[Jacob-list] Genetics?

[Hobsickle at aol.com]

The gene is not for 2 horns, 4 horns, or 6 horns, but for two horns or  
multiple horns.  Technically, the gene is not even for horn number, but is  somehow 
tied to migration of cells in the developing embryo.  For some  reason (that 
I haven't found an explanation for) the gene causes some of the  cell masses 
in the developing embryo separate improperly.  These cells are  the cells that 
will result in the horns (and other parts of the skull--hence the  associated 
split eyelids and notched eye orbits).  As the embryo continues  to develop 
the separated cell masses grow back together so the skull is intact,  but the 
horn buds don't quite make it back together (usually).
 
The gene is dominant in its cell mass slitting action, but sometimes the  
masses don't split far enough to remain separated, thus growing back  together, 
producing a two-horned animal.  (These two-horned sheep with the  multi-horn 
gene often have a crease, line, etc., running longways through the  horn.)  This 
explains why multiple horns usually appear to  be genetically dominant, but 
sometimes seem to be recessive.   The "splitting cell mass" that was their real 
action is never  seen.
 
Because the multiple horns are coming from the same amount of embryonic  
tissue, each successive split results in smaller horns, though the splitting  
appears to be somewhat random, so there can be evenly sized daughter cell masses  
(and hence evenly sized horns) or unevenly sized masses.
 
I expect that there are probably genetic factors that determine how far the  
cell masses split and how much they grow back together, but to my knowledge 
the  factors have not yet been determined.  There may also be genetic factors  
that determine how many pieces the cell masses divide into (4 vs. 6 horns), but 
 those factors are also unknown to me.
 
-Dan

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