How exactly do you determine a dog’s breed mix?

To determine breed ancestry, we look at long stretches of the genome in dogs of known breed from our reference panel. When we see identical long stretches of DNA in two different individuals, this means they were inherited from the same, somewhat recent, ancestor. 

If we add up all the stretches of DNA that match perfectly to one and only one breed, we identify how much DNA must have been inherited from that breed. We are then able to do this for any other breed found in the dog, and from all that, we can understand what percentage of the dog's ancestry came from each breed.