KCBS_740 SAN FRANCISCO (KCBS) — The Doomsday clock is ticking for a popular and beloved breed of dog because humans have tinkered too much with its look.

Those loveable English bulldogs all droopy skin, downturned mouth, and squatty legs – just right for apartment living.

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“It is the fourth most popular breed in the United States,” says Niels Pederson, a professor emeritus at the UC Davis School of Veterinary Medicine.

Niels says bulldogs probably have more health problems than any other breed.

“People seem to be more enamored with the appearance of the dog than with its health,” Pedersen said.

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The dogs at one time looked more like wolves, but have been bred to create their current appearance, leading to breathing and reproductive problems, and greatly shortened life spans of only about six years.

Pedersen says the breed cannot be changed from within.

“It would have to be out-crossed with other breeds to change the physical characteristics and bring back the better health,” Pedersen said.

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While the bulldog is the worst case of breeding gone bad, the life span of many pure breeds of dogs has decreased over the last decade.