This is one of my favorite things. Look at what they achieved with foxes in less than a century, then think of the tens of thousands of years we've domesticated dogs. The whole field of "dog science" is both fascinating and heartwarming.
Foxes in Class I are friendly toward experimenters, wagging their tails and whining. In the sixth generation bred for tameness we had to add an even higher-scoring category. Members of Class IE, the "domesticated elite," are eager to establish human contact, whimpering to attract attention and sniffing and licking experimenters like dogs. They start displaying this kind of behavior before they are one month old.
It took them a long time to get them to mostly reproduce with those traits (the modern ones only reach 80%), but the traits appeared in 6 generations.