Pugs are deformed because centuries of selective breeding compressed their skull into a shape that no longer supports normal breathing, eye placement, or spinal alignment. The flat face, bulging eyes, and curly tail that define the breed are all the result of specific genetic mutations that were deliberately selected for appearance, with severe consequences for the dogs’ health. Roughly 65% of pugs have clinically significant airway obstruction, and many live with chronic pain, skin infections, or eye problems directly caused by their body shape.
The Gene Behind the Flat Face
The pug’s dramatically shortened skull traces back to a mutation in a gene called SMOC2, which controls how the facial skeleton develops. In pugs, a small piece of parasitic DNA inserted itself into this gene and disrupts its normal function, causing the gene to produce faulty instructions. The result: dogs that carry two copies of this mutation produce about five times less SMOC2 protein than dogs without it. Since the protein is essential for building the bones of the muzzle and jaw, less of it means a drastically shorter face.
The effect is dose-dependent. Dogs with one copy of the mutation have moderately shortened faces, while dogs with two copies have the extreme flat-faced look seen in pugs. This single genetic change accounts for about 36% of all facial length variation across dog breeds, making it the most powerful known driver of brachycephaly. The mutation is found on about 92% of the chromosomes in flat-faced breeds, compared to just 2% in dogs with normal-length snouts.
Why Their Breathing Is So Compromised
Shrinking the skull didn’t shrink the soft tissue inside it. Pugs still have roughly the same amount of tissue in their airways as a dog with a normal-length snout, but it’s crammed into a fraction of the space. This creates a cascade of problems known as brachycephalic obstructive airway syndrome, or BOAS.
Three structural abnormalities drive most of the obstruction. First, pugs are born with abnormally narrow nostrils that can collapse inward when they inhale. Second, the soft palate at the back of the mouth is too long for the shortened skull, so it drapes over the airway entrance and blocks airflow. Third, some pugs have a windpipe that is proportionally too narrow for their body size, limiting how much air they can move even when the upper airway is clear.
A 2017 study that graded respiratory function in pugs found that nearly 65% were clinically affected, meaning they needed either medical management or surgery. About 20% fell into the most severe category, requiring immediate surgical intervention. Only 5% of the pugs tested had completely normal breathing. The remaining 30% showed mild signs but weren’t yet considered clinical cases. These numbers mean that a majority of pugs spend their lives working harder than normal to get enough air.
The Screw Tail and Spinal Deformities
The pug’s tightly curled “screw tail” isn’t just a cosmetic quirk. It’s caused by a mutation in a gene called DVL2, which plays a role in the WNT signaling pathway, a fundamental system that guides how vertebrae form during embryonic development. Researchers identified this as a frameshift mutation, meaning it scrambles the gene’s instructions from a certain point onward.
This mutation doesn’t just affect the tail vertebrae. It’s associated with malformations throughout the spine, particularly in the thoracic (mid-back) region. These wedge-shaped or fused vertebrae, called hemivertebrae, can compress the spinal cord. In mild cases, a dog might show no symptoms. In moderate to severe cases, the compression causes pain, weakness in the hind legs, or loss of coordination. The most severe cases can lead to paralysis and euthanasia. The same DVL2 mutation is also linked to higher rates of cleft palate and congenital heart disease in screw-tailed breeds.
Shallow Eye Sockets and Corneal Damage
Pugs’ prominent, round eyes are a direct consequence of the shortened skull. The bone structure behind a normal dog’s eyes forms a deep socket that holds the eyeball securely in place. In pugs, these sockets are extremely shallow, so the eyes sit further forward and are more exposed. The wide gap between the upper and lower eyelids makes things worse, leaving more of the eye’s surface unprotected.
This combination creates real vulnerability. Even moderate pressure around the head or neck, something as routine as being held firmly, can displace the eyeball forward in severe cases. The exposed cornea is also prone to scratches, ulcers, and chronic irritation. Many pugs need corrective procedures for eyelids that roll inward and scrape against the eye’s surface, or for lashes that grow in the wrong direction. Left untreated, these issues can cause pain and vision loss.
Skin Fold Infections
The deep facial wrinkles that give pugs their expressive look create warm, moist pockets where skin presses against skin with no airflow. These folds trap moisture, dead skin cells, and bacteria, creating an ideal environment for infection. The result is a condition called skin fold dermatitis, which ranges from mild redness and odor to severe ulceration that penetrates into deeper layers of skin.
Bacteria and yeast thrive in these folds, producing toxins that irritate and break down the skin barrier, allowing the infection to spread deeper. Chronic, recurring infections can permanently darken and thicken the skin. Pugs with pronounced facial folds often need daily cleaning of these creases to keep infections at bay, a maintenance routine that exists solely because the breed’s facial structure traps moisture against itself.
Breeding Difficulties
The pug’s altered skull proportions also complicate reproduction. Puppies with disproportionately large, round heads relative to the mother’s pelvic width can become stuck during birth, a complication called fetomaternal disproportion. Across all dog breeds, difficult births occur in roughly 5 to 16% of pregnancies, and 60 to 80% of those end in cesarean section. Brachycephalic breeds face higher rates of these complications because the trait that defines the breed, a wide, round skull, is the same trait that makes natural delivery harder.
Regulatory Response and Breeding Reform
The severity of these health problems has prompted some countries to act. The Netherlands introduced rules using a traffic-light system based on muzzle length relative to skull size. Dogs with a muzzle shorter than one-third of their skull length are banned from breeding entirely. Dogs with muzzles between one-third and one-half the skull length can only breed if they meet additional health criteria. Only dogs whose muzzle is at least half the length of their skull get unrestricted breeding approval. For context, many modern pugs fall well below that one-third threshold.
Some breeders have pursued a different approach, crossing pugs with longer-snouted breeds to create what’s sometimes called a “retro pug” or “retro mops,” aiming to restore the more moderate facial proportions pugs had a century ago. Historical images of pugs from the 1800s show dogs with noticeably longer muzzles than today’s breed standard. These retro breeding programs remain controversial in the show world, and it’s still unclear whether they’ll gain mainstream acceptance among pug owners. But the underlying message is hard to argue with: the traits that define pugs today are the same traits causing them to suffer, and the only way to fix that is to change what the breed looks like.