On Tuesday, May 14, we said goodbye to one of the Internet’s most beloved stars. Grumpy Cat, everyone’s favorite peeved puss, passed away aged 7 due to complications from a urinary tract infection. Her relatable look of disdain brought her millions of fans, but what was behind her unique visage?

Grumpy Cat, whose real name was Tardar Sauce, first leapt to fame as a kitten in 2012 and since garnered millions of followers on Facebook, Instagram, and Twitter. Her facial features were due to an underbite and a condition called feline dwarfism.

Feline dwarfism results from a genetic mutation that leads to abnormal development of the bones and cartilage. Technically called achondroplasia, the condition is caused by a mutation in the fibroblast growth receptor gene, which provides the instructions for making a kind of fibroblast growth factor, a protein involved in normal development. The most noticeable result of the mutation is unusually short legs, but other symptoms include an abnormally large head, an underbite, and bowed legs.

The severity of feline dwarfism can vary, and many cats learn to live with their condition. However, they are at a higher risk of certain ailments such as arthritis and obesity.  

While feline dwarfism is the result of a genetic mishap that causes suffering, it is actively encouraged in certain breeds like the munchkin cat. Also called the “sausage cat”, this relatively new breed is highly controversial as it encourages the breeding of animals with physical deformities that impact their health. Sausage cats' canine counterparts are no better; the dachshund’s deformed shape can lead to nasty muscular and spinal issues.

Another cat breed with a genetic mutation is the Scottish fold. Descended from a cat called Susie found on a Scottish farm in 1961, these cats have small, forward-folding ears thanks to a genetic abnormality called osteochondrodysplasia that affects their cartilage. Thanks to its "cute" factor, the breed has gained huge popularity with both celebrities – Tay Tay and Ed Sheeran, we’re looking at you – and the online community. Osteochondrodysplasia affects cartilage throughout the cats' bodies, putting them at risk of a short, painful life.

"The cartilage is deformed and it is not supporting the ears,” Gudrun Ravetz, the president of the British Veterinary Association, told BBC News in 2017.

"These genetic mutations, which all the Scottish fold cats will have, develop into lifelong incurable and painful diseases such as a type of arthritis."

"We should not be breeding pets that can have these problems."

On the other end of the ear spectrum are American curl cats. These have upright ears that bend backwards at the top. This is again caused by a cartilage mutation, but it's currently unclear whether this breed is at risk of the same health issues as Scottish folds. However, their exposed ears are fragile and put them at greater risk of infection.

One of the most bizarre-looking cat breeds humans have selectively bred is the aptly named sphynx cat, bred from cats with a rare, natural mutation that makes them almost entirely hairless. Cats have fur for a reason and this breed can suffer from sun damage and skin cancer. They are also at risk of oils building up on their skin and need to be bathed regularly.

Although humans have been selectively breeding animals for centuries, the rise of social media has led to a growing obsession with critters that look cute at the expense of their health and quality of life. Pugs, bulldogs, and Persian cats suffer serious respiratory problems due to their squashed, brachycephalic skulls, for example.

If you’re planning on purchasing a furry friend, be sure to research the breed it belongs to. A healthy pet will live a long and happy life.