Although tail suppression (or tail length variety) is not the sole characteristic feature of the breed, the chief defining one of the Manx cat is its absence of a tail to having a tail of long length, or tail of any length between the two extremes. This is a naturally occurring, cat body-type mutation of the spine, caused by a dominant gene. As with the sometimes-tail-suppressed Schipperke dog and Old English Sheepdog, tail suppression does not "breed true" in Manx cats. Attempting to force the tailless trait to breed true by continually breeding tailless Manx cats to tailless Manx cats has led to increased negative, even fatal genetic disorders "(see below)". Tail length is random throughout a litter of kittens. Manx to non-Manx breeding will usually produce some Manx-type tail varieties in kittens. Whether the shorter tailed kittens of these varieties are labeled Manx is up to the breed standard consulted. Manx cats' tails are classified according to proportional tail length as kittens (the proportion does not change after birth):