Forensics, explained
Clear, practitioner-written answers to the questions people actually ask about forensic DNA, evidence handling, and the science behind the headlines.
Step-by-step: How PCR amplification copies a single drop of blood
A crime-scene sample can be vanishingly small. PCR turns a few cells' worth of DNA into billions of copies. Here's the reaction, cycle by cycle.
Read article →How do investigators collect 'Touch DNA' without contaminating it?
Touch DNA can be just a few skin cells. Because modern testing amplifies anything, collecting it without adding your own DNA is a discipline. Here's how it's done.
Read →What is the difference between CODIS and public ancestry DNA databases?
CODIS and consumer ancestry databases are both 'DNA databases', but they are built from different markers, for opposite purposes. Here's how they actually differ.
Read →Loops, whorls and arches: how are fingerprints actually classified?
Every fingerprint falls into one of three pattern families — and examiners read three levels of detail within them. Here's how fingerprint classification works.
Read →How do fire investigators determine where a fire started?
Finding a fire's origin and cause is detective work grounded in fire science — burn patterns, char depth, and a disciplined method that guards against bias.
Read →How do examiners match a bullet to a specific gun?
A fired bullet and cartridge case carry microscopic marks from the gun that fired them. Here's how firearms examiners compare them — and where the science is debated.
Read →What can a forensic toxicology test detect — and for how long?
Forensic toxicology finds drugs, alcohol and poisons in the body — but what it detects depends on the sample and the timing. Here's what a tox screen can and can't do.
Read →Locard's exchange principle: what 'every contact leaves a trace' really means
The founding idea of trace evidence is that two objects in contact always exchange material. Here's what that means in practice — and where its limits lie.
Read →The prosecutor's fallacy: how a DNA statistic gets misused in court
A '1 in a billion' DNA match does not mean a 1 in a billion chance of innocence. Confusing the two is the prosecutor's fallacy — and it has swayed real trials.
Read →How is time of death estimated?
There is no clock that reads the moment of death. Instead, forensic scientists combine body cooling, rigor and livor mortis, decomposition and insects into an estimate.
Read →Chain of custody: how evidence integrity is protected — and challenged
An unbroken chain of custody is what proves the evidence tested is the evidence collected. It's unglamorous, and it can win or lose a case on its own.
Read →Can you really get DNA from a fingerprint?
A fingerprint is a ridge pattern — but the residue that forms it can also contain DNA. So can you get both? Sometimes, but there's a trade-off.
Read →Mitochondrial DNA: solving cold cases when standard DNA fails
When a sample is too old or degraded for standard DNA testing — old bones, rootless hair, teeth — mitochondrial DNA can still deliver answers. Here's how.
Read →Gunshot residue: what GSR can and can't prove
Gunshot residue can show a person was near a firing gun — but it doesn't prove they pulled the trigger. Here's what GSR is, how it's found, and its limits.
Read →Forensic entomology: how insects help estimate time of death
When a body has been dead for days or weeks, the insects colonising it become a clock. Here's how forensic entomologists read them.
Read →What does a forensic scientist actually do?
Forensic science isn't one job — it's a family of disciplines united by one duty: to recover, analyse and interpret evidence objectively for the court.
Read →STR profiling: how a DNA profile is actually read
A DNA profile is really just a short list of numbers. Here's what those numbers are, how they're read from a graph, and what a 'match' means.
Read →What is a DNA mixture, and why is it so hard to interpret?
Real samples often contain DNA from several people at once. Untangling these mixtures is one of the hardest — and most contested — tasks in the DNA lab.
Read →Familial DNA searching: catching offenders through their relatives
When a crime-scene profile isn't in the database, a near-match to a relative can still crack the case. Here's how familial searching works — and why it's debated.
Read →How are latent fingerprints developed? Powders, ninhydrin and superglue
Most crime-scene fingerprints are invisible. Making them visible depends on the surface — and choosing the wrong method can destroy the mark or its DNA.
Read →Are fingerprints really unique? The science and the debate
Fingerprint identification rests on two claims: that prints are unique and permanent. Here's the evidence behind them — and where modern scrutiny has landed.
Read →What is an accelerant, and how do investigators detect it?
An accelerant is anything used to start or speed a fire. Finding one points toward arson — but detecting it, and interpreting its absence, takes care.
Read →What can a spent cartridge case tell investigators?
The empty case ejected when a gun fires carries a set of microscopic marks that can link it to a specific firearm — and to other shootings.
Read →How is blood alcohol concentration measured?
From roadside breath tests to definitive lab analysis of blood, here's how forensic science measures alcohol — and why the method and timing matter.
Read →How is glass evidence analysed and compared?
