DNA profiling methods have become faster, more sensitive, and more user-friendly since the first murderer was caught with help from genetic evidence
The name Colin Pitchfork might not be as recognizable as Charles Manson or Jeffrey Dahmer. But for those in the forensic science community, it’s a name that holds weight. Pitchfork was the first murderer to be caught using DNA analysis.
When 15-year-old Dawn Ashworth was raped and murdered in Leicestershire, England, in late July 1986, Alec Jeffreys was a genetics professor at the nearby University of Leicester. A few years earlier, he had discovered that patterns in some regions of a person’s DNA could be used to distinguish one individual from another.
So far, Jeffreys had put his DNA pattern recognition technique to work in paternity and immigration cases, but now the police wanted him to help solve Ashworth’s murder as well as a similar one that happened in 1983. The police already had a suspect, Richard Buckland, who had even confessed to Ashworth’s murder. When Jeffreys analyzed DNA samples from the 1983 and 1986 crime scenes and from Buckland, he found matching DNA from both crime scenes—but the recovered DNA didn’t match Buckland’s genetic code.
In an attempt to find the real culprit—the one whose DNA had been left behind—the police undertook a genetic dragnet. They obtained blood and saliva samples from more than 4,000 men in the Leicestershire area between the ages of 17 and 34 and had Jeffreys analyze the DNA. They didn’t find a match until a man was overheard saying he’d been paid to pose as someone else and provide false samples. The person trying to evade the DNA dragnet was Colin Pitchfork. When Pitchfork’s DNA was analyzed, it matched the crime scene samples. Pitchfork was arrested on Sept. 19, 1987, and convicted and sentenced to life in prison the following January.
Although DNA evidence alone is not enough to secure a conviction today, DNA profiling has become the gold standard in forensic science since that first case 30 years ago. Despite being dogged by sample processing delays because of forensic lab backlogs, the technique has gotten progressively faster and more sensitive: Today, investigators can retrieve DNA profiles from skin cells left behind when a criminal merely touches a surface. This improved sensitivity combined with new data analysis approaches has made it possible for investigators to identify and distinguish multiple individuals from the DNA in a mixed sample. And it’s made possible efforts that are underway to develop user-friendly instruments that can run and analyze samples in less than two hours.
Then and now
DNA contains regions in which short sequences of bases are repeated multiple times. These repeats are found in many spots—or loci—throughout the genome. Because the exact number of repeats at any particular locus varies from person to person, forensic scientists can use these markers, called short tandem repeats (STRs), to identify individuals. What’s more, people inherit chromosomes from both of their parents, so individuals have two sets of STRs, each of which can have different numbers of repeats at the same locus. This pair of alleles can therefore provide even more specificity to a person’s DNA profile.
During DNA profiling, cells are collected and broken open to gain access to their DNA. Then forensic scientists copy the DNA regions of interest and measure the length of the repeat sequences at multiple loci. The length rather than the exact sequence of the repeats serves as a marker for DNA profiles because repeat length is sufficient for distinguishing among individuals. Although many STR loci dot the human genome, forensic scientists choose to analyze a small set of markers, rarely more than one locus per chromosome. Picking loci that are distant from one another ups the likelihood that the number of repeats at one locus is inherited independently of the number of repeats at another locus, thereby increasing the rarity of any particular DNA profile.
During the Leicestershire-area dragnet, Jeffreys used a type of repeat unit different from the ones used today. Those so-called minisatellites contained repeat segments that were dozens or even hundreds of bases long, says John M. Butler, special assistant to the director for forensic science at the U.S. National Institute of Standards & Technology. The overall DNA fragment at each locus could be tens of thousands of bases long, adds Tracey Dawson Cruz, a professor of forensic science at Virginia Commonwealth University (VCU).
References:
Celia Henry Arnaud (October 15, 2020). Thirty years of DNA forensics: How DNA has revolutionized criminal investigations [Blog post]. Retrieved from https://buff.ly/2Hxv8nW
