Some cats are born without tails because of genetic mutations that disrupt how the spine develops in the womb. The most well-known cause is a mutation in a gene called Brachyury (also known as the T-box gene), which controls the formation of tail vertebrae during embryonic development. When this gene is altered, the tail either fails to form entirely or develops only partially. A completely different gene mutation produces the short, kinked tails seen in Asian cat breeds like the Japanese Bobtail. So “tailless” cats aren’t all tailless for the same reason.
The Manx Mutation: A Dominant Gene With Serious Consequences
The Manx cat, originating from the Isle of Man, is the breed most associated with taillessness. Researchers have identified four distinct mutations in the Brachyury gene that cause the trait: three are single-letter deletions in the gene’s code, and one is a more complex duplication-deletion. Each of these errors causes the genetic instructions to be cut short, producing a truncated protein that can’t do its full job of building tail vertebrae.
The mutation follows an autosomal dominant inheritance pattern, meaning a kitten only needs one copy of the mutated gene (from one parent) to be born with a shortened or absent tail. This is unusual. Most genetic traits require both parents to pass along a copy before the trait shows up. With the Manx gene, just one copy is enough.
There’s a grim flip side to this dominance. Kittens that inherit two copies of the mutated gene (one from each parent) are so severely affected that they die early in fetal development. The Manx gene is classified as a lethal gene for this reason. This is why litters from two short-tailed Manx parents tend to be smaller than average: some embryos simply don’t survive.
Why Tail Length Varies Even Within a Litter
One of the more surprising things about tailless cat genetics is how much variation a single litter can produce. Even when both parents carry the mutation, their kittens can range from completely tailless to nearly full-tailed. Breeders and enthusiasts use four informal categories to describe the spectrum:
- Rumpy: completely absent tail, no visible vertebrae at the base of the spine
- Rumpy riser: a tiny nub of cartilage or one to three vertebrae, barely visible
- Stumpy: a short but noticeable tail stump, sometimes slightly irregular
- Longy: roughly half a normal tail length
Researchers documented a case where a single rumpy-riser male produced kittens representing all four tail types in one litter. This suggests that the Brachyury mutation doesn’t act alone. Other genes and possibly environmental factors during pregnancy interact with it to determine exactly how much tail each kitten develops. Two kittens with the same parents and the same core mutation can look very different.
Japanese Bobtails Have a Completely Different Gene
Not all short-tailed cats carry the Manx mutation. The Japanese Bobtail and other Asian bobtail breeds owe their stubby, kinked tails to a mutation in a different gene called HES7, located on a different chromosome entirely. Researchers confirmed this by sequencing Asian short-tailed cats and finding none of the four Brachyury mutations associated with the Manx.
The HES7 mutation is a single-letter change that swaps one amino acid for another at a spot in the protein that has remained the same across many species over millions of years of evolution. Unlike the Manx mutation, the Japanese Bobtail version appears to be fixed in the breed, meaning all Japanese Bobtails are homozygous (carrying two copies). This is a critical difference: two copies of the Manx gene are lethal, but two copies of the HES7 mutation are perfectly compatible with life. Japanese Bobtails don’t face the same embryonic lethality or the spinal complications common in Manx cats.
Manx Syndrome and Spinal Problems
Because the Brachyury gene affects more than just the tail, some Manx kittens are born with serious developmental problems collectively called Manx syndrome. The tail is really just the end of the spine, so disruptions to tail development can also affect the spinal cord, the surrounding vertebrae, and the nerves that branch out from the lower spine. These are the same nerves that control the bladder, bowel, and hind legs.
Affected cats may be born with spina bifida, a condition where the spine doesn’t close properly during development. Others develop sacrocaudal dysgenesis, meaning the vertebrae at the base of the spine are malformed or fused. The practical consequences can be severe: urinary incontinence, inability to control bowel movements, and weakness or reduced reflexes in the back legs. These problems stem from nerve damage or underdevelopment in the lower spinal cord, and they’re present from birth even though they may not become obvious until a kitten starts walking.
Not every tailless Manx develops these problems. Cats with partial tails (stumpies and longies) generally face lower risk than completely tailless rumpies. But the correlation between shorter tails and more spinal involvement is real, and it’s the reason some veterinary welfare organizations have raised concerns about breeding for the rumpy body type specifically.
How Tailless Cats Compensate Physically
A cat’s tail serves as a counterbalance during jumping, climbing, and quick directional changes. Cats born without one don’t simply stumble through life, though. Research on balance in tailless cats found that they develop anatomical and behavioral adaptations to compensate for the missing “fifth limb.” Manx cats, for instance, are reported to have proportionally longer hind legs than tailed cats, which likely helps with stability and the powerful, rabbit-like hopping gait the breed is known for.
Cats that lose their tails to injury later in life tend to have a harder adjustment period than cats born without one. When the tail never existed, the brain and body develop together without it, building balance strategies from the start rather than having to relearn them.
Tail Loss From Non-Genetic Causes
Not every kitten born with a shortened tail carries a genetic mutation. Trauma during pregnancy, constriction of blood supply, or infections can damage developing tissue and result in a shortened or absent tail at birth. These cases are sporadic and not heritable. If a stray cat produces a single tailless kitten in an otherwise normal litter with no breed history of bobtails, an in-utero developmental accident is a more likely explanation than a spontaneous genetic mutation.