Achondroplasic Dwarfism in Domestic Animals
Some people may know about achondroplasia (and hypochondroplasia - the more severe form) because it is the most common form of dwarfism in humans (out of over 100 different varieties of human dwarfism.) It disrupts the process of turning cartilage into bone matter and in doing so typically creates someone with shorter arms and legs an often a larger skull than is usual. Depending on the time period and culture this could be a positive thing (with an invitation for a peasant to join the royal court for example) to a negative thing (with it being classified today as a medical condition.) But achondroplasia is not exclusive to humans by any means. It's a very common genetic mutation among domestic animals and has been used for hundreds, if not thousands, of years to produce an assortment of short domestic creatures. Below I hope to enlighten you with a little history and maybe a touch of scientific knowledge so that you too can understand why we have made this gene into something desirable in our pets and livestock.
Why is Achondroplasia so Common in Animals?
As I stated above there are many things that can cause dwarfism in both people and animals and achondroplasia is just one of them. It's a genetic mutation that has been historically easy to manipulate once it's found in an animal population. The reason for this is because it's a dominant gene which means you only need the original dwarfed animal to create more, as it will likely pass these qualities down to at least some of its offspring, no matter who and what the other parent is. This eliminates the need to breed for several generations to get carriers before getting a baby that expresses the gene in full which is usually how these things go. With that being said you may wonder where the original dwarfed animal comes from. Well, it would come from something called spontaneous mutation. This is when something happens at random in the genes causing things to be slightly different than its parents. For example in cats this could be a curly-haired kitten showing up in a litter bred from two straight-haired parents. If someone takes an interest in the spontaneous mutation (be it achondroplasia or something else) then it can be basis of new breeds being created. Below are some breeds of animals who are perfect examples of this.
Achondroplasia in Cats
Currently there is one breed of cat that carries this gene, they're called Munchkins. These cats look more like a ferret with a cat's head than anything. They stand close to the ground with very short legs, and what appears to be their knees are actually their wrists. This breed has been used to make short versions of almost all the other recognized breeds which means there are a lot of people out there that find this trait attractive. The first Munchkin showed up as a spontaneous mutation in a litter of barn cats. He was a male named Blackberry who grew up to have quite the career as a stud sending his short offspring in all directions. At first people didn't know what to think of these new low-riding kitties. There were rumors they were half raccoon. Let me assure you they are 100% feline.
Munchkin breeders normally use both short legged and long legged cats in their breeding program for a variety of reasons. The first is that a kitten that gets a double dose of the gene (one from each short parent) has a pretty high risk of dying during the pregnancy. Although a 25% fetal death rate would be normal in such a breeding it could also be as high as 50% in some cases. These fetuses die early in the pregnancy and are reabsorbed causing litters to be comparatively small. It's also not recommended to inbreed anything quite to that level. Since the breed started with one individual it's good to expand the gene pool a little! And finally crossing the breed with long legged cats is a good way to prevent other tag-along genetic diseases from being locked into the breed.
Achondroplasia in Dogs
I would love to start this section with a story about a breed of dog that's now extinct, the Turnspit Dog, because it's story is so fascinating. At least since the 1500's these dogs were bred to help the servants in the kitchens of royal houses and castles. The dogs were short, bow-legged, with severe expressions and worse temperaments. They were a working dog of the lowliest sort. Before these dogs peasant children were employed to turn spits next to the scorching heat of the fire in order for the meat on the spit to cook evenly but this fell out of practice when someone invented a hamster wheel sort of contraption that they could hook up to the spit. Now instead of peasant children these poor pooches were put into the wheel and made to run for hours at a stretch in the horrendous heat. They were also used as foot warmers during Sunday church services but they seemed to have been rarely if ever kept as pets due to their salty temperaments. I'm not sure I'd be super friendly either if this was my life but this is how it was. And so in the 1800's when mechanical spits were invented these dogs fell along the wayside and quickly became extinct. There is one stuffed specimen and a few illustrations in existence to mark their passing in our history.
