How Reliable Is DNA In Identifying Suspects?

BJason Felch and Maura Dolan, Los Angeles Times, July 20, 2008

State crime lab analyst Kathryn Troyer was running tests on Arizona’s DNA database when she stumbled across two felons with remarkably similar genetic profiles.

The men matched at nine of the 13 locations on chromosomes, or loci, commonly used to distinguish people.

The FBI estimated the odds of unrelated people sharing those genetic markers to be as remote as 1 in 113 billion. But the mug shots of the two felons suggested that they were not related: One was black, the other white.

In the years after her 2001 discovery, Troyer found dozens of similar matches—each seeming to defy impossible odds.

As word spread, these findings by a little-known lab worker raised questions about the accuracy of the FBI’s DNA statistics and ignited a legal fight over whether the nation’s genetic databases ought to be opened to wider scrutiny.

The FBI laboratory, which administers the national DNA database system, tried to stop distribution of Troyer’s results and began an aggressive behind-the-scenes campaign to block similar searches elsewhere, even those ordered by courts, a Times investigation found.

At stake is the credibility of the compelling odds often cited in DNA cases, which can suggest an all but certain link between a suspect and a crime scene.

When DNA from such clues as blood or skin cells matches a suspect’s genetic profile, it can seal his fate with a jury, even in the absence of other evidence. As questions arise about the reliability of ballistic, bite-mark and even fingerprint analysis, genetic evidence has emerged as the forensic gold standard, often portrayed in courtrooms as unassailable.

But DNA “matches” are not always what they appear to be. Although a person’s genetic makeup is unique, his genetic profile—just a tiny sliver of the full genome—may not be. Siblings often share genetic markers at several locations, and even unrelated people can share some by coincidence.

No one knows precisely how rare DNA profiles are. The odds presented in court are the FBI’s best estimates.

The Arizona search was, in effect, the first test of those estimates in a large state database, and the results were surprising, even to some experts.

Defense attorneys seized on the Arizona discoveries as evidence that genetic profiles match more often than the official statistics imply—and are far from unique, as the FBI has sometimes suggested.

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Indeed, experts generally agree that most—but not all—of the Arizona matches were to be expected statistically because of the unusual way Troyer searched for them.

In a typical criminal case, investigators look for matches to a specific profile. But the Arizona search looked for any matches among all the thousands of profiles in the database, greatly increasing the odds of finding them.

As a result, Thomas Callaghan, head of the FBI’s CODIS unit, has dismissed Troyer’s findings as “misleading” and “meaningless.”

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Concerned about errors

From her first discovery in 2001, Troyer and her colleagues in the Arizona Department of Public Safety’s Phoenix DNA lab were intrigued.

At the time, many states looked at only nine or fewer loci when searching for suspects. (States now commonly attempt to compare 13 loci, though often fewer are available from old or contaminated crime scene evidence.)

Based on Troyer’s results, she and her colleagues believed that a nine-locus match could point investigators to the wrong person.

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But Bruce Budowle, an FBI scientist who specializes in forensic DNA, told colleagues of Troyer that such coincidental matches were to be expected.

Three years later, Bicka Barlow, a San Francisco defense attorney, came across a description of Troyer’s poster on the Internet.

Its implications became clear as she prepared to defend a client accused of a 20-year-old rape and murder.

A database search had found a nine-locus match between his DNA profile and semen found in the victim’s body. Based on FBI estimates, the prosecutor said the odds of a coincidental match were as remote as 1 in 108 trillion.

Recalling the Arizona discovery, Barlow wondered if there might be similar coincidental matches in California’s database—the world’s third-largest, with 360,000 DNA profiles at the time. The attorney called Troyer in Phoenix to learn more.

Troyer seemed eager to talk about her discovery, which still had her puzzled, Barlow recalled. The analyst told Barlow she had searched the growing Arizona database since the conference and found more pairs of profiles matching at nine and even 10 loci.

Encouraged, Barlow subpoenaed a new search of the Arizona database. Among about 65,000 felons, there were 122 pairs that matched at nine of 13 loci. Twenty pairs matched at 10 loci. One matched at 11 and one at 12, though both later proved to belong to relatives.

Barlow was stunned. At the time, such matches were almost unheard of.

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Arizona officials obtained a court order to prevent Barlow from sharing the results with anyone else.

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Soon, defense lawyers in other states were seeking what came to be known as “Arizona searches.”

‘Don’t panic’

For years, DNA’s strength in the courtroom has been the brute power of its numbers. It’s hard to argue with odds like 1 in 100 billion.

Troyer’s discovery threatened to turn the tables on prosecutors. At first blush, the Arizona matches appeared to contradict those statistics and the popular notion that DNA profiles, like DNA, were essentially unique.

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Many of the Arizona matches were predictable, Myers [Steven Myers, a senior DNA analyst at the California Department of Justice] said, given the type of search Troyer had conducted.

In a database search for a criminal case, a crime scene sample would have been compared to every profile in the database—about 65,000 comparisons. But Troyer compared all 65,000 profiles in Arizona’s database to each other, resulting in about 2 billion comparisons. Each comparison made it more likely she would find a match.

When this “database effect” was considered, about 100 of the 144 matches Troyer had found were to be expected statistically, Myers found.

Troyer’s search also looked for matches at any of 13 genetic locations, while in a real criminal case the analyst would look for a particular profile—making a match far less likely.

Further, any nonmatching markers would immediately rule out a suspect. In the case of the black and white men who matched at nine loci, the four loci that differed—if available from crime scene evidence—would have ensured that the wrong man was not implicated.

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Some scientists are not satisfied by any of these explanations. They wonder whether Troyer’s findings signal flaws in the complex assumptions that underlie the FBI’s rarity estimates.

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