In the 1990s, FBI scientists estimated the rarity of each genetic marker by extrapolating from sample populations of a few hundred people from various ethnic or racial groups. The estimates for each marker are multiplied across all 13 loci to come up with a rarity estimate for the entire profile.
These estimates make assumptions about how populations mate and whether genetic markers are independent of each other. They also don't account for relatives.
Bruce Weir, a statistician at the University of Washington who has studied the issue, said these assumptions should be tested empirically in the national database system.
"Instead of saying we predict there will be a match, let's open it up and look," Weir said.
Some experts predict that given the rapid growth of CODIS, such a search would produce one or more examples of unrelated people who are identical at all 13 loci.
Such a discovery was once unimaginable.
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'Dire consequences'
In January 2006, not long after Barlow distributed the results of the court-ordered search in Arizona, the FBI sent out a nationwide alert to crime labs warning of similar defense requests.
Soon after, the bureau's arguments against the searches were being made in courtrooms around the country.
In California, Michael Chamberlain, a state Department of Justice official, persuaded judges that such a search could have "dire consequences" -- violating the privacy of convicted offenders, shutting down the database for days and risking the state's expulsion from the FBI's national DNA system. All this for a search whose results would be irrelevant and misleading to jurors, Chamberlain argued.
When similar arguments were made in an Arizona case, the judge ruled that the search would be "nothing more than an interesting deep sea fishing expedition."
But in Illinois and Maryland, courts ordered the searches to proceed, despite opposition from the FBI and state officials at every turn.
In July 2006, after Chicago-area defense attorneys sought a database search on behalf of a murder suspect, the FBI's Callaghan held a telephone conference with Illinois crime lab officials.
The topic was "how to fight this," according to lab officials' summary of the conversation, which later became part of the court record.
Callaghan suggested they tell the judge that Illinois could be disconnected from the national database system, the summary shows. Callaghan then told the lab officials "it would in fact be unlikely that IL would be disconnected," according to the summary.