With that all being said it may be helpful to know what achondroplasia does to the look of a dog because not all small dogs have this gene by any means. In most it will create short often bowed legs, deep heavy chests, and sometimes they may show up with very large heads, and bulkier heavier bodies. However the gene doesn't effect the growth of the spine so these animals may look elongated and disproportional. Most often these qualities are avoided in working breeds as it may (or may not) cause a dog to run slower, drown under their own weight, be prone to obesity and broken backs, or have trouble birthing puppies. This has caused some breeders to say we should stop breeding them but people still do for all sorts of reasons.
Dachunds are probably the best example. They didn't need to be fast to do their job they just had to be short and fearless as they were originally employed to chase cornered badgers and other animals out of their dens. Corgis were a great way to have a herding dog that didn't need quite as much food as a normal sized herding dog. They were so low to the ground that instead of scaring cattle into their desired corners they nipped at their heals until the job was done. Other dogs to share this gene are Basset Hounds, Pekinese, Shitzus, and English and French Bulldogs as well as some others.
Dachshund Running in Slow Motion
Achondroplasia in Rabbits
Achondroplasia is a bit of a mystery in rabbits. It has been reported as happening at the very least in the Havana breed but it's not seen very often because these dwarfed offspring either die before or shortly after birth. There is something about hereditary achondroplasia that is lethal to rabbits.
Achondroplasia in Chickens
Believe it or not this gene has showed up naturally in one breed of chickens, the Scots Dumpy. This Scottish breed of chicken has had a lot of names over the years including the ever adorable creepies, crawlers, and stumpies. These short and stout chickens are the same size as most large fowl weighing up to seven pounds in adulthood but with the unusual characteristic of standing only two inches off the ground giving them an elongated stout posture.
This breed has an ancestry that goes back at least as far as the 11th century and likely long before then but today it is considered a breed in danger of extinction as it has not maintained popularity and breeders often complain of the same 25% fetal death that occurs before their eggs hatch that I have mentioned in the Munchkin cats. Still many people find the weird waddling gait to be worth a watch.
And achondroplasia can also be induced in chicken embryos in a laboratory setting when scientists inject 0.6 mg of thallium sulfate into each egg between the 5th and 8th day of development. This may give insight into the condition as a whole, especially in cases that are environmentally, not genetically, caused.
Acondroplasia in Goats
Although there are several breeds of dwarf goats only the Nigerian Dwarf and the African Pygmy show the achondroplasia gene in the United States. These goats are often used for milking or are used as pets but in Africa they are also very commonly eaten as well. It's not clear if these goats were intentionally bred because of their short stature of it they were merely allowed to breed until they reached their current robust population, but either way they have become very common in both Africa and the United States. They may be slightly more prone to obesity than regular goats but it's not clear if this is because of the achondroplasia or just because Americans really love feeding treats to their pets. Who knows!
Achondroplasia in Cows
Although achondroplasia frequently pops up at random, usually in beef cattle breeds, this trait is usually seen as a defect and culled. The only breed of cattle I could find which encouraged it was the Irish Dexter, a type of miniature cow, which clearly displayed the shortened legs, the broadened skull, and with an unfortunate propensity to birth "bulldog cattle" which were grossly mutated offspring with even shorter legs and often cranial deformities. These calves usually die before adulthood. Curiously although these cows clearly have acondroplasia scientific studies have shown it is not from the same gene mutation as in humans. This opens the possibility that many of the animals listed above may in fact have a series of different gene mutations, not any singular one, that just happens to have the same effect.
Achondroplasia in Horses
Achondroplasia in horses is an interesting concept because it's usually found in already miniaturized breeds but are not the source of the miniaturization. Miniature horses were bred through dozens, perhaps hundreds, of generations to be increasingly small by breeding the smallest mares to the smallest studs that were available. Curiously it was after this that the achondroplasia showed up and stranger still it is not the defining feature of any one breed of miniature horse, instead it shows up in 25-50% of all of the combined breeds. They have shorter upper legs and ears than their peers. They have a higher tendency to be born with contracted legs and loose tendons and can suffer higher rates of arthritis.
Achondroplasia is a very old gene (or series of genes) that has been bred into animals for almost as long. Sometimes this comes at a cost, other times this results in perfectly healthy animals, and it is up to both breeders and the pet owners who buy from them to decide whether or not these are features we want to continue encouraging or not. I hope you found this article fun and informative. If you'd like to read some of my other genetic articles feel free to take a gander. I've listed some below.