Mandibular Prognathism is the technical term for a Hapsburg Jaw. It refers to a tendency for the mandible, or the jaw to outgrown the upper jaw, thus resulting in a large chin. Commonly it is known as an underbite. It can be inherited as a part of a more severe syndrome (for example Crouzon’s syndrome), or develop during life due to disorders such as acromegaly. However in the vast majority of people it is inherited as a dominant genetic disorder with variable penetrance. Essentially this means that if you inherit a copy of the gene from one parent, you will develop a large jaw of some description depending how strongly that copy of the gene is expressed (penetrates). In most cases mandibular prognathism is a relatively benign condition. On occasion, however, it can cause problems with talking or chewing due to an abnormal jaw alignment. Charles II of Spain, for example, was apparently so disfigured that he was unable to chew.
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