An adequate airway is THE most important factor in a child's facial development. Genetics determines factors such as hair color, eye color, and height. In contrast, it is altered oral posture usually caused by an altered nasal airway which determines whether or not the face will be well balanced.
Too often children grow up as mouthbreathers due to allergies, obstructions in the airway such as enlarged tonsils or adenoids, or sinus problems. Mouthbreathing allows the child to get the air he or she needs, but it alters "proper oral posture" and causes changes in the child's growth pattern. (See Proper Oral Posture)
If a child grows up as a mouthbreather without proper oral posture the growth tends to be in a downward and backward direction rather than a forward and downward direction. Downward and backward growth results in a long lower face and recessive chin. Different alterations in oral posture produce facial and tooth changes that are unique and different from the genetically determined pattern for an individual.
Pictured below is a 10 year old boy who was developing normally. He was a nosebreather,and his face is well balanced. Not long after this picture was taken he was given a gerbil which he kept in his room. Unfortunately, he was very allergic to the gerbil and went from being a nosebreather to a mouthbreather. Note the changes in his face by the time he is 17: the chin is back (recessive), the cheeks are flat, the lips are flaccid, and the nose appears to stick out.
Studies have been done with monkeys to show the effects of mouthbreathing on growth. When the noses of perfectly normal growing monkeys were plugged, their faces began to grow backward and downward rather than forward and downward. ("Neuromuscular and Morphological Adaptations in Experimentally Induced Oral Respiration" Nasorspiration Function and Craniofacial Growth).
Mouthbreathers continue to have facial changes occur throughout life. Depending on the extent of the mouthbreathing, the chin may continue to become more recessive bringing the soft tissue drape of the cheeks and nose downward. This can result in the cartilage of the nose being pulled down making it appear as if there is a bump in the nose where the nose becomes bony. Some of the downward & backward change may be masked by tilting the head in an unconscious effort to open the airway. This results in a forehead that slopes backward, but the chin does not appear as recessive.
Mouthbreathing also contributes to an unstable orthodontic result because the forces of the tongue and cheeks are unbalanced. (See Proper Oral Posture).