analysis: definition, numbers, impact and communication Most human errors can be caught and fixed, while heavy contamination of crime samples often can’t be undone.28PubMed. Error rates in forensic DNA analysis: definition, numbers, impact and communication Many errors happen after testing, when results are interpreted or reported.29PubMed. Error rates in forensic DNA analysis: definition, numbers, impact and communication Uncertainty about interpretation and the chance of error can greatly weaken a match’s real value.30PubMed. Subjective interpretation, laboratory error and the value of forensic DNA evidence: three case studies
Biology can add surprises. A bone marrow transplant recipient can carry both their own and the donor’s DNA, so their profile may look like a two-person mixture though it comes from one person.31Digital Commons @ New Haven. The Impact of Bone Marrow Transplantation on Forensic Human Identification and Genetic Genealogy Testing
When you hear about a DNA match, a few questions are worth asking. Was the sample a clear single profile or a faint mixture? Could innocent contact or transfer explain it? Was it handled to avoid contamination? Was the result checked by a second analyst?
Special Tests for Male DNA and Hair
In sexual assaults without semen, male skin cells can still be present but swamped by the victim’s DNA.32PubMed Central. Y-STR Databases—Application in Sexual Crimes Standard testing reads mainly the dominant DNA, so labs instead test repeat regions only on the male Y chromosome, called Y-STRs.33PubMed Central. Y-STR Databases—Application in Sexual Crimes Women don’t carry a Y chromosome, so the victim’s DNA is effectively invisible to the test. A male profile can be recovered even when female DNA outnumbers it more than 1,000 to 1.34PubMed Central. Y-STR Databases—Application in Sexual Crimes
The tradeoff is that Y-STRs pass from father to son mostly unchanged. A suspect’s father, brother, son and paternal uncles will usually share the profile, so a match points to a family line, not one man.35PubMed Central. Forensic use of Y-chromosome DNA: a general overview A man whose Y profile clearly differs can generally be ruled out as the source of the male DNA.
Hair without a root has little standard DNA but holds mitochondrial DNA, which is passed down from mothers. In one homicide case, about 9 in 10 hairs under 1 cm gave a mitochondrial profile or species result.36PubMed Central. Mitochondrial DNA analysis of 114 hairs measuring less than 1 cm from a 19-year-old homicide Improved methods can even recover standard DNA from old rootless hairs.37PubMed Central. Improved DNA Extraction and Illumina Sequencing of DNA Recovered from Aged Rootless Hair Shafts Found in Relics Associated with the Romanov Family Mother-line DNA is shared by siblings and maternal relatives, so like the male-line test, it mostly excludes people or narrows a search.
Reading Appearance, Age and Family Ties
When a profile matches no one, DNA can predict appearance, ancestry and age to give investigators leads.38PubMed. Recent advances in Forensic DNA Phenotyping of appearance, ancestry and age Validated tests estimate eye, hair and skin color and continental ancestry.39PubMed Central. The Use of Forensic DNA Phenotyping in Predicting Appearance and Biogeographic Ancestry Accuracy varies, with hair color least reliable and some eye and skin predictions very good.40PubMed Central. The Use of Forensic DNA Phenotyping in Predicting Appearance and Biogeographic Ancestry Newer tools add freckles, hair shape and baldness, using about 524 markers.41PubMed Central. Forensic DNA Phenotyping: Genes and Genetic Variants for Eye Color Prediction
These tools look at specific places in the DNA linked with pigment, such as genes involved in melanin production. The result is a probability, such as a high chance of brown eyes, not a photo. Hair color can change with age and dye hides the natural shade, so a predicted color may not match what a witness saw. Some traits depend on many genes plus environment, which caps accuracy, and predicted ancestry describes broad genetic background, not how a person looks or identifies. Investigators use predictions to prioritize, then confirm with a standard profile.
Chemical tags on DNA change with age, allowing an estimated age range.42PubMed Central. Getting the chronological age out of DNA: using insights of age-dependent DNA methylation for forensic DNA applications That can narrow suspects, identify bodies, or check a living person’s age.43PubMed Central. Getting the chronological age out of DNA: using insights of age-dependent DNA methylation for forensic DNA applications
Investigators can also upload crime-scene DNA to public genealogy sites like GEDmatch to find relatives.44PubMed Central. Law enforcement use of genetic genealogy databases in criminal investigations: Nomenclature, definition and scope This approach identified the Golden State Killer.45PubMed Central. Law enforcement use of genetic genealogy databases in criminal investigations: Nomenclature, definition and scope Such leads still require a direct DNA match before anyone is treated as the source.
Rapid DNA and Your Privacy
Fully automated machines can produce a profile from a swab in 84 minutes without hands-on work.46PubMed Central. Fully integrated, fully automated generation of short tandem repeat profiles In testing on over 2,000 cheek swabs, results agreed with standard results over 99.99% of the time.47PubMed. Developmental validation of the ANDE™ rapid DNA system with FlexPlex™ assay for arrestee and reference buccal swab processing and database searching Field use is validated for blood and saliva, though, not every crime-scene sample.48PubMed Central. Introducing a Rapid DNA Analysis Procedure for Crime Scene Samples Outside of the Laboratory—A Field Experiment
The machine performs the extraction, copying, sizing and reading inside a sealed cartridge, so no technician handles the sample. The speed comes from automating the steps, not from changing the method. Cheek swabs and clean blood contain plentiful DNA, which suits automation, whereas crime scene items are often messy, mixed or degraded. When a run fails or looks poor, it generally has to go to a regular lab for expert review.
On privacy, US law generally treats DNA you leave behind, like a cigarette butt, as abandoned.49PubMed Central. Is It Ethical to Use Genealogy Data to Solve Crimes? DNA uploaded to genealogy sites can likely be searched by police, whether or not you’re aware of it.50PubMed Central. Is It Ethical to Use Genealogy Data to Solve Crimes?
Before uploading to such a site, consider that your upload also exposes relatives who never consented. Check the site’s settings for opting out of police matching, and keep in mind that deleting data later may not undo past searches.