The term ‘purebred’ refers to a dog which is born of two dogs which are registered as being that particular breed. A dog can not become kennel club registered as, for example, a border collie if its parents are not both a border collie. Therefore, that dog will not be able to sire a litter of purebred puppies. Purebred dogs have been bred for many centuries for specific traits and characteristics. When you breed a purebred dog with another breed, these characteristics are ‘diluted’ meaning that there is an introduction of other traits. Therefore, the puppies would be less like the breed standard as the parent.