Chondrodystrophy Type 1 IVDD (CDDY) indicates a short-legged phenotype, as well as abnormal and premature degeneration of interverebral discs leading to increased risk of intervertebral disc herniation. This means that the dog will have short legs and a stiffened spine - the spine of which may go on to degenerate, even in young dogs, causing inflammation, hemorrage, severe pain, and neurological problems. Inheritance is autosomal dominant for intervertebral disc disease, but semi-dominant for height.

There are a number of breeds currently undergoing research related to this condition, and a comprehensive study in 2019 investigated breed distribution of the mutation. Understanding the allele frequencies is key to recommendations in testing and how to use test results. "For those breeds in which CDDY has been documented to segregate, in other words, both the CDDY allele and the normal allele are present in the breed, genetic testing is recommended. The results of this test can assist in breeding decisions and should be shared with the animal's veterinarian to assist in clinical decisions. The CDDY allele has been found to be present in mixed breed dogs and therefore testing is recommended for these animals as well. For breeds where the CDDY allele is fixed in the population, in other words only the CDDY is present or allele frequency is 1.0, genetic testing for CDDY is not necessary, in these cases it is reasonable to assume that all dogs in that breed will be homozygous for CDDY (CDDY/CDDY)." [UC Davis/VGL, 2020] A current list of mutation frequency can be found from VHL: https://vgl.ucdavis.edu/test/cddy-cdpa. Other GTPs may also be able to provide frequency of mutation information.

"Chondrodystrophy in dogs is defined by dysplastic, shortened long bones and premature degeneration and calcification of intervertebral discs... Long bone length in dogs is a unique example of multiple disease-causing retrocopies of the same parental gene in a mammalian species... Extensive examination of growth plates has been performed on many of these short-legged dog breeds (dachshund, Pekingese, French bulldog, spaniels, beagle), as these breeds are also prone to intervertebral disc disease (IVDD). Histopathological analysis of the bones of puppies from these breeds demonstrated that their short stature is due to defects in endochondral ossification, the process whereby cartilage is replaced with bone, in the developing limb. The long bone growth plates show disorganization of the proliferative zone and reduction in the depth of the maturation zone. In addition to the long bones, similar but more subtle changes exist in endochondral ossification of the vertebral bodies...In chondrodystrophic dogs, the nucleus pulposus is gradually replaced by chondrocyte-like cells in chondroid metaplasia (or metamorphosis) that occurs between birth and 1 y of age. Recent studies have shown that in advanced stages of degeneration in nonchondrodystrophoid dogs there is also replacement of notochordal cells by chondrocyte-like cells, similar to the changes observed in chondrodystrophoid dogs, although this happens at an older age. Hansen described the two different types of canine IVD prolapse as type I and type II. Type I occurs exclusively in chondrodystrophic breeds and is characterized by premature degeneration of all discs in young dogs. In type I disc disease, the calcified nucleus pulposus may undergo an explosive herniation through the annulus fibrous into the vertebral canal, resulting in inflammation and hemorrhage and causing severe pain and neurological dysfunction..." [Brown et. al, 2017]

The application of this test will vary by breed. There is research (Batcher et. al) that indicates some populations of breeds, such as dachshund varieties, are "fixed" for this mutation. This means, in practice, that there are very few dogs in the population who do not carry the mutation, and therefore it may not be possible to breed away from - e.g. all dogs who are tested are "affected." It is strongly recommended that you review the breed-specfic references provided here and/or contact your local breed club for support in breeding decisions. Many breed clubs strongly recommend you undertake a clinical examination before breeding. This is extremely valuable in identifying lower-risk dogs. If you are testing for information on your own dog, and not for breeding plans, it is important to remember that a dog who tests affected for this mutation is at risk for developing the condition, but the test is not an indication of severity, age of onset, or that the disease will manifest in the dog's lifetime. There is research indicating that there could be other factors (such as environmental factors) that impact disease development. Your breed club, breed advisor, veterinarian, or test provider should be able to provide more information.lved in test development. Please refer to OMIA 157 and 189 for OMIA information. As always, this information may change or evolve as the genetics are better understood.