Our Canine Breed Inheritance Infographic explains how purebred and mixed breed puppies inherit DNA from their parents.
Each dog has 78 chromosomes. Humans have 46.
Puppies will randomly inherit 39 chromosomes from the Mother and 39 chromosomes from the Father.
If a purebred dog is bred with another purebred dog of a different breed then their offspring can only be 50% of the breeds from each parent.
A purebred Labrador Retriever who breeds with a purebred Border Collie can only have puppies that are half Labrador and half Border Collie.
If these 50/50 dogs continue to breed with other mixed breed dogs the inheritance is spread out. After only a few generations each puppy from the same litter can have a different breed composition depending on the breeds available for inheritance.