Throughout a chilly night in April 1996, more than 50 search-and-rescue team members and 6 specially trained dogs combed the New Mexico foothills of the Sangre de Cristo Mountains and the banks of the Rio Grande searching for a missing 3-year-old girl. The child, accompanied by three neighborhood dogs, had wandered away that evening from her home in Pilar, a community 17 miles south of Taos. The next morning, a search-and-rescue dog named Samson spotted the child under a tree surrounded by the three dogs, who had kept her warm all night. Samson raced back to his handler, alerted him to the find, and led him to the child.

Dogs have been helping people for thousands of years. They were first used to guard property, help people hunt, and pull sleds or carts. In more recent times, dogs have been trained to sniff out drugs, explosives, illegal food substances, and even termites; to help people who are blind, deaf, or disabled, or who have epileptic seizures; and to enrich the lives of people living in institutions or troubled by emotional or psychological problems.

What qualities make dogs work so well, and so willingly, with people at such a wide variety of tasks? And how do certain kinds of dogs pass on to their offspring specific traits—such as Newfoundlands with their love of water or border collies with their instinct to herd anything in sight? Scientists have been working to find the answers to these questions for many years. They have learned much about the ancestors of dogs and how to choose and train the best dogs for specific tasks. Then, in the 1990's, researchers began trying to determine how dogs' behavior may be coded in their genes.