[Jacob-list] reply on scurs

[Grose]

Assuming:
a) That we are interested genetic scurs...  [Remember, we can not select for those characteristics that have no genetic basis.]
b) That the polycerate gene is a single gene that only codes for splitting of the horn bud...

How the heck can one set of horns on a 4 horned sheep be "normal" and the other set "scurs"?

I have looked at what little information I could find on the genetics of horns and have concluded only that the more I know, the more I understand what it is that I don't know. I have seen true scurs in non-Jacob sheep and it does not look like simple weak horns. Genetic scurring should occur equally in males and females, and in two and multiple horned sheep. What we are seeing does not do this. 
                         However...
This does not mean that we need to tolerate overly weak/scurs routinely in the breed registries. What was tolerable when animal numbers were small may not be tolerable now. Now, there may be over-riding considerations for a particular animals, but we should certainly be moving away from weak horns on a breed that is has horns as a primary defining characteristic.

I have three final observations to make:

1) Within similar animals, horn mass seems to be roughly the same between 2, 4, and 6 horned sheep. Polycerate animals wind up with the last horn to be formed losing out.
2) Horn mass is directly proportional to bone mass. Little under-grown sheep have crappy horns ("scurs"). Big round boned sheep have "good" horns. As in most things, we need to shoot for mediocrity in our breeding programs.
3) It is very difficult to separate environmental factors such as founder or injury from truly genetic factors. Chances are, however, that stronger horns will be more immune from environmental insult than weaker horns.

Neal Grose
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