There are three common and valued genetic conditions in dogs that result in stunted growth: pituitary (ateliotic) dwarfism which results in proportional minis, micromelic achondroplasia which results in shortened limbs, and brachycephalic achondroplasia which shortens the head.
All of these conditions are genetic disorders and all of them are definitive sine-qua-non features of some breeds, often in combination. These disorders aren’t accidental and unwanted, they are written into the breed standards. The breed wardens don’t want to breed these conditions out, they demand they breed true.
Most of the toy breeds are ateliotic dwarfs: Chihuahuas, Boston Terriers, Italian Greyhounds, Maltese, Miniature Pinschers, Miniature Spaniels, Pomeranians, Toy Poodles, Yorkies, etc. This form of dwarfism is caused by a deficiency in somatropin which results in stunted growth of all somatic cells in the body. Ateliosis is a recessive allele.
If you’re a self-styled expert with a bad case of illusory superiority who has written more books than he’s read, you might say, “‘Achondroplasia’ literally means ‘an absence of good shape,'” but that’s not what the literal Greek means. “Chondro” is Greek for cartilage and “Plasia” means growth or change from the Greek word for moulding. Thus, the literal meaning of Achondroplasia is “Defective growth of cartilage” and that’s pretty much what it is. In general, it serves as the name for a cluster of similar disorders of the cartilage and bone (osteochondrodysplasias).
The most obvious effects of Achondroplasia occur in the long bones of the leg and “micromelic” means short limbs. Micromelic breeds include: Basset Hounds, Bulldogs, Corgis, Dachshunds, Lhasa Apsos, Scottish Terriers, Shetland Sheepdogs, etc. Micromelic Achondroplasia is a dominant allele with incomplete penetrance.
Another form of Achondroplasia that doesn’t lead to what we commonly consider to be dwarfism but which likewise results in insufficient growth of bone is Brachycephalic Achondroplasia which shortens bones in the skull. In dogs, shortening of the mid-face and maxilla and shortening of the lower jaw are inherited separately. Boxers have a shortened upper jaw, but their lower jaw is normal and they are normal sized in all other respects; whereas Boston Terriers have both upper and lower brachycephalism and are also ateliotic dwarfs, so they are proportional but small with smooshed faces.
Bulldogs have midface and upperjaw brachycephaly so their lower jaw juts out and they have trouble breathing, and they also have micromelic achondroplasia so their legs are short and bowed while their trunk is not significantly
Miniature Dachshunds carry both forms of body dwarfism, but their faces are unaffected.
And what do you get when you combine all three disorders? A Pug.
The most completely achondroplastic dog breeds are the toy imports of East Asia origin (bracycephalic + micromelic achondroplastic + ateliotic): the Pekingese, the Shih Tzu, and the Pug. The body-forms of these triply achondroplastic breeds represent the simultaneous superposition of all three varities of achondroplasia–micromelic, maxillary, and mandibular–on a midget frame. Thus, the Pug’s disproportionately stumpy legs, tending to bow (genu varum), result from micromelic achondroplasia. The Pug’s bulging forehead (frontal bossing); large, staring eyes (exophthalmos); pronounced stop (recessed nasion); and short midface (midface hypoplasia) all reflect maxillary achondroplasia. The Pug’s short lower jaw expresses mandibular achondroplasia. The Pug’s extraordinarily flat face and crowded dentition are accidence of the simultaneous operation of maxillary and mandibular achondroplasia.
– For the Love of Dachshunds, Robert Hutchinson
None of these disorders exist without other health complications like luxating patellas, arthritis, cataracts, and shortened lifespans. But, none of these disorders can be removed from breeds without fundamentally altering the breed itself. The Miniature Dachshund would simply be a Dachshund if you removed the pituitary dwarfism, but most of the other dogs no longer have a perfect corresponding wildtype breed.
For that reason, if the disease is to go, the breed is to go. I won’t count on that happening any time soon, if ever.
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