At first it seems an easy answer. A Mongrel (or mutt) is simply a dog that doesn't fit in any breed. But now comes another question: what is a breed? Biologists describe a breed as the subgroup of a species that shares similar characteristics and signs and when mated together reproduce their kind.
Such distinctive and unique traits distinguish the breed from other subgroups of the same species. For instance, the Doberman and the Poodle belong to a species, but their characteristics are so marked that we don't have any doubt as to what breed belongs each of them. However, the offspring of a crossing of the Doberman with the Poodle would be completely different from any of their parents and we no longer could speak of a breed. Why?
Dog breeds, as we know them today, exist since the XIX Century. Previously dogs of different appearance and size existed, but they were valued for the practical use rather than for aesthetic reasons. Aesthetic aspects began to be valued later, after the first world canine exhibition that took place in England in 1859. Only then the first dog breed clubs were founded.
The dogs’ breed clubs are the entities responsible for the determination of the typology of each dog breed (specific size, physical build, general appearance, head shape, eye colour, ears and tail style, etc. Prior to becoming a recognized breed served by a registry, a new breed has to travel a long way. Breeders have to meet some standards for several dog generations, like the parentage. Regrettably, some of the breeds developed by humans serve more to satisfy aesthetic values. Some not so responsible breeders don’t have any concern for the physical build of the animals, this resulting in some breeds possessing malformations or exaggerated physical characteristics which cause the animals to suffer.
 
In breed out crossings the extreme characteristics become weaker, which means that a mongrel can be a more balanced dog. Besides, and bearing in mind that any breed is a mixture of other breeds, couldn’t we say that the mongrel is indeed a “pedigreed” dog?
Are mongrels better dogs?
We cannot be 100% sure. However, the probability of a mongrel being stronger, more flexible in behavior and having a stronger adaptation capacity is great. This can be explained by the fact that when there is an out crossing there is a sort of blood revitalization that inhibits the negative effects of consanguineous crossing. This does not mean that mongrels are automatically healthy and good-mannered, but that there is a significant probability that they are so: their gene pool is much more diversified than that of the purebreds.
We think there is no need to say much more. After all mongrel owners know that “class” has nothing to do with breed!