[Jacob-list] mismatched bite.

[Jacobflock at aol.com]

In a message dated 3/10/2003 12:04:49 PM Central Standard Time, 
oberlef at supernet.com writes:

> Listers-
>  Can anyone out there help me remember which way lamb mandibles grow?  That 
> is, is it the lower mandible that grows faster than the upper one? A 
> mismatched bite (with the teeth...lower jaw... coming up and past the upper 
> pad).   I think I remember hearing it was better to see a lamb whose bite 
> did not meet having the lower jaw be too short rather than too long. Too 
> bad this is a really pretty lamb.
>  Katrina Lefever
> 


Peter Spellmeyer ... where are you?

Two conditions of jaw misalignment are (1) brachygnathia, short lower jaw, 
sometimes called "parrot mouth", and (2) brachygnathia superior, the maxilla, 
the top bone, appears too short, the mandible (jaw) is too long, sometimes 
called "bulldog" or "monkey jaw". The maxilla has some special "parts" ...the 
palatine bone is the section that supports the "gum pad", there is also a 
bone for the nasal cavity and a section called the lacrimal bone which runs 
from the eye to the nose.   

There are a number of possibilities for brachygnathia superior or "bulldog" 
mouth:: a normal maxilla (top part) and long mandible (jaw), too short a 
maxilla and a normal mandible.  The maxilla has several "parts" ... one or 
more of those parts may be too short.

Degree of misalignment or distance may be significant.  A study by Ercanbrack 
and Knight on 12,000 jaws over 10 years used the folowing grades ... slight 
undershot or overshot as 0.1 cm or less in front or behind the dental pad ... 
 plain undershot or overshot as between 0.1cm and 0.5 cm. ... bad was greater 
than 0.5cm.  I'm not sure how many Jacobs would pass a test of exact 
alignment ... perhaps alignment defined in terms of pastoral survival is more 
on point, i.e., can they live on pasture and thrive.

Since sheep back teeth (molars) are hypsodont, they keep growing through life 
and some "parrot mouth" or lower jaw "realignment" may occur ...  A slight 
"parrot mouth" condition may be corrected by age ... if it is slight.   Your 
assumption about allowing the young "parrot mouth" to mature is correct.  
However, this does not seem to occur with "bulldog mouth" which takes me back 
to the top bone, the maxilla and the various areas of the maxilla.

If the "bulldog mouth" case is "obvious", you might get a sense of the source 
of the difference by comparing the length of the maxilla (top from eyes to 
nose) with other sheep in the flock.  If a lamb's top of the nose is too 
short compared to the others ... a difference is observed ... one can check a 
little further by looking at the palatine bone (top gum plate) to see if 
there is a shortness ... or is it truly a mandible that is too long.  

Another area to look at is the lacrimal area (tear duct area of the eye to 
nose), is it shorter than others?  Does either sire or dam or their parents 
have an evidently "bulbous" sinus? ...  The nasal sinus bone sits "on top" of 
the maxilla and may be short with short lacrimal bones and thus a shorter 
maxilla.  

The problem of jaw misalignment seems to be a recessive.  There was a study 
done on some commercial breeds that concluded that jaw deformities were 
associated with level of inbreeding.