A munchkin Sphynx cat with heterochromia iridum.
A munchkin Sphynx cat with heterochromia iridum. (Photo: Linn Currie/Shutterstock)

From Grumpy Cat to Lil Bub, it seems like every day there's a new Internet "It Cat" that steals our hearts with its unbearably adorable looks or personality. While many of these charismatic kitties rise to fame from humble beginnings as rescue animals, countless others are purposefully bred for their unique, squee-worthy characteristics.

At its root, there is nothing new about this practice. Selective breeding has been going on for thousands of years — it's how all domesticated animals were developed. When a mutated trait is appealing or useful to humans, these individuals are intentionally bred to produce more offspring that exhibit the trait.

That said, while many mutations can be quite benign, serious ethical concerns arise when animals are specifically bred for aesthetic traits that are painful or debilitating.

"In cat breeds, physical mutations that were previously allowed to perish are now being developed merely for the sake of difference," Roger Tabor, biologist and author of "The Rise of the Cats," explains. "Not all are harmful, but some are achieved at considerable cost to the cat."

One example of a controversial cat "breed" are twisty cats. Also known as squittens or kangaroo cats, these felines are born with unusually short forelegs that are the result of conditions such as radial hypoplasia, radial aplasia, radial agenesis or foreleg micromelia. Due to their short forelegs, they often sit in an upright posture reminiscent of a kangaroo or squirrel.

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In her essay on the ethics of twisty cat breeding, British cat care expert Sarah Hartwell explains that "the deformity causes locomotory problems for the cats which must either hop on their back legs like kangaroos or use their almost useless front flippers to hobble along. The foot and/or claws may also be deformed, causing further discomfort."

There is nothing inherently wrong with kitties who are born with physical challenges like this. After all, disabled cats can make just as wonderful of companions as able-bodied cats. However, it's an entirely different matter when cats are purposefully bred for a deformity in the interest of making money off a market for "cute" disabled pets.

Not all mutations are as extreme or debilitating as the ones twisty cats exhibit, though some fancy cat breeds do experience higher incidences of certain medical conditions. Continue reading below to learn about some of the most unusual (yet wildly popular) genetic mutations in cats.

Scottish fold cats

The distinctive ears of a Scottish Fold.
The distinctive ears of a Scottish Fold. (Photo: Andrey Tairov/Shutterstock)

These cute owl-like felines are known for their folded lop ears, which are the result of a mutation that affects cartilage and bone development. The official term for their condition is osteochondrodysplasia.

Great care must be taken in the breeding of Scottish folds. In addition to their increased risk of polycystic kidney disease and cardiomyopathy, some Scottish folds are susceptible to painful degenerative joint diseases. For homozygous folds (individuals that possess not one but two copies of the fold gene), these severe joint issues generally occur very early in life. Because of this, it is considered unethical to breed homozygous folds.

Curly tailed cats

While the kitty in the video above exhibits an especially dramatic curled tail, curly cat tails can vary greatly in shape and size — simple loops, tight "piggy" corkscrews, kinked knots or even tails that lay nearly flat across the cat's back.

These curly tails are typically random one-off mutations, but cats that specifically carry their tail in a curved arch over their back are often classified as American ringtails. This unofficial breed originates from a male cat named Solomon, who was found in 1998 as a stray kitten underneath a school in Fremont, California.

Munchkin cats

The short, stout physique of a munchkin cat.
The short, stout physique of a munchkin cat. (Photo: otsphoto/Shutterstock)

The genetic mutation responsible for the munchkin cat's appearance is often referred to as pseudoachondroplasia, which is characterized by short limbs and a head that remains proportional. Unlike many short-legged dogs, munchkin cats are remarkably healthy, though they sometime exhibit lordosis (excessive spine curvature) and pectus excavatum (hollowed chest) in their later years.

What's especially interesting about this breed is the mutation was once a viable, naturally occurring trait that appeared long before it became the mark of a trendy "designer" pet. Hartwell explains short-legged feral cats were observed in England throughout the 1930s:

"In 1944, four generations of feral short-legged cats were documented in the Veterinary Record by Dr. H.E. Williams-Jones. He reported the case of an 8 1/2-year-old black female (described as having lived an extremely healthy life) who was normal in every way apart from her short legs. Her mother, grandmother and some of her own offspring were similar in appearance. [...] These cats were one of many established bloodlines that disappeared during World War II and the few surviving individuals had been neutered, losing the mutation altogether."

