It’s highly possible that Cymric cats became so rare because of the same gene that gives them and the Manx cats their distinctive lack of tail. It appears that the gene is also lethal to them. Breeding two tailless Manx or Cymrics together results in congenital disabilities, also known as the Manx Syndrome. These defects include spinal deformities, gaps in the vertebrae, fused vertebrae, and bowel or bladder dysfunctions. Kittens who inherit two copies of the tailless gene die before birth and are reabsorbed in the womb, which makes born litters usually smaller. The Manx syndrome can appear up to the first 6 months of age and is as well present in cats with only one copy of the gene, so normally breeders will keep the kittens longer before selling them.