Chondrodystrophy CDDY (FGF4-18) which causes short legs and the risk of developing Intervertebral Disc Disease (IVDD).
Chondrodysplasia CDPA (FGF4-12), which causes the short legged phenotype in a number of breeds.
Chondrodystrophy (CDDY with IVDD Risk) is a trait that is common to many dog breeds and it is characterised by shorter legs due to shorter long bones. CDDY can also be associated with Intervertebral Disc Disease (IVDD) due to premature degeneration of the intervertbral disc. The intervertebral disc lie between the vertebrae and it is made of a cartilage which separate vertebrae from each other, absorb shocks and allow slight movement of the vertebrae. In affected dogs, premature calcification of part of the disc at early age (from birth to 1 year of age) results in degeneration of all discs in young dogs. These abnormal discs are susceptible to herniation into the spinal canal where the inflammation, and hemorrhage can cause severe pain and neurological dysfunction. CDDY is inherited as a semi-dominant trait which means that dogs with 2 copies of the mutation are smaller than dogs with only 1 copy. As for IVDD, the inheritance follows a dominant mode, meaning that 1 copy of CDDY mutation is sufficient to predispose dogs to IVDD.
The second mutation CDPA explains the short-legged phenotype known as chondrodysplasia (CDPA)