Study coauthor Heidi Parker says this gene may turn on growth mechanisms at the wrong time during foetal development, stunting the growth of long bones in the leg and making them curvy. The trait affects only the legs, unlike the small-all-over effect seen in miniature or toy breeds, such as poodles [ Reuters ]. Having joined the genetic repertoire of dogs, the gene was available for selection by dog breeders whenever they wanted to develop a downsized breed.
The basset hound, for example, was bred for its short legs so people on horseback could keep up with it during hunting, Dr. Parker said [ The New York Times ]. In the study , published in Science , Parker and her colleagues compared the genomes of 95 short-legged dogs from the eight breeds with the genomes of dogs from 64 breeds without the trait.
The team found that this mystery region exactly matched a gene called fibroblast growth factor FGF4. That was puzzling, for FGF4 normally sits at a very different location, some distance away on the dog genome. In fact, Parker found that the short-legged breeds have two copies and the one associated with their abnormal growth has been inserted in an unusual site. The DNA molecule can then be inserted back into the genome it was originally transcribed from, often in a new place.
Without these promoters, most inserted genes just sit around doing nothing, slowly building up debilitating mutations. They can go on to produce functional proteins and these active copies are called retrogenes. Parker found clear evidence that the extra FGF4 of dachshunds and corgis is indeed a retrogene.
When genes are transcribed into mRNA, useless chunks of sequence called introns are edited out of the transcript. An mRNA molecule that is then reverse-transcribed would produce a shorter, more streamlined stretch of DNA than its original gene. They suggest that with two copies of the gene, the dogs overproduce the FGF4 protein, which docks at, and turns on, another protein called FGFR3.
In humans, overactive FGFR3 is responsible for virtually every case of achondroplasia , the most common type of dwarfism. Nonetheless, the sequence of the FGF4 retrogene strongly implies that it arose only once, very early on in the evolution of domestic dogs and before they split into the various modern breeds.
Read on to find out what height miniature and standard Dachshunds are, why they were bred to have such short legs, and the genetic reason behind them.
On average, Miniature Dachshunds stand 13 to 18 centimetres in height to their withers. Standard Dachshunds stand between 20 to 22 centimetres in height to their withers.
They are very short in stature and have their tiny legs to thank for this! In fact, their characteristically short legs and long bodies are the defining features of the Dachshund breed. Dachshunds were originally bred as hunting dogs over three hundred years ago. Their hunting involved following the scent of burrowing animals and diving into holes to hunt them.
A Dachshunds short legs enabled them to enter into the burrows of badgers and other small animals. Their small size allowed Dachshunds to fit into tiny holes that other dog breeds with longer legs would just be too tall to dive into! So, Dachshunds were the perfect dog for this type of hunting. Their short legs meant they could crawl into the burrows without any issues.
And their brave attitude meant they would successfully capture their prey! Being so close to the ground helped Dachshunds with their ability to track the scents of their prey. It allowed them to scrabble through the undergrowth too. This made them the perfect height for hunting burrowing creatures. She added researchers should start looking in humans to see if the FGF4 gene plays a role in a form of dwarfism called hypochondroplasia, which represents about one-third of the unexplained cases of dwarfism in humans.
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