The Remarkable Rise of DNA Evidence in Criminal Cases
The Discovery of DNA Fingerprinting
In 1953, James Watson and Francis Crick famously uncovered the double helix structure of DNA, the genetic blueprint for life. This Nobel Prize-winning work enabled researchers to understand how DNA encodes biological information. But it wasn’t until 1984 that Dr. Alec Jeffreys first realized DNA could be used to identify individuals in forensic investigations.
Jeffreys, a geneticist at the University of Leicester, was studying hereditary diseases using a technique called restriction fragment length polymorphism (RFLP) analysis. This method looks at variable lengths of DNA sequence that differ between individuals. During an experiment, Jeffreys was astonished to find a DNA profile that was unique to one of his lab technicians. He immediately grasped the significance: an individual’s DNA profile could serve as a genetic fingerprint for identification purposes.
Jeffreys helped develop RFLP analysis into the first DNA profiling system, launching the field of forensic DNA typing. Genetic fingerprints obtained from blood, semen, skin or other crime scene evidence could now be compared to DNA from criminal suspects. DNA matches provide an extremely high statistical probability of shared identity, allowing investigators to include or exclude potential sources of biological evidence.
Early DNA Profiling Cases
Forensic DNA analysis debuted in criminal investigations in the mid-1980s, when British police asked Dr. Jeffreys to assist in two high-profile murder cases. Police had no suspect in the rape and murder of 15-year-old Lynda Mann in 1983. But three years later, another 15-year-old girl, Dawn Ashworth, was similarly sexually assaulted and strangled in the same village. Police suspected a serial offender was responsible.
Jeffreys used his newly developed DNA profiling technique to analyze semen samples from both crime scenes. This revealed the same perpetrator was linked to both murders – but police still lacked a suspect. In a bold move, investigators screened over 5,000 local men to look for DNA matches. This mass screening identified a 17-year-old male with matching DNA, who confessed to Dawn’s murder but denied killing Lynda. He was convicted of one of the murders, but many ethical concerns were raised over privacy violations during this DNA dragnet approach.
Just a year later in 1988, DNA evidence exonerated a wrongly convicted individual for the first time. Gary Dotson had served 10 years in prison for a rape conviction based on a victim’s testimony. But DNA tests proved the semen from the crime scene did not match Dotson, corroborating his innocence. This case demonstrated DNA’s power to overturn wrongful convictions based on less reliable evidence.
The Legal Challenges to DNA Evidence
As DNA profiling emerged in the late 1980s, its admissibility as legal evidence faced numerous court challenges. Defense attorneys argued DNA results lacked accepted scientific standards and scrutinized the validity of statistical calculations used to estimate match probabilities. Prosecutors worked to establish reliability and prevent exclusion of this incriminating new tool.
Several key appellate cases ultimately upheld DNA evidence as admissible in court. In United States v. Jakobetz (1991), the 2nd U.S. Circuit Court of Appeals affirmed that DNA fingerprinting met the Frye “general acceptance” standard for expert scientific testimony. People v. Castro (1989) was the first state supreme court ruling upholding DNA evidence, establishing its acceptance under the Frye test in New York courts.
But the most critical case was Daubert v. Merrell Dow Pharmaceuticals (1993), in which the U.S. Supreme Court outlined more flexible guidelines for admitting scientific expert testimony. This superseded the rigid Frye standard, requiring only that evidence be scientifically valid and reliable. Under Daubert, judges must assess factors like peer review, error rates, standards, and general acceptance when evaluating admissibility of innovative scientific techniques like DNA profiling.
The DNA Identification Act of 1994
By the early 1990s, courts were increasingly admitting DNA results as evidence, but no universal standards governed forensic DNA labs. In response, Congress passed the DNA Identification Act in 1994 to regulate quality assurance, accreditation and proficiency testing in DNA analysis nationwide. The Act authorized creation of the FBI’s Combined DNA Index System (CODIS), a massive DNA profile databank linking national, state and local forensic laboratories.
CODIS enables law enforcement to match DNA from crime scenes against known offender profiles and link serial cases together. This system now contains over 14 million offender profiles and has aided over 500,000 investigations. The DNA Identification Act ensured the reliability and utility of DNA evidence on a national scale.
PCR Analysis Revolutionizes DNA Profiling
Just as DNA profiling gained widespread courtroom admissibility and legal support, new technology transformed the field. The development of the polymerase chain reaction (PCR) in the 1980s provided a major breakthrough in DNA analysis. PCR enables targeted amplification of small DNA samples to generate millions of copies for profiling. This allowed DNA testing on samples as minute as a single hair follicle or tiny trace of body fluid or skin cells.
PCR also permitted analysis of shorter repetitive DNA sequences called short tandem repeats (STRs). STR profiling replaced RFLP analysis by the late 1990s to become the standard forensic DNA method. PCR-STR testing is extremely sensitive, fast, and cost-effective, requiring only small samples. This opened up tremendous new forensic applications for DNA evidence.
21st Century DNA Analysis Capabilities
Continuing advances now allow DNA profiling from just a few cells, enabling touch DNA analysis of skin cell traces left on objects someone handled. New rapid DNA instruments can generate STR profiles in under two hours for quick suspect identification. Automated extraction, quantification and interpretation systems streamline the DNA analysis workflow.
Sophisticated software can now even predict physical appearance and biogeographic ancestry from DNA samples through forensic DNA phenotyping. Next-generation sequencing platforms enable full DNA sequencing for the most information from ultra-degraded or complex samples. As technology progresses, forensic DNA typing becomes ever faster, more sensitive and more powerful.
DNA’s Impact on Criminal Justice
In just four decades, DNA profiling has transformed from a novel science into a core forensic tool that has radically changed how crimes are investigated and prosecuted. DNA evidence provides unparalleled identification accuracy that enables decisive criminal links while avoiding reliance on less reliable circumstantial evidence.
It provides definitive evidence to settle uncertainty, prevent wrongful convictions, resolve cold cases or exonerate the wrongly accused. DNA analysis has developed into the gold standard of forensic science and become a vital asset in the pursuit of justice.