Skeletal dysplasia 2 (SD2) is an inherited disorder that causes a mild form of disproportionate dwarfism consisting of short legs with normal body length and width.
Phenotype: Skeletal dysplasia 2 (SD2) is an inherited disorder that causes a mild form of disproportionate dwarfism consisting of short legs with normal body length and width. The leg bones of affected dogs are shorter, thicker, and slightly curved; in most cases, the front legs are slightly more affected than the hind legs. The height of affected dogs is variable, and there is an overlap in shoulder height between small non-affected individuals and tall affected individuals. The SD2 mutation shows incomplete penetrance: not all dogs with two copies of the defective gene will show obvious physical characteristics of SD2.
Mode of Inheritance: Autosomal recessive (incomplete penetrance)
Alleles: N = Normal, SD2 = Skeletal dysplasia 2
Breeds appropriate for testing: Labrador Retriever, Labradoodle
Explanation of Results:
Dogs with N/N genotype will not have skeletal dysplasia 2 and cannot transmit this SD2 variant to their offspring.
Dogs with N/SD2 genotype will not be affected by skeletal dysplasia 2, but are carriers. They may transmit this SD2 variant to 50% of their offspring. Matings between two carriers are predicted to produce 25% skeletal dysplasia 2-affected puppies.
Dogs with SD2/SD2 genotype may develop skeletal dysplasia 2, and will transmit this SD2 variant to all of their offspring.
Skeletal dysplasia 2 (SD2) is an inherited musculoskeletal disorder that causes a form of mild disproportionate dwarfism in affected dogs – their body length and width are normal, but their legs are shorter than normal. Radiological findings typically include shortened and sometimes slightly curved long bones with relatively wide epiphyses (rounded ends of long bones). The front legs are affected more frequently than hind legs. The disorder shows a subtle phenotype in which the height of affected dogs may overlap with that of small normal dogs; no ocular, auditory or secondary joint symptoms are associated with SD2. This disorder has been reported to occur more frequently in the “working” line of Labrador Retrievers, and less frequently in the “show” line.
SD2 in Labrador Retrievers is caused by a single nucleotide mutation (c.143G>C) on the collagen alpha-2(XI) chain (COL11A2) gene. The mode of inheritance for this disease is autosomal recessive, which means that males and females are equally affected and that two copies of the mutation are needed to cause the defect. Moreover, this mutation shows incomplete penetrance, and therefore not all affected dogs will show obvious physical characteristics of SD2. Screening of a random set of Labradors Retrievers at the VGL determined that the SD2 disease allele is rare in this breed, with frequency less than 1%.
Testing for SD2 can assist clinicians, owners, and breeders in identifying affected and carrier dogs. Breeders can use results from the test as a tool for selection of mating pairs to avoid producing affected dogs. Of special note is that, because of the incomplete penetrance, not all affected dogs with show obvious signs of SD2, thus making genetic testing for this disorder important to detect dogs carrying the mutation.