Dog Underbite Explained: The Science, Genetics, & What
What causes a dog underbite? Explore the genetics, evolutionary origins, health implications, and breed-specific data behind canine mandibular prognathism.
Is a Dog Underbite Normal?
Yes — in certain breeds, a dog underbite (mandibular prognathism) is a normal, breed-standard trait. Brachycephalic breeds like Bulldogs, Pugs, and Shih Tzus are genetically predisposed to underbites due to selective breeding for shortened skulls. In non-brachycephalic breeds, an underbite may indicate a developmental anomaly, genetic mutation, or dental disease. While many dogs with underbites live healthy lives, the condition can cause chewing difficulties, accelerated dental wear, and periodontal disease if not monitored. Always have a veterinarian evaluate your dog's bite alignment during routine exams.
The Genetics Behind Dog Underbites: What Science Tells Us
A dog underbite — clinically termed mandibular prognathism or mandibular mesioclusion — occurs when the lower jaw (mandible) extends beyond the upper jaw (maxilla), causing the lower incisors to sit in front of the upper incisors when the mouth is closed. This is the inverse of the typical canine bite, where upper incisors slightly overlap the lowers (a "scissor bite").
The Molecular Mechanism
Research published in PLoS Genetics identified that brachycephalic skull shape is strongly associated with variants in the BMP3 gene (Bone Morphogenetic Protein 3). This gene regulates craniofacial bone development during embryogenesis. In brachycephalic breeds, BMP3 variants cause the maxilla to develop shorter than the mandible, producing the characteristic flat face and, frequently, an underbite.
Additionally, a 2017 study in Nature Communications found that the SMOC2 gene plays a role in brachycephaly. Dogs carrying specific SMOC2 variants showed significantly reduced maxillary length relative to mandibular length. The interaction between these genes and others (including Wnt signaling pathway genes) creates a spectrum of severity — from a mild underbite barely noticeable to owners, to severe prognathism causing functional impairment.
The inheritance pattern is polygenic — meaning multiple genes contribute — which is why two Bulldogs with underbites can produce puppies with varying degrees of jaw misalignment. This polygenic basis also explains why underbites occasionally appear in breeds where they are not standard: spontaneous combinations of these gene variants can occur in any lineage.
In non-brachycephalic dogs, an underbite may result from:
- Developmental trauma to the maxillary growth plates during puppyhood (typically before 6 months of age)
- Nutritional imbalances — particularly calcium-phosphorus ratio disruption during skeletal growth
- Congenital anomalies unrelated to breed standard, including cleft palate repair complications
- Retained deciduous teeth that alter jaw growth trajectory
Evolutionary Context: Did Ancestral Dogs Have Underbites?
What Wolves Tell Us About Jaw Alignment
Gray wolves (Canis lupus) — the primary ancestors of domestic dogs — virtually never display mandibular prognathism in wild populations. The wolf skull is dolichocephalic (long-muzzled), optimized for a powerful scissor bite that delivers precise killing force to prey. An underbite in a wild canid would compromise hunting efficiency, reducing bite force by an estimated 15-30% at the incisors due to misaligned leverage.
The underbite in domestic dogs is therefore a product of artificial selection, not natural selection. Beginning roughly 200-400 years ago, breeders in Europe and Asia began selecting for shortened muzzles in companion and guard dogs. The shortened maxilla — driven by the BMP3 and SMOC2 variants mentioned above — was aesthetically preferred but carried the underbite as a byproduct. No known evolutionary advantage exists for the underbite trait; it persists solely because humans selected for the overall brachycephalic head shape.
Interestingly, archaeological evidence from ancient dog remains shows that extreme brachycephaly (and the associated underbites) was rare before the 1800s. Skull measurements from Victorian-era breed standardization suggest that the modern Bulldog's degree of prognathism is significantly more pronounced than that of its 17th-century ancestors — a consequence of intensifying breed standards over generations.
Physiological Impact: How an Underbite Affects Your Dog's Body
Understanding the downstream effects of mandibular prognathism requires looking at how the canine masticatory (chewing) system functions.
| Severity | Mandibular Overjet | Functional Impact | Dental Risk Level |
|---|---|---|---|
| Mild | 1-3 mm | Minimal; normal chewing, no soft tissue trauma | Low — monitor during annual exams |
| Moderate | 4-8 mm | Some difficulty with hard kibble; possible lip trapping | Moderate — biannual dental checks recommended |
| Severe | 9+ mm | Impaired prehension; chronic gingival trauma; difficulty drinking | High — veterinary dental specialist referral likely needed |
The primary concern is malocclusion-induced periodontal disease. When the lower canine teeth contact the upper gingiva or hard palate instead of fitting into the normal interdental space, they create chronic soft tissue trauma. According to the Merck Veterinary Manual, this can lead to oronasal fistulas — abnormal openings between the mouth and nasal cavity — particularly in severe cases where lower canines penetrate the palatal tissue.
