Breeders of dairy cows are particularly excited about the gene chips. Currently, they can't tell if a bull produces high-quality progeny--meaning cows that make lots of milk--until the bull's female calves grow up. If scientists can find a genetic pattern that quickly and cheaply identifies desirable bulls, the breeding process would be much more efficient. "We're looking at changing the costs from tens of thousands to under one thousand dollars," says Curt Van Tassell, a geneticist at the U.S. Department of Agriculture in Beltsville, MD, and a collaborator on the chip project. Van Tassell and the team plan to start such an analysis as soon as they finish designing the chip. "We hope to have genetic prediction machinery within a year of having collected data, with a higher-resolution model in two years," he says.

The chip could also shed light on how breeding has shaped the bovine genome. Missouri-Columbia's Taylor plans to characterize the genetic variation in different breeds of cattle, creating a bovine map that's much like the human HapMap released last year, which mapped the genetic diversity of people from all over the world (see "A New Map for Health"). The researchers also plan to look at related species, such as bison, water buffalo, and the now-extinct auroch, an ancestor of modern cattle.

Early results of the bovine genetic testing suggest that breeding for particular qualities, such as high milk production, hasn't selected for two or three specific variants associated with that trait. Rather, years of breeding have produced selection pressure across the genome in a complex pattern. "Small differences in many genes leads to big differences in underlying phenotype," says Taylor. "It's much more subtle than you might think."