Two dogs selected for defect by the American Kennel Club.
The Chihuahua is bred too small, and will probably have a hole in the top of its skull where the plates do not fuse. It's called a Molera and the skull does not close because the brain is bigger than the box which is supposed to contain it. Serious -- and painful -- neurological issues frequently follow.
The chihuahua's teeth will be another big problem -- the jaw will be too small to hold them, and so they fall out and some are likely to grow sideways and need pulling.
The Great Dane is bred too big and will likely be dead at age 6 due to heart and hip issues, as well as a wide variety of inherited problems, or gastric torsion.
This is a breed of dog that will, on average, show its first inherited defect before it even stops growing.