Despite their rarity in the fancy, tailless rats are actually a very old variety. Papers regarding tailless rats appeared as early as 1915, but it was in the 1980s that they emerged as a variety in the modern fancy. While tailless rats do occasionally crop up in random litters in the UK, they are not recognised and are indeed banned from the grounds of many UK shows on welfare grounds.
The tailless gene in the fancy appears to be a recessive, but the extent of the taillessness in each rat can vary wildly. In simple terms, the tail in a rat is an extension of the spine, and the tailless gene makes this shorter. So in the least extreme examples, you end up with a rat that simply has a shorter-than-normal tail. In the most extreme examples, you end up with rats who cannot walk properly due to spinal deformities. The ideal example would have this end just after the pelvis, to allow the rat proper locomotion without crippling the rat.
Genetics: Recessive stubby gene, plus modifers to control the tail length – stst
Description: Tailless rats have a shortened body length which is overall more “rounded” similar to that of a rabbit. Their hind legs should be slightly longer then their front legs, so that when in a resting position the back appears slightly sloped to the front. In totally tailless or “rumpy” manx rats, the judge should feel for where the spine ends. The spine should end just short of where the tail would have begun if it existed. The further up the spine ends- the less desirable. Spines that end just where the hips begin should be faulted. The tailless rat may be smaller then the average rat- this shall not be faulted, but in a close match, larger shall be preferred.