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Scientists Explain How Familial DNA Testing Nabbed Alleged Serial Killer

A quarter-century of conventional detective work failed to track down the killer responsible for the deaths of at least 10 young women in south Los Angeles dating back to the mid-1980s. But a discarded piece of pizza and a relatively new method of DNA testing has finally cracked the case, police announced last week. On 7 July, L.A. police arrested Lonnie Franklin Jr., 57, a former garage attendant and sanitation worker they suspect is the serial killer nicknamed the "Grim Sleeper."

Since 2008, California has allowed so-called familial DNA searches, in which investigators look for close-but-not-exact matches between DNA evidence collected at crime scenes and the state's data bank of DNA collected from 1.3 million convicted felons. The method has a longer history in the United Kingdom, where it led to a conviction in a murder case in 2004. In Colorado, the only other U.S. state to allow it, the method led to a guilty plea in a car-theft case in Denver last year.

On Friday, ScienceInsider spoke with two scientists involved with the DNA search, senior criminalist Steven Myers and case-work laboratory manager Gary Sims, both based at the Jan Bashinski DNA Laboratory in Richmond, California. They explained that the searches initially focus on 15 regions of DNA on 13 chromosomes. These include the 13 regions used by the Federal Bureau of Investigation's Combined DNA Index System (CODIS) and two additional regions used in California. All of these regions contain genetic stutters called short tandem repeats, in which a pattern of base pairs repeats itself over and over. The number of repeats varies from person to person, and two people who are related are likely to have the same number of repeats at more of these sites. The lab's analysis also considers how frequently a given variation occurs in the general population: two people who share a rare variation are more likely to be related than are two people who share a common one.

Sims and Myers explained that the lab's software uses this information to generate a ranked list of the convicted felons in the DNA database who are most likely to be first-order relatives—parents, children, or full siblings—of the person a DNA sample came from. (The statistics aren't strong enough to identify more distant relatives, they say.) When both individuals in question are male, the lab also looks at a similar number of short tandem repeats on the Y chromosome, which should be an exact match between fathers and sons and between full brothers.

California allows familial DNA searches only for violent crimes in which the perpetrator is still believed to be a danger to society. Sims and Myers say they have run 10 searches so far. The first nine came up empty, including a 2008 search with DNA evidence from the Grim Sleeper crime scene. "We did not find anybody in the database who we thought was a potential relative," says Sims.

However, a second search in April 2010 did turn up a potential match: a young man named Christopher Franklin who was convicted last year on a felony weapons charge. The DNA search, along with the dates of the murders cast suspicion on Christopher Franklin's father. After an internal review of the evidence, investigators at the Bashinski lab notified the L.A. police, who followed the elder Franklin and eventually got a DNA sample from a discarded piece of pizza. Lonnie Franklin's DNA matched DNA from the crime scenes, and police arrested him at his home last week.

Although it remains to be seen whether Franklin will be convicted, Sims says the lab is proud of its work. "To put this whole thing together ... and see it pay off is very gratifying," he says. "Nobody popped champagne bottles or anything like that, but we all feel like we earned our pay."

The apparent success of familial DNA testing in such a high-profile case may encourage other states to adopt it, but civil-liberties groups and some legal scholars have concerns about privacy and ethics issues. Stanford Law School professor Hank Greely is concerned about what he sees as a high likelihood of false-positive tests. In a database with DNA from a million or more individuals, several hundred people might have a close enough match on the CODIS markers to suggest a blood relationship, Greely estimates. If those matches result in false accusations, the burden of being falsely accused would fall disproportionately to African-Americans, who are overrepresented in the U.S. prison population. "It does raise some concerns about discrimination," he says. Even so, Greely says California's requirement for Y chromosome testing is a good safeguard. "Unlike some folks, I'm not opposed to the technique and especially not to its use in this case," he says.