The study of genetics is a miraculous thing. Scientists in the lab can pinpoint genetic markers for various traits, enhancing the ones they want to see more of and discarding the ones they don't. On commercial farms, these genetic modifications have enabled farmers to breed cows that produce more milk or chickens with more meat.

But geneticists have also made cosmetic changes to animals over the years, honing in on the characteristics that humans find most appealing. And when it comes to our canine companions, these selections haven't always been what's best for the breed. In fact, as these photos show, many of these changes have led to a laundry list of health issues and complications.

In a post on the dog behavior blog, Science and Dogs, the author found images of various dog breeds from the 1915 book, "Dogs of All Nations" by W.E. Mason and compared them to modern-day images of the same breed. The results show in detail how some breeds have been altered to the point that they're now almost unrecognizable from their former selves.

Bull Terrier, then and now

Once a handsome breed, the bull terrier of today has been bred for fighting, with a mutated skull and thickened abdomen. (Photo: Science and Dogs)

Veterinarian Dr. Taylor Truitt has paid close attention to the changes in various breed over the years. "I believe the two breeds I see the largest bastardization in their physical form ... are the old English bulldog and the AKC German shepherd," said Truitt. (She distinguishes the German shepherd dog certified by the American Kennel Club from the various other lines of German shepherd.)

German Shepherd Dog, then and nowThe sloped back of the modern German shepherd dog has led to health issues. (Photo: Science and Dogs)

Put the AKC German shepherd dog next to its ancestor. and the two animals look like two different breeds, Truitt noted, adding that the lean form and sloped back of today's German shepherds are responsible for the breed's orthopedic woes.

As for the English bulldogs, Truitt points to the short, squat frame of the modern animal and how the desired genetic traits "helped create a breed with more congenital defects than I care to think about," she commented.

English Bulldog, then and nowThe English bulldog has been bred for physical traits that have left him vulnerable to a number of health conditions and diseases. This is one of the unhealthiest purebred dogs out there, with an average lifespan of only 6 to 7 years. (Photo: Science and Dogs)

These certainly aren't the only cases where humans have meddled with the genetic makeup of canine companions in ways that have been harmful to the overall health of the breed. Take a look at these examples:

Pug, then and nowAll of the desirable traits that make pugs 'cute,' have also left them predisposed to a long list of health complications. And that 'desirable' curled tail is a genetic mutation that in serious cases can lead to paralysis. (Photo: Science and Dogs)

Basset Hound, then and nowThe basset hound's droopy eyes and ears and excessive skin can lead to infections as well as vision and hearing issues. It's lowered structure has also led to changes in leg alignment that cause problems within the spine. (Photo: Science and Dogs)

Boxer, then and nowThe modern-day boxer has difficulty breathing and one of the highest cancer rates of all dogs. (Photo: Science and Dogs)

Dachshund, then and nowAs these images show, dachshunds have been pulled and stretched to the point that their back and limbs no longer work for a dog their size. They have the highest risk of any breed for a spinal issue called intervertebral disc disease, which can lead to paralysis. (Photo: Science and Dogs)

The pressure to create the "perfect" specimen for a breed has increased over the last several decades, in part thanks to standards derived by the American Kennel Club dictating everything from the correct color for a dog's eyes to the slope of the back to the size of the paws. Truitt says that some breed enthusiasts — such as groups that support Jack Russell terriers — are moving away from these guidelines to protect the health of the animals. But others see any kind of cross-breeding as an attempt to destroy the "purity" of their breed.