Additionally, misaligned jaws alter the temporomandibular joint (TMJ) loading pattern. Over years, this uneven force distribution can accelerate joint degeneration, potentially contributing to TMJ osteoarthritis in senior dogs with lifelong underbites.
Breed Variations: Which Dogs Are Predisposed to Underbites?
The American Kennel Club breed standards explicitly describe an underbite as correct or acceptable for several breeds. Here is a comparison of underbite prevalence and severity across commonly affected breeds:
| Breed | Underbite in Breed Standard? | Estimated Prevalence | Typical Severity |
|---|---|---|---|
| English Bulldog | Yes — required | ~95% | Moderate to severe |
| Pug | Yes — slightly undershot preferred | ~80% | Mild to moderate |
| Shih Tzu | Yes — undershot bite specified | ~75% | Mild |
| Boxer | Yes — undershot required | ~85% | Moderate |
| French Bulldog | Yes — slightly undershot | ~80% | Mild to moderate |
| Lhasa Apso | Yes — slight undershot acceptable | ~50% | Mild |
| Pekingese | Yes — undershot specified | ~70% | Moderate |
| Boston Terrier | Yes — even or slightly undershot | ~40% | Mild |
Notably, an underbite can also appear unexpectedly in breeds where it is not standard — including Labrador Retrievers, German Shepherds, and mixed-breed dogs. In these cases, the underbite is considered a fault in conformation terms but more importantly warrants veterinary evaluation to rule out underlying developmental issues.
Practical Implications: Living With a Dog That Has an Underbite
Daily Care Adjustments
- Diet texture: Dogs with moderate-to-severe underbites may struggle with large, hard kibble pieces. Consider smaller kibble size (look for brands offering "small bite" formulas with pieces under 10mm diameter) or lightly moistening dry food with warm water for 2-3 minutes before serving.
- Dental hygiene: Brush teeth daily using a veterinary enzymatic toothpaste (e.g., CET Poultry Flavor, approximately $12-15 per tube). Pay particular attention to the lower canine teeth and any teeth that contact soft tissue abnormally. Budget $300-600 annually for professional dental cleanings under anesthesia — underbite dogs typically need these more frequently than dogs with normal occlusion.
- Chew toy selection: Avoid extremely hard chews (antlers, weight-bearing bones, nylon bones rated "extreme") that can fracture already-stressed teeth. Opt for rubber toys with some flex (Kong Classic, $15-22) or dental chews with the Veterinary Oral Health Council (VOHC) seal of acceptance.
- Monitoring: Check weekly for signs of oral discomfort: pawing at the mouth, dropping food, blood-tinged drool, or reluctance to chew on the preferred side.
- Orthodontic intervention: In severe cases causing pain, veterinary dentists may recommend extraction of traumatizing teeth, crown reduction with vital pulp therapy, or (rarely) orthodontic movement. Costs for specialist dental procedures range from $800 to $3,500+ depending on complexity and geographic region.
Common Misconceptions About Dog Underbites
| Myth | Fact |
|---|---|
| "An underbite means my dog is aggressive." | Jaw alignment has zero correlation with temperament. Aggression is influenced by genetics, socialization, training history, and environment — not skeletal conformation. |
| "Puppies will grow out of an underbite." | While jaw proportions can shift slightly during growth (especially between 8-16 weeks), a true skeletal underbite present at 6 months of age is almost always permanent. Do not "wait and see" — get a veterinary assessment. |
| "Underbites are purely cosmetic and never cause health problems." | Moderate to severe underbites can cause chronic pain, periodontal disease, oronasal fistulas, and TMJ degeneration. Regular veterinary dental monitoring is essential. |
| "You can fix an underbite with braces, just like humans." | Veterinary orthodontics exists but is rarely used for underbites. Dogs don't care about aesthetics — treatment is only pursued when the malocclusion causes pain or functional impairment, and extraction is often preferred over orthodontic devices. |
| "Breeding two dogs with underbites guarantees underbite puppies." | Because the trait is polygenic, outcomes are variable. However, breeding two affected dogs increases the probability and potential severity. Responsible breeders prioritize overall health and function over extreme conformation. |
How Is a Dog Underbite Different from a Human Underbite?
While the basic skeletal concept is the same — mandible protruding beyond the maxilla — the implications differ significantly between species:
- Functional priority: Humans with underbites primarily face speech and aesthetic concerns. Dogs rely on their bite for food prehension, grooming, and play — making functional impairment more impactful on daily welfare.
- Treatment approach: Human orthodontics routinely corrects underbites with braces, headgear, or orthognathic surgery over 1-3 years. In dogs, treatment is only pursued when there is documented pain or tissue damage, and the goal is comfort rather than cosmetic alignment.
