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How Forensics Follows The DNA Trail

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Colin Pitchfork might not be as recognizable a name as Ted Bundy or Jeffrey Dahmer, but he does occupy a certain significance in the annals of crime: He is the first person to be convicted based on DNA fingerprinting, apprehended for the rape and murder of two young girls in 1986 after being betrayed by the bodily fluids he had left at the scene.

DNA fingerprinting not only led to Pitchfork’s arrest, but also exonerated a young teenage boy who had initially confessed to the murders. Since 1989, 317 people have been exonerated by DNA evidence after being convicted of crimes, according to the Innocence Project; of those reopened cases, 154 have led to the identification of the true perpetrator (or at least a more likely suspect).

Current Practice: Telltale Little Pieces
As laboratory techniques advance, investigators are finding new ways to use DNA evidence. The initial technique used for DNA fingerprinting, developed by British geneticist Alec Jeffreys in the late 1970s (and initially primarily used for paternity testing), was called restriction fragment length polymorphisms (RFLP) analysis. In this technique, an investigator uses an enzyme to break the DNA sample at certain specific points in the genetic code. The lengths of these resulting pieces will vary among individuals, thanks to a feature of our genomes called variable number tandem repeats (VNTRs), which are stretches of DNA where a certain sequence of around 10 to 100 letters in the genetic code repeats a unique number of times.

“The reality of the early days of DNA technology was that the technique could be applied only to the most serious cases,” Toronto forensic scientists Raymond Prime and Jonathan Newman wrote in The Police Chief magazine. “In the beginning, the availability of technical specialists was limited, the sample size needed for the RFLP analysis was rather large, and the length of time needed to complete the laboratory analysis narrowly restricted the application of the technique.”

Today, police departments use a quicker test called short tandem repeats (STR) typing, which characterizes a DNA sample using shorter sequences, usually consisting of 2 to 6 base pairs. Because a person can be profiled in this way using a much smaller sample of genetic material, STR quickly became the method of choice.

Future Practice: Widening The Scope
Why focus on these areas of DNA “stuttering,” and not the actual genes of the sample? The answer is primarily practical: From person to person, the amount of genetic material that varies is relatively miniscule; variable repetitive sequences are an easy and reliable tool to use in comparing a suspect to a sample. But whole genomes are not necessarily totally useless in investigations. There may be information that investigators could glean from examining the entire sequence from a sample, especially if there’s no suspect in hand. A future forensic scientist might find genes at the crime scene that point toward a perpetrator with a particular eye or hair color, or ethnicity. By examining the telomeres—the caps on the ends of chromosome strands that degrade over time—it might be possible to deduce someone’s age from their genetic sequence. Every drop of blood could be a tiny sketch artist drawing a profile for investigators.

The world recently got a preview of this sort of whole genome investigation with the purported identification of late 19th-century serial killer Jack the Ripper, identified as Polish immigrant Aaron Kosminski (one of the initial suspects picked up by the police) by Liverpool John Moores University molecular biologist Jari Louhelainen and author Russell Edwards. Louhelainen took biological samples taken from a shawl supposedly belonging to Ripper victim Catherine Eddowes, and used whole genome amplification to open up the suspect’s entire DNA library for analysis. Louhelainen wrote in the Daily Mail that the genome of the suspect provided enough information to establish that he was probably of Russian Jewish ancestry and even had dark hair. But many scientists are reserving judgment on the strength of Russell and Louhelainen’s work until a second, independent researcher verifies the results.

Experts are still skeptical that this kind of detailed DNA profiling will be feasible for routine use any time soon. The relationship between the genome and physical traits is complex; human eye color alone is thought to be governed by 16 or more genes. Still, in future criminal investigations, “one important shift may be the scope of what’s looked at to guess who a DNA sample came from,” says Nathan Pearson, senior director of scientific engagement and public outreach at the New York Genome Center. “Going forward, we’ll likely scan chromosomes more thoroughly to tell definitively who was there—especially when trying to disentangle the DNA of one person from another [in a potentially mixed sample].”

Whole genome sequencing has also been too costly to be widely used in forensics, but the process is becoming cheaper and faster all the time. In January 2014, San Diego-based sequencing company Illumina announced that its new HiSeq X Ten system would deliver a full human genome sequence for less than $1,000, a long-sought benchmark achievement. The NY Genome Center was one of the first buyers of the HiSeq X Ten, but the price tag (estimated at $10 million by Nature) is beyond the reach of most police departments.

Scientists are also getting better and better at recovering genetic information from older samples. It’s still a lot of hard work—DNA, like any organic matter, degrades over time. But given the right sample, researchers can reconstruct incredibly ancient sequences, such as the genome of an ancient horse that lived 700,000 years ago.

“Ten years ago, people were skeptical that we would ever be able to read much DNA from a Neanderthal bone,” Pearson says. “The dogma was that it had degraded too much. But lab methods improve; people find clever ways to salvage DNA, copy it, even repair it. Similarly, when we look at forensics from crime scenes or mass graves, there’s more hope now that we can recover more DNA.”

Future Concerns: A Brave, Scary New World
As with any piece of technology, the power of DNA profiling comes with a potential for abuse. There are a growing number of DNA databases maintained by governments, academic institutions, and non-governmental organizations, and the security of this genetic information—and the ethics of how the data is collected and used—are potent concerns.

Many of these databases “involve vulnerable populations, including children, sex workers, and persons whose legal or resident status may be questioned. When government authorities request voluntary DNA samples, prospective participants may feel coerced to participate,” Duke University researchers Joyce Kim and Sara Katsanis wrote in a 2013 article for the journal Trends in Genetics.

In particular, there’s a risk that advancements in DNA fingerprinting may do more harm than good for communities of color. Already, abuses have surfaced: Khalil Muhammad, Director of the Schomburg Center for Research in Black Culture, remembers one incident in Virginia back in the early 2000s where the police in Charlottesville asked nearly 200 black men to submit a cheek swab DNA sample for their collection in pursuit of a serial rapist. “If there had been a white serial rapist on the loose, would they attempt to swab the cheeks of every white male in Albermarle County?” Muhammad asks. “The answer is no, of course.”

As this technology advances, Muhammad says, scientists need to be more attuned to the social implications of their work. “The solution to wrestling with the thorny questions of genetic profiling is having as much historical context for these technologies as possible,” he says. “Technology has always been subject to human will and interpretation. It is not neutral, because it is for our use, and therefore is implicated in the range of human reactions.”

Like any forensic scientific technique, analyzing DNA evidence is limited by human error. Remarkably, Colin Pitchfork nearly escaped the DNA dragnet set for him, after he paid a friend to give a sample in his stead. It was only when a woman overheard Pitchfork’s accomplice bragging about the deed that the police were able to zero in on Pitchfork and confront him with the DNA evidence. But for a pair of sharp ears, the first DNA fingerprinting conviction might have gone nowhere at all.

Want to read more about crime, punishment, and science? The World Science Festival event The Science of Justice: A Matter of Opinion covered the psychology of faulty witness identifications, false confessions, and more. And you can see flawed perception in action in this live demonstration of a crime and photo lineup.


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