they have a gene that dictates this. Read this http://www.isle-of-man.com/manxnotebook/histor y/agricul/manxcat.htm about the Manx cat. The factual bit is near the end but the preamble makes good reading..a mid 19th century treatise indicating it was a cross between a cat and rabbit!!
It is an other one of those horrible mutations that supposedly cat lovers have seen as an desirable trait and purposely bred it on. A lot of progeny from the Manx breed have other deformities which people don't generally see.