What it is
The same gene that shortens the tail can affect the spine too: vertebrae are missing, fused or misshapen, and the end of the spinal cord and the nerves leaving it develop incompletely. These nerves control the back legs, the bladder and the bowel.
Signs appear from birth or in the first months of life, as a kitten starts to walk and learns to use the litter box. In some kittens the changes are barely visible; in others they get in the way every day.
A cat like this may need more help from you: keeping an eye on whether it passes stools regularly, keeping its hindquarters clean, choosing a litter box with low sides. Any changes you notice are worth discussing with a vet.
Signs you may notice
A kitten hops with both back legs together like a rabbit, its hindquarters look weak, it cannot control its urine or stools, or it struggles to pass stools.
When to see a vet
Book a visit if a kitten walks oddly, cannot control its urine or stools, or struggles to pass stools. When getting a kitten, ask the breeder about the parents' health testing.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.