Why do people have over-jet and class II? Is it a genetic issue, or is it due to environmental problems such as mouth-breathing?​

The answer to this questions is rather complex and multifactorial.

We are actually talking about skeletal class II and it’s development. It is multifactorial, and it cannot be really simplified if we want to understand the problem correctly.

#1. Diet. The development of class II started with the start of farming around 10,000 years ago. With more soft processed foods, chewing forces dramatically decreased, which led to the rise to the overjet.

After the industrial revolution this problem has increased as more and more soft foods got introduces into our diet.

#2 Some orthodontic problems, for example, class II div 2 has a strong genetic component, but the overjet appeared in history so fast in 10,000 years that it’s development is probably not due to evolution. Class II div 2 however definitely has a strong genetic component – see case frequency in certain areas like Germany, eastern Europe, and Holland.

#3 Environment has a huge impact on the development of skeletal class II, especially poor airway and nasal congestion and tongue and swallowing postures and patternes.

Once, often a child develops allergies, airway problems (large adenoids, and or tonsils) nasal congestion, mouth-breathing and a reverse tongue thrust swallow, this often gives rise to a down and backswing of the mandible and a vertical long face convex growth pattern (see adenoid face syndrome). If the nasal congestion and mouth-breathing development happened early in life when the bones are very soft and remodel easily, the changes to a small retrognathic (backset) jaw – convex profile with often a long face can be very dramatic.

If you are interested you should read Prof. Larsen’s books. https://anthropology.osu.edu/people/larsen.53

Here are some examples on the topic from my teaching slides.