- Growth timing: Human jaw growth continues into the late teens. Dogs reach skeletal maturity between 10-18 months (breed-dependent), meaning the window for interceptive orthodontics is much shorter — typically 3-6 months of age for procedures like extracting problematic deciduous teeth that may influence jaw growth direction.
- Breed standards: No human culture selects for underbites. In dogs, multiple breeds have been actively selected for this trait over centuries, creating a unique situation where a "defect" in one context is a desired trait in another.
When a Dog Underbite Signals a Problem
Red Flags: Consult Your Veterinarian If You Notice
- Sudden change in bite alignment in an adult dog (may indicate fracture, tumor, or bone disease)
- Dropping food or difficulty picking up treats consistently over more than 3 days
- Blood in saliva or on chew toys, indicating soft tissue trauma from misaligned teeth
- Foul oral odor (beyond typical "dog breath") suggesting periodontal infection
- Nasal discharge or sneezing after eating — a potential sign of oronasal fistula from a lower canine penetrating the palate
- Reluctance to open the mouth or yelping when yawning, suggesting TMJ pain
- An underbite appearing in a breed where it is not standard — especially if the dog is under 12 months old, as this warrants developmental assessment
Always consult your veterinarian for evaluation of bite alignment changes or oral discomfort. Early intervention prevents more costly and invasive treatment later.
Frequently Asked Questions
Is a dog underbite normal?
Yes, in brachycephalic breeds like Bulldogs, Pugs, Boxers, and Shih Tzus, an underbite is a normal, breed-standard trait present in the majority of individuals. In non-brachycephalic breeds, it is atypical and should be evaluated by a veterinarian to rule out developmental abnormalities or dental disease.
Why do some dogs have underbites?
Dog underbites result from genetic variants — primarily in the BMP3 and SMOC2 genes — that shorten the maxilla (upper jaw) relative to the mandible (lower jaw). These variants were selected for during breed development of brachycephalic dogs. In non-standard breeds, underbites can result from developmental trauma, nutritional imbalances during growth, or spontaneous genetic combinations.
Can dogs with underbites eat normally?
Most dogs with mild-to-moderate underbites eat without significant difficulty. Dogs with severe prognathism (9+ mm overjet) may struggle with large kibble or hard treats and benefit from smaller food pieces, moistened kibble, or wet food formulations. Monitor your dog's eating habits — consistent food dropping or one-sided chewing warrants veterinary assessment.
How do dogs with underbites process food differently?
Dogs don't chew with the lateral grinding motion humans use — they use a vertical "guillotine" bite to shear food. An underbite reduces incisor overlap, making it harder to grip and tear food. The carnassial teeth (premolars/molars used for shearing) are usually less affected, so most food processing remains functional unless the malocclusion is severe.
What does it mean when my dog suddenly develops an underbite?
A sudden change in bite alignment in an adult dog is abnormal and requires prompt veterinary evaluation. Potential causes include mandibular or maxillary fractures, oral tumors (particularly squamous cell carcinoma or fibrosarcoma), metabolic bone disease, or advanced periodontal disease causing tooth loss and jaw shift. This is especially urgent if accompanied by facial swelling, pain, or appetite changes.
How much does it cost to treat a problematic dog underbite?
Costs vary significantly by procedure and region. A veterinary dental cleaning with assessment typically runs $300-600. Extraction of individual teeth causing soft tissue trauma costs $200-500 per tooth. Crown reduction with vital pulp therapy ranges from $800-1,500 per tooth. Full orthodontic assessment and treatment by a board-certified veterinary dentist (DACVD or AVDC diplomate) can exceed $3,000-5,000. Pet insurance with dental coverage may offset some costs, though pre-existing conditions are typically excluded.
Should I avoid breeding my dog if it has an underbite?
If your dog is a brachycephalic breed where underbites are standard, the underbite alone is not a reason to exclude from breeding — but overall health, breathing capacity (evaluate for BOAS), and responsible breeding practices should guide that decision. If your dog is a non-brachycephalic breed with an unexpected underbite, breeding is generally discouraged as this may pass on the trait to offspring. Always consult with a veterinarian and, ideally, a veterinary geneticist before making breeding decisions.
Sources
- Hoekstra, L. et al. (2013). "A genetic link between brachycephaly and BMP3 in dogs." PLoS Genetics. ncbi.nlm.nih.gov/pmc/articles/PMC3933510/
- Marchant, T.W. et al. (2017). "A SMOC2 variant is associated with brachycephaly in dogs." Nature Communications. ncbi.nlm.nih.gov/pmc/articles/PMC5607137/
- Merck Veterinary Manual — Malocclusion in Small Animals. merckvetmanual.com
- American Kennel Club Breed Standards. akc.org/dog-breeds/
Jonas Cole
All our authors care for dogs every day — read more of their work on the authors page.