Sphynx cats

A Sphynx cat keeps its soft, wrinkly skin warm in a blanket.
A Sphynx cat keeps its soft, wrinkly skin warm in a blanket. (Photo: Andrea Izzotti/Shutterstock)

With their chamois-like hairlessness, Sphynx cats are definitely one of the world's most visually arresting cat breeds. However, this genetic mutation can sometimes work against them.

Due to their lack of protective fur, Sphynx cats are extra sensitive to sunlight and are more likely to develop skin cancer than other cats. And while you might assume that a cat with no hair would require little to no grooming, the opposite is true. Sphynx cats require regular baths as they are prone to oil build-up on their skin. Luckily, they make up for their high maintenance with heat-seeking snuggles and affection!

American curl cats

An American curl cat.
An American curl cat. (Photo: Zanna Holstova/Shutterstock)

Now that you're familiar with Scottish folds, it's time to introduce you to the American curl, which was first discovered in a family of strays in 1981. Instead of having ears that are lopped forward, this cat breed boasts a genetic mutation that produces horn-like ears curled away from its face.

Because the curled gene is so dominant, it is easily inherited by offspring of a curled and non-curled mating pair. This has allowed for the development of a large, diverse genetic pool that produces generally healthy curled individuals. Despite this, the cats' relatively "open" ears do require regular cleaning to prevent infections.

Rex cats

Cornish rex cat with curly fur.
Cornish rex cat with curly fur. (Photo: Imageman/Shutterstock)

Poodles and sheep aren't the only domestic animals with curly fur! Rex cats, which are named for the rex mutation, are born with oval-shaped hair follicles that produce curly fur. There are numerous rex cat breeds, with the most common being the Cornish Rex, Devon Rex, Selkirk Rex and LaPerm. And unlike other fancy breeds, there are no major health issues that correspond with curly cat fur.

Bobbed and tailless cats

A fluffy bobtail cat.
A fluffy bobtail cat. (Photo: Nataliya Kuznetsova/Shutterstock)

There are several breeds of bobbed and tailless cats from around the world, with the most well-known being the Manx cat — a breed that originated on the Isle of Man. Other breeds with shortened tails include the Japanese bobtail, the long-haired Cymric and the American bobtail.

What these kitties lack for in tails is generally made up in their large, bunny-like hind legs. Sadly, sometimes the tailless gene goes a little too far. In some cases, a cat will be born with "Manx syndrome," which is a condition that causes the spine to shorten too much, resulting in spina bifida.

Polydactyl cats

Cat paws exhibiting polydactylism.
Cat paws exhibiting polydactylism. (Photo: Philip Kopylov/Shutterstock)

The average cat boasts a total of 18 toes (5 toes per front paw and 4 toes per hind paw), but polydactyl cats have been known to have as many as eight toes per paw. One of the most well-known colonies of polydactyl cats is located at Ernest Hemmingway's former home in Key West, Florida.

While feline polydactylism itself is fairly harmless, a similar polydactyl trait can be found in instances of feline radial hypoplasia, which results in the severely disabled "twisty cats."

Lykoi cats

A Lykoi cat.
A Lykoi cat. (Photo: Seregraff/Shutterstock)

Named for the Greek word for "wolf," Lykoi cats get their thin-haired, wiry appearance from a natural genetic mutation that causes them to develop without a thick, furry undercoat. The little hair they do have is shed each year, leaving them looking like Sphynx cats for several months. This periodic "molting" is what lends them the nickname, "werewolf cat."

Odd-eyed cats

White cat with heterochromia iridum.
White cat with heterochromia iridum. (Photo: Helen Bloom)

Odd-eyed cats are felines with heterochromia iridum, meaning an individual has one blue eye and one eye that is either green, yellow or brown. The intriguing feature occurs most often in white cats, though it can manifest in cats of any color as long as they possess the white spotting gene. This gene, which is responsible for tuxedo and bicolor coats, can prevent melanin granules from taking root in one of the eyes. Heterochromia iridum is a fairly benign mutation, though deafness does occur in a significant number of odd-eyed cats with all-white coats.

Flat-faced cats

The flat-nosed profile of a Persian cat.
The flat-nosed profile of a Persian cat. (Photo: Linn Currie/Shutterstock)

Persian cats have long been a symbol of luxury in the pet world, but after centuries of breeding for shows, these felines have developed more than a distinctly flat face. Just like many flat-faced dog breeds (pugs, bulldogs, etc), these gorgeous kitties have their fair share of health problems. Due to their exaggerated brachycephalic skulls, Persians grapple with respiratory issues, birthing difficulties and eye infections. The average life expectancy of a flat-faced Persian is just between 10 and 12.5 years — markedly lower than the average cat.