Formally known as mandibular brachygnathism, "parrot mouth" in a dog results in an overbite, whereby the upper jaw protrudes above the lower jaw, resembling a parrot's beak. Also known as malocclusion, an overbite is usually hereditary, based on the genetic growth rates of the jaws. With parrot mouth, the lower jaw doesn't grow as long as the upper jaw. Uncorrected, it can interfere with a dog's chewing ability.
Parrot mouth usually affects breeds with long, narrow muzzles. Affected breeds range in size from that of the little dachshund to that of the larger Irish wolfhound. The condition is found in many herding breeds, including German shepherds and various collies. Canines with parrot mouth can't be shown in dog show conformation classes.
Since a parrot mouth is apparent at an early age, a veterinarian specializing in dentistry can help correct this hereditary defect in your puppy. To do so, the vet extracts certain teeth on the lower jaw -- either the deciduous canine teeth or incisors -- in a procedure known as selective extraction therapy. Removing these teeth allows the jaw to grow out as much as genetically possible. The permanent teeth will still erupt, so your grown dog won't be without these teeth on the lower jaw.