Every purebred dog is famous for a certain characteristic, such as a “pug” face, large wrinkles, floppy ears or perky tails. In order to get these aesthetically pleasing looks, dog breeders pick the two dogs that are closest to the desired feature. Then they breed them, select from their puppies and another dog, breed them, and so on until the desired characteristic is “perfect”. Unfortunately, most of these characteristics when bred to the extreme, have severe health consequences for the dogs.
The dogs chosen for purebreds have to be from the same gene pool. One gene pool is limiting enough, allowing the same genetic problems to continually be passed on. But often, though highly discouraged, breeders inbreed their own dogs. Dog clubs don't help matters, usually requiring that the dogs are bred within the same club, thus limiting or closing off gene pool variety.
Bulldogs are bred to have flat noses. Breeders have made their noses so flat for that classic bulldog face that they often have shortened air passages. Their nasal openings can also be pinched or partially closed rather than being wide open. Both of these conditions may cause breathing obstruction making the animal work twice as hard to get air.
Great Danes, bred to be sleek, large, and long, often suffer from joint problems that can be debilitating. Hypertrophic Osteodystrophy, or "swollen joints," is a common disease of large dogs. Additionally, due to the fact that Great Danes grow rapidly, they are likely to develop Panosteitis which causes lumps and inflammation.
Dalmatians are another breed that suffer from genetic problems. Bred for their popular black and white spotted coat, it is estimated that one in every three dogs suffer from skin issues. Susceptible to sunburn and skin cancer, they often have skin allergies that can develop into bacterial infections or worse.
Those are just a very few of the breeds that can, and often do, develop diseases or other problems from selective breeding. If you really want a purebred dog, have the breeder and the animal thoroughly checked out. Medical expenses for these dogs can be frighteningly high, while suffering for the animal is a challenge and often inhumane. It is important to make sure the dog is healthy and the breeder reputable. It may also be worthwhile to reconsider a purebred altogether.