DNA was first used to solve a crime in 1986, when British police asked geneticist Alec Jeffreys to help identify a man who had raped and murdered two girls in Leicestershire, England. That case, which led to the conviction of Colin Pitchfork, launched a revolution in criminal justice that has only accelerated in the four decades since.
The 1986 Case That Started It All
In 1984, Alec Jeffreys and his colleagues at the University of Leicester discovered that certain regions of human DNA vary enormously from person to person. They developed a method to read those variations, creating what Jeffreys called a “DNA fingerprint.” Two years later, local police investigating the murders of two teenage girls asked him to apply the technique to crime scene evidence.
Police collected blood samples from thousands of men in the area in what became the first genetic dragnet. Pitchfork nearly evaded detection by convincing a friend to submit a blood sample on his behalf. When that deception was uncovered, Pitchfork’s own DNA was tested and matched the crime scene samples. He was convicted in 1988, and the case proved to the world that biological evidence could identify a specific individual with near certainty.
DNA Evidence Crosses the Atlantic
The United States wasn’t far behind. In 1987, Tommie Lee Andrews became the first American convicted using DNA evidence. On February 21 of that year, a stranger broke into a Florida woman’s home at knifepoint, burglarizing and raping her. DNA from semen recovered at the crime scene matched blood drawn from Andrews, a serial rapist. He was sentenced to 22 years in prison for rape, aggravated burglary, and burglary. The case established a legal precedent: American courts would accept DNA as admissible evidence.
The First DNA Exoneration
DNA didn’t just put people behind bars. It also started getting innocent people out. On August 14, 1989, a Cook County court in Chicago vacated Gary Dotson’s 1979 rape conviction and dismissed the charges against him. Dotson had spent a decade in prison for a crime he didn’t commit. He was the first person in the United States cleared by DNA identification technology, opening the door for hundreds of wrongful conviction reversals in the years that followed.
Building a National DNA Database
As DNA profiling proved its value case by case, law enforcement saw the need for a centralized system. The FBI Laboratory launched its Combined DNA Index System, known as CODIS, as a pilot project in 1990, initially serving just 14 state and local laboratories. The DNA Identification Act of 1994 then formalized the FBI’s authority to establish a National DNA Index System for law enforcement purposes. CODIS allows investigators to compare DNA from unsolved crime scenes against a growing database of offender profiles, linking cases that would otherwise remain unconnected.
From Weeks in the Lab to Hours at a Station
The early DNA profiling method, called RFLP, required large, high-quality samples and took weeks to process. By the early 1990s, a faster approach called STR typing began replacing it. STR profiling works with much smaller or degraded samples and pairs with a copying technique that can amplify tiny traces of genetic material into usable quantities. This shift was so significant that many of the recommendations issued in a major 1992 report on forensic DNA standards became unnecessary almost immediately.
Modern forensic labs can detect DNA at astonishingly low concentrations. Current quantification technology picks up DNA at levels below 5 picograms per microliter, which is roughly the amount found in a single cell. Standard analysis needs about 225 picograms of DNA per test, while specialized low-template protocols can work with as little as 22.5 picograms. In practical terms, this means a partial fingerprint, a few skin cells on a doorknob, or a trace of saliva on a stamp can yield a usable profile.
The latest leap is Rapid DNA technology. Authorized by the Rapid DNA Act of 2017, these fully automated machines can generate a DNA profile from a cheek swab in one to two hours, with no laboratory and no human interpretation required. They were designed for police booking stations, allowing officers to compare an arrestee’s DNA against CODIS during the booking process itself.
Genetic Genealogy and Cold Cases
Perhaps the most dramatic recent development came in 2018 with the arrest of the Golden State Killer, a serial rapist and murderer who had eluded police for decades. Investigators uploaded crime scene DNA to GEDmatch, a public genealogy database where people voluntarily share their genetic data. The DNA matched a probable fourth cousin of the suspect. Working with a genealogist, police reconstructed a family tree from that distant match and narrowed the search to Joseph James DeAngelo, who was subsequently arrested and convicted.
This technique, called investigative genetic genealogy, doesn’t require the suspect to be in any law enforcement database at all. It only needs a relative, even a distant one, to have uploaded their data to a public genealogy site. Since the Golden State Killer case, the method has been applied to an increasing number of cold cases, solving murders and sexual assaults that had gone unanswered for decades.
How Reliable DNA Evidence Really Is
Modern DNA profiles are extraordinarily precise. Courts have heard match probabilities as high as one in 19 billion, meaning the chance that a random unrelated person would share the same profile is vanishingly small. With current STR technology analyzing multiple genetic locations simultaneously, some calculations produce probabilities in the trillions or beyond. For context, the entire world population is around 8 billion, so a one-in-19-billion match probability already exceeds the number of people on Earth.
That said, DNA evidence is not without challenges. Mixed samples containing DNA from two or more people remain difficult to interpret. When multiple contributors are present, some genetic markers from minor contributors can be completely obscured by the dominant profile, creating ambiguity about how many people contributed and which markers belong to whom. Tiny samples also introduce random effects: genetic markers can drop out entirely or foreign DNA can be introduced during collection or lab handling. With current technology, it is impossible to determine the number of contributors in a mixed DNA sample with 100% certainty. Most forensic mixed samples involve four or fewer profiles, but untangling them requires specialized software that has only recently been widely standardized.
A Timeline of Key Milestones
- 1984: Alec Jeffreys develops the first DNA fingerprinting method
- 1986: DNA is used in a criminal investigation for the first time (Pitchfork case, UK)
- 1987: Tommie Lee Andrews becomes the first American convicted using DNA evidence
- 1989: Gary Dotson becomes the first person exonerated by DNA testing
- 1990: FBI launches the CODIS database as a pilot project
- 1994: Congress authorizes the National DNA Index System
- Early 1990s: STR typing begins replacing the slower RFLP method
- 2017: Rapid DNA Act authorizes automated DNA processing at booking stations
- 2018: Genetic genealogy solves the Golden State Killer cold case