The Basset Hound is a medium to large sized dog that stands on short, curved legs. Its legs are disproportionately short relative to its body size. It is one of more than a dozen modern breeds including the dachshund and corgi that are deliberately bred to have a genetic condition known as chondrodysplasia. This condition, a type of dwarfism, causes stunted leg growth in dogs.
In a study conducted by the National Human Genome Research Institute (NHGRI), researchers discovered that dog breeds with short, curved legs have an extra copy of a specific gene. This gene is responsible for producing a growth protein referred to as FGF4 (fibroblast growth factor 4). The study suggests that having this extra gene causes an overproduction of FGF4 resulting in short legs.
One might logically conclude that an extra dose of growth protein should cause dogs to have extra long legs. But, scientists believe that too much actually causes proteins responsible for controlling growth to become over activated during fetal development. This in turn disturbs bone growth and the end result is short, curved legs.
Interestingly, researchers have determined that the disproportionately short legs of at least 19 dog breeds can be traced back to one mutational event in the canine genome. In layman's terms, here's what happened many years ago...
A gene involved in growth was duplicated in a slightly altered form. This gene, actually called a retrogene, was inserted back into the dog genome in a different place than the original. It happened to be inserted at an optimal place enabling it to be functional, meaning it had the capability to produce growth proteins. Consequently, dogs that ended up with this extra gene had short legs with a curved appearance.
It's unclear exactly when this mutation occurred. But, it probably happened after dogs first became domesticated and before the development of modern dog breeds, anywhere between 300 and 15,000 years ago.
This single evolutionary event enabled dogs with a form of dwarfism to be produced and at some point they were. Of course early on, man found these low set dogs to be quite useful and through selective breeding produced many distinct, short-legged breeds, including the Basset Hound, to suit his various needs.
The results of this study are significant because the findings are the first example of a retrogene that has spurred significant and long-lasting variation within a single species.
Now that we have that all cleared up, I'd like to know what caused Basset Hounds to have extra long ears!
During the latter half of the 1700's some experts in France surmised that breeds with crooked legs had an inherited form of a disease similar to rickets, which at the time was a common bone disease affecting children. Little did they realize that this disease is usually caused by a vitamin D deficiency and had nothing to do with the evolution of dog legs.