Deoxyribonucleic acid (DNA) testing, which identifies individuals based on unique patterns within their genetic code, represents one of the most significant advances in forensic science. The technology provides a powerful method for human identification by analyzing specific, highly variable sequences of DNA. Its introduction into the United States revolutionized the justice system, offering an unprecedented tool for linking suspects to crimes, exonerating the wrongfully convicted, and establishing biological relationships. The journey from a laboratory discovery to a common courtroom tool involved a rapid evolution of both the science and its legal acceptance.
The Foundation of DNA Fingerprinting
The scientific groundwork for forensic DNA testing was established in the United Kingdom in 1984 by geneticist Sir Alec Jeffreys. He discovered that certain regions of the human genome contain sequences of bases that are repeated multiple times, and the number of these repetitions varies dramatically between individuals. This breakthrough allowed for the creation of a unique genetic profile, which he termed “DNA fingerprinting.”
The initial technique used to analyze these differences was Restriction Fragment Length Polymorphism (RFLP). This method involved cutting DNA samples with specific enzymes, separating the resulting fragments by size, and visualizing them to create a distinctive pattern of bands. The variable regions examined were known as Variable Number Tandem Repeats (VNTRs), which contain longer repeating units of DNA.
This RFLP process required a relatively large and non-degraded biological sample, a limitation that would later spur technological development.
Initial Forensic Adoption in the United States
The power of DNA fingerprinting quickly crossed the Atlantic, leading to its first applications in US criminal justice in the late 1980s. The first person convicted in the United States using RFLP-based DNA evidence was Tommie Lee Andrews in Florida, who was found guilty of rape in 1987. This conviction signaled the arrival of the new technology in American courtrooms, establishing its potential as a tool for criminal identification.
The early admissibility of this novel science faced rigorous legal scrutiny under standards such as the Frye rule, which required new scientific methods to be generally accepted within the relevant scientific community. A pivotal moment occurred in 1989 with the New York case People v. Castro, where a lengthy pretrial hearing was held to determine the reliability of the RFLP technique.
The court ultimately affirmed the general scientific validity of DNA testing but concluded that the specific laboratory procedures used in that case were flawed, highlighting the need for standardization in its application.
The state adoption of DNA technology also began in this period. Virginia implemented RFLP-based DNA testing in its state crime laboratory in 1989. This action made Virginia the first state to adopt a policy of using the genetic analysis method in criminal investigations. The successful but often-challenged introduction of RFLP in the late 1980s laid the necessary legal and procedural foundation for the widespread acceptance that followed.
The Shift to Modern Testing Methods
The widespread use of RFLP was severely limited by its requirement for large, pristine DNA samples and the slow, labor-intensive nature of the process. A major technological advancement was the transition to Polymerase Chain Reaction (PCR) in the late 1980s, which allowed scientists to create millions of copies of a specific DNA segment from only a minute amount of original material. This amplification process was a game-changer, enabling the testing of small or degraded samples often recovered from crime scenes, which RFLP could not handle.
The shift was completed with the adoption of Short Tandem Repeats (STRs) as the standard marker system in the early 1990s. Unlike the longer VNTRs used in RFLP, STRs involve much shorter repeating sequences of two to six base pairs, making them ideal for PCR amplification. Analyzing multiple STR loci simultaneously provides an extremely high power of discrimination, even when the DNA is partially broken down.
This new methodology, combining PCR with STR analysis, significantly reduced the time, sample size, and quality required for a successful DNA profile. The transition to this more robust, automated, and sensitive system made DNA testing a practical, routine procedure for forensic laboratories across the country. The adoption of STR analysis solidified its position as the preferred method for forensic DNA analysis.
Creating Centralized DNA Databases
With the new, reliable STR methodology in place, the United States government began institutionalizing DNA testing through the creation of centralized databases. The Federal Bureau of Investigation (FBI) started a pilot program for the Combined DNA Index System (CODIS) in 1990. This system was designed to allow federal, state, and local forensic laboratories to electronically exchange and compare DNA profiles.
The legal framework for a national database was formalized with the passage of the DNA Identification Act of 1994, which authorized the FBI to establish the National DNA Index System (NDIS). This national level of CODIS became fully operational in October 1998, connecting participating laboratories across the country.
CODIS contains profiles from crime scenes (Forensic Index) and convicted offenders (Offender Index), allowing investigators to link cold cases to each other and to known individuals. The system’s core function is generating investigative leads, as matches made between the indexes can link serial crimes across different jurisdictions. The initial framework required the collection of DNA samples from convicted offenders, and subsequent legislation expanded the categories of individuals whose profiles would be included. The systematic storage and comparison of DNA profiles through CODIS marked the final step in establishing DNA analysis as a permanent, systematic tool in US law enforcement.