From ever-tinier bits of tissue, crime fighters are tweezing out the DNA evidence they need to identify and convict violent offenders. The same technology has saved hundreds, possibly thousands of innocents from jail or even execution.
But some bioethicists warn there might be a dark side to the expanding use of genetics by criminal investigators.
Police are using "racial markers" from crime-scene DNA to steer investigators toward the likely race or ethnicity of unidentified suspects. But medical geneticists say there's not much evidence that those markers can reliably predict a person's ancestry or appearance.
Genetic inferences about a suspect's race have led to a least a dozen "dragnets" in the U.S. Thousands of innocent people have been asked to "volunteer" their DNA for comparison with a suspect's. Some who refused have been served with search warrants.
In Louisiana, police gathered DNA from 1,200 men during a DNA dragnet. They were eliminated as suspects, but their samples went into the state's criminal database anyway.
"Too often, the mere availability of data and technology, rather than ethical considerations of social needs, drives its use in unintended ways," says Mildred K. Cho of the Stanford Center for Biomedical Ethics, writing in a special issue of the journal Nature Genetics with Pamela Sankar of the University of Pennsylvania Center for Bioethics.
They want medical geneticists and experts in legal, ethical and social issues to look harder at how forensic geneticists fight crime.
Particularly in light of past injustices in the volatile arena of race and genetics, Cho says, scientists must "see that the science they do, and the technologies they develop, are used appropriately."
Their paper, in a recent issue of Nature Genetics devoted to race and genetics, is part of a growing scientific debate over the implications of human genome research for our 400-year-old concepts of race.
Research shows that our relatively recent origin as a species - and millenniums of mixing our DNA - have made us 99.9 percent identical at the genetic level. Most of the remaining variation is between individuals, not ancestral groups. The outward traits we use to judge "race" have little deeper biological significance and are easily misread.
Police first used DNA forensically to establish identity. Tissue samples found at crime scenes are compared with DNA from known suspects or victims. A match can establish individual identity to a high degree of probability.
But forensic scientists have gone further. They're using genetic "markers," their term for specific DNA sequences that appear more often among broad "ancestral" groups, to assign a likely ethnicity to unknown tissue samples.
That helps police "create suspects where there were none," the authors argue. They say this racial and ethnic typing is done with too little regard for statistical uncertainties, the broad range of genetic variation within traditional "races" or the overlap between them.
"Attributing racial and ethnic labels to samples, a subject of considerable and still unresolved debate in medical genetics, seems well on its way to acceptance in forensics and the courtroom," Cho and Sankar state in their paper.
Last year, police in Louisiana were looking for a serial killer whom eyewitnesses identified as white. They collected DNA samples from more than 1,200 white men but found no match to crime-scene DNA.
Then, DNAPrint, a small genomic testing company in Florida, offered a test that it had used in more than 65 investigations. It translates DNA markers into a prediction of the "donor's" likely racial mix.
The results, the company says, suggested that the killer had 85 percent African ancestry and "moderately dark" skin. Detectives switched their search to the local African-American community and quickly focused on Derrick Todd Lee. They matched his DNA to the crimes and have since won two convictions.
Such racial inferences from DNA go too far, Cho says. "We don't know whether the DNA can actually tell you anything about somebody's race or physical characteristics with that much certainty. Then to obtain samples is an intrusion on their privacy, and I don't think it meets the criteria for probable cause," she says.
Tony Frudakis, DNAPrint's founder and chief science officer, agrees it's a fallacy to "bin" people into traditional racial groups. "Each person is better described as residing along a continuum of ancestry," he says. "Most people are a mixture of ancestry."
But he says his tests use plenty of "high-quality" genetic markers that his research has found to sort clearly by ancestry. The results are not absolutes, he says, but probable ancestral mixes. That provides clues to skin shade that are of use to police.