American scientists have discovered why some dog breeds have very short legs. In fact breeds as dachshunds, corgis or basset hounds have an extra copy of a normal gene to thank for their diminutive stature. Dogs come in a wide variety of body shapes and sizes and researchers focused on eight breeds known to have chondrodysplasia — short legs relative to body size with curved and heavier-bones than normal and compared their genomes to the ones from breeds not harboring that trait. They pinpointed an extra stretch of DNA on chromosome 18 in each dog from short-legged breeds, but in none of 204 control dogs examined. This extra stretch of DNA turned out to have a sequence almost identical to another gene important for limb development, called FGF4. Located at the opposite end of chromosome 18 in dogs, the original FGF4 gene was duplicated at some point in the dog lineage, creating a new copy elsewhere called the fgf4 retrogene. In rare cases, messenger RNA — molecules made from DNA that carry information to cellular machinery, which then makes proteins—can get turned back into DNA. If this DNA then gets plopped back into the genome in a new neighborhood, and conditions are right for this genetic new kid on the block to become active, the extra DNA becomes a retrogene. Most pieces of DNA that hop around the genome are not functional, in part because they may land in inhospitable places in the genome. But this retrogene landed in a sweet spot that allowed it to change dog leg length. This retrogene is a dominant allele, meaning that only one copy is needed for chondrodysplasia to appear. As none of the wolves tested had this retrogene, this gene duplication probably happened after dogs first domestication and before division of early dogs into modern breeds so anywhere between 300 and 15,000 years ago.
Summarized from the article of Laura Sanders: Retrogene causes short legs in man’s best friends
Sciencenews. August, 2009. Vol.176 #4 (p.8)
Original scientific article: Parker HG et al. An expressed fgf4 retrogene is associated with breed-defining chondrodysplasia in domestic dogs. Science 2009. 325:995-8.