When a window breaks, tiny fragments fly back onto whoever broke it. Here's how forensic scientists compare crime-scene glass to fragments on a suspect.
Read →How is a shoeprint matched to a shoe?
Footwear marks are one of the most common — and underused — forms of evidence. Here's how they're recovered and compared, from tread pattern to unique wear.
Read →What is a likelihood ratio in forensic science?
Modern forensic interpretation increasingly reports a likelihood ratio rather than a flat 'match'. Here's what it means and why courts prefer it.
Read →What can a forensic anthropologist tell from a skeleton?
Given only bones, a forensic anthropologist can often estimate who a person was and what happened to them. Here's what a skeleton can reveal.
Read →How is blood detected at a crime scene?
Blood isn't always visible — especially when someone has tried to clean it up. Presumptive tests like luminol and Kastle-Meyer reveal it, with important caveats.
Read →Bloodstain pattern analysis: what spatter can — and can't — reveal
The shape and distribution of bloodstains can help reconstruct a violent event. Here's what bloodstain pattern analysis reads, and where its limits lie.
Read →What is ACE-V, the method behind every fingerprint comparison?
Fingerprint examiners don't just eyeball a match. They follow a four-stage method — Analysis, Comparison, Evaluation, Verification — designed to be disciplined and checkable.
Read →What are minutiae? The ridge features examiners actually compare
Fingerprint identification comes down to tiny features called minutiae — ridge endings, bifurcations and more. Here's what they are and how they're used.
Read →AFIS explained: how automated systems search millions of fingerprints
An AFIS can compare a crime-scene mark against tens of millions of prints in minutes — but it doesn't declare matches. Here's how the technology really works.
Read →Patent, latent and plastic prints: the three types of fingerprints
Not all crime-scene fingerprints are invisible. They come in three kinds — patent, latent and plastic — and each is found and recovered differently.
Read →How are fingerprints taken? Ink cards vs live scan
Capturing someone's fingerprints for the record — 'ten-printing' — has moved from ink and paper to digital live scan. Here's how each works and why quality matters.
Read →Can fingerprints be erased or altered?
Criminals have burned, cut and acid-treated their fingertips to escape identification. Does it work? Almost never — and here's the biology of why.
Read →How long do fingerprints last at a crime scene?
A latent fingerprint might survive for hours or for years. What decides it? The surface, the environment, and what the print is made of.
Read →The Brandon Mayfield case: how a fingerprint 'match' went wrong
In 2004 the FBI wrongly matched an American lawyer to the Madrid bombings on fingerprint evidence. The case reshaped how examiners work — here's what happened.
Read →How reliable is fingerprint evidence in court?
Fingerprint evidence is powerful, but not infallible. Here's what studies actually show about its error rate — and how examiners should present it.
Read →What is a fingerprint actually made of?
A latent fingerprint is a chemical deposit — sweat, oils and more. What's in it explains why some development methods work on some surfaces and not others.
Read →How are tool marks matched to a specific tool?
A jemmy, a bolt cutter, a screwdriver — each leaves marks that can be traced back to it. Here's how examiners compare tool marks, and the two kinds they read.
Read →Tyre track evidence: what tread marks reveal
Tyre marks at a scene can identify a vehicle's type — and sometimes the exact tyre. Here's what investigators read from a track, and how they recover it.
Read →Casting impressions: how 3D marks are preserved
A footwear or tyre mark pressed into soil is fragile 3D evidence. Casting captures it permanently — here's how it's done, and why photography comes first.
Read →Do identical twins have the same fingerprints?
Identical twins share the same DNA — so do they share fingerprints? No. Here's why, and what it reveals about how fingerprints form.
Read →Fingerprints, face and iris: how do biometrics compare?
Fingerprints, facial recognition and iris scans all identify people — but they differ in accuracy, permanence and how easily they're fooled. A practical comparison.
Read →Blood grouping vs DNA: why serology wasn't enough
Before DNA, forensic scientists identified blood and body fluids by their type and protein markers. It could exclude — but rarely identify. Here's the story.
Read →What is trace evidence? A beginner's guide
Fibres, glass, hair, paint, soil — trace evidence is the small stuff that links people, objects and places. Here's what it is and why it matters.
Read →How do investigators process a crime scene?
Working a crime scene is a disciplined sequence, not a rummage. Here are the stages — secure, document, search, collect — and why order matters.
Read →Cognitive bias in forensic science
Even skilled examiners can be swayed by context and expectation. Here's how cognitive bias affects forensic conclusions — and what's being done about it.
Read →How does a drug test actually work?
Most drug testing is two-stage: a fast screen, then a definitive confirmation. Here's what each step does, what it detects, and why both are needed.
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