"More information: https://www.vet.cam.ac.uk/boas/about-... The French bulldog, bulldog, pug, pekingese, shih tzu, Japanese chin, boxer and Boston terrier are all examples of brachycephalic breeds. The most distinctive feature of these breeds is their short muzzle. Brachycephalic dogs have been bred for centuries to possess a normal-sized lower jaw, and a disproportionately shorter upper jaw.
In recent decades, breeding selection for extreme brachycephalic features has resulted in dogs that are predisposed to upper airway tract obstruction and subsequent respiratory distress, among several other health issues. Although not all brachycephalic dogs suffer clinical signs, the incidence and severity of the respiratory disorders has increased. The respiratory disease related to brachycephalic confirmation is called brachycephalic obstructive airway syndrome (BOAS)."