In other cases, detectives can secretly collect DNA from a relative of a suspect by testing an item that the relative threw in the trash.
Maryland's new law states that when police officers test the DNA of "third parties" – anyone other than the suspect – they must first obtain written consent, unless a judge approves a fraudulent collection.
Investigators cannot use any of the genetic information collected, either from the suspect or from third parties, to learn more about a person's psychological characteristics or predispositions to illness. At the end of the examination, all genetic and genealogical records created for this purpose must be deleted from the databases.
Perhaps most momentous, Maryland investigators interested in genetic genealogy must first try their luck with a government-run DNA database called Codis, whose profiles use far fewer genetic markers.
Mr Holes said this part of the law could have tragic consequences. In ancient cases, he pointed out, DNA evidence is often badly degraded and fragile, and every DNA test uses up some of this valuable sample. "Essentially, the law could make me kill my case," he said. Given the speed with which DNA technology is advancing, it is unwise for a law to mandate the use of a certain type of test.
However, other experts have identified this determination as critical, as the potential invasion of privacy is far more serious for genetic genealogy, which gives law enforcement access to hundreds of thousands of genetic markers, than it is for Codis, which only uses about two dozen markers.
These searches are "the equivalent of the government going through all of your medical records and all of your family records just to identify you," said Leah Larkin, a genetic genealogist who runs a consulting firm in the San Francisco Bay Area that is largely focused is to help adoptees and others find their biological relatives. "I don't think people really appreciate how much there is in your genetic data."