Purebreds often carry genetic disease. Heterosis is the phenomenon where crossing two purebreds lines can produce descendants with superior genetic foundation. This increased health and vigor does not create a superior breed, but the advantages obtained from it are what produce hybrid vigor. This goal in this scenario is not to create a new breed, but to create a happy and healthy pet.

Heterosis effect results in a healthier, more vigorous dog with a reduced chance of genetic disease. It is well known in all domestic animal breedings, hybrids 50%-50% mixes of two different breeds will raise the chances of having less genetic diseases because all doubling of detrimental effects will stop in the first generation. The genetic term for this is HETEROSIS EFFECT. This effect often gives non-related individuals stronger descendants than purebreds.

The purpose of these hybrids is not and should never be to develop a new breed. Once one goes beyond first generation purebred to purebred, you loose the heterosis effect, which is the goal for most hybrid breeders. The mother should always be the bigger of the two, to avoid puppies getting too big and complicating the delivery for the mother. It is only in this first generation, the dogs could get the benefit of the greatly demanded HETEROSIS effect, and avoid genetic diseases, which are common among purebreds and inbreeds.

 

I got the above information at dog breed info.