If both parents contribute the folded gene or parent are not healthy, then the kitten is likely to suffer from genetic mutatio- congenital osteodystrophy, a genetic condition that causes deformities of the vertebral, leg and tail bones due to cartilage thickening (a little like osteoarthritis, a degenerative joint disease in humans). So if you buy a kitten, check his body. The tail shouldn’t be small , short and thick .This disease in not curable.Always confirm the bloodline before purchasing a kitten.