Inbreeding or line breeding, breeding within the same breed type, can lead to immune system, fertility, and mental health problems. Extremes of breed conformation lead to inherent health problems which are not so much a defect as an inevitable characteristic of that breed standard, For example:
Doodles, because of the crossing of 2 breeds, have none of these issues, and have the benefit of hybrid vigor, that the first cross between two unrelated purebred lines is healthier and grows better than either parent line