Sunday, November 18, 2012

Jay's Jutting Jaw

Jay Leno’s distinctively large chin has been something of a trademark for the comedian. The title of his 1996 autobiography is ‘Leading with my Chin’.   However you may be surprised to learn that the Tonight show host’s distinctively large mandible is most likely the result of an inherited genetic condition.
It is perhaps not surprising to hear that inbreeding is not the most sensible concept. Dog breeders have known this for centuries.
Yet for some odd reason the royal families of the world have not only felt it necessary to breed with close family members but they have also done so frequently. 
The Russian royal family is reasonably well known for its heritage of haemophilia. The Hapsburg Jaw is less well known but is once again related to frequent marriages between related individuals. 
The Hapsburg lineage has been a ruling house of Europe for some six centuries. They primarily acted as rulers of Austria but relatives have filled roles as Kings of Germany, Hungary, Croatia, Spain, Portugal, Bohemia and Transylvania. There were also two Holy Roman Emperors, namely Maximilian I and Charles V.
Generations of ruling notwithstanding, the Hapsburg Jaw is perhaps their largest claim to fame amongst the genetics community. 
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. 
As you may imagine, there is also an industry thriving on cosmetic alterations to the jaw. Mandibular Prognathism gives less scrupulous cosmetic surgeons the ideal diagnosis to perform facial surgery.  (Arnold Schwarzneggar is someone famed to have had his corrected).
Now I’m not suggesting that Jay Leno is a result of inbreeding.  Nor am I suggesting he is related to royalty (although it is certainly possible). However, as the following pictures demonstrate he very clearly has something in common with them. 
Take a look, and judge for yourself..

Maximilian I, Holy Roman Emperor
(Public domain image - sourced from wikipedia)

Charles II of Spain 
(Public Domain Image, sourced from wikipedia)                                    

Phillip IV of Spain                                                       
(Public domain image: Sourced from wikipedia)

(Public Domain Image: sourced from wikipedia)
And Jay...

(1)    Wikipedia (Online) Habsburg, Wikimedia Foundation Inc.
(last update: 6/4/07, accessed 10/4/07)
(2) OMIM (Online) Prognathism, Mandibular, NCBI
(last update: 18/3/2004, accessed 10/4/07)

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