Skeletal dysplasia 2 – Dwarfism (SD2)
Skeletal dysplasia 2 (SD2) is a form of dwarfism known as the mild disproportionate dwarfism. It is characterized by disproportion is body parts; one or more body parts are smaller in comparison to overall size. Short legs, which are on of typical characteristics of dwarfism, are an expected standard in some breeds, but in many other breeds are an undesirable trait, especially among working breeds, such as Labrador retriever. Skeletal dysplasia 2 (SD2) is caused by a mutation in COL11A2 gene and is inherited in an autosomal recessive manner.
Skeletal Dysplasia 2 (SD2) Characteristics
Fédération Cynologique Internationale established international breed standars accepted worldwide. Following are the valid standards for the Labrador retrievers: shoulder height in males should be 56-57 cm and 54-56 cm in females. Dogs with an inherited SD2 develope short legs and front legs are commonly more severe affected than the back legs. While the legs develope shorter than the standards for a particular breed, body’s development occurs normal. Most male Labrador retriever dogs affected by SD2 are under 55 cm at the shoulder and females are often shorter than 50 cm. However, in consideration should be taken the variability in blood lines among Labrador retrievers and also in physical display of the SD2, since it can happen that affected dogs from large Labrador blood lines can appear the same size as unaffected dogs from smaller blood lines.
Skeletal dysplasia 2 can be easily mistaken with retinal dysplasia/oculoskeletal dysplasia (RD/OSD). Oculoskeletal dysplasia (OSD) an inherited disease, whose phenotype displays as short legs and severe ocular defects. It is known OSD is caused by a mutation in the COL9A3 gene encoding the α3-chain of collagen type IX. In contrast to retinal dysplasia/oculoskeletal dysplasia (RD/OSD), skeletal dysplasia 2 (SD2) in Labrador Retrievers has a very subtle phenotype without any obvious ocular or auditory involvement. Also in researches made, the owners of affected dogs did not report signs of degenerative joint disease in their affected dogs, which was in accordance to the radiological findings. This leads to conslusion that SD2 does not cause joint problems, in contract to OSD.
SD2 Origin and spreading
While working with affected dogs, for purposes of researching, it was established that all affected dogs were related to each other and had the same ancestor,a male Labrador Retriever born in 1966. This male was a working dog that became a popular sire and contributed to the spreading of SD2 gene in the population. It is suspected that this exact male is the actual founder of the mutation. If this is true, mutation would have occured less than 10 generatons ago. Knowing all this, it is easy to conclude that SD2 is a relatively rare and young trait.