[Jacob-list] More than four

[Neal and Louise Grose]

Linda's ram's rack is not unusual for us. We have had a larger than average number of sheep with more than four true horns. My feeling is that we make too much of the difference between "scurs", "weak horns" and "true horns". (I am not talking about the true scurs that look almost like scabs. These are sometimes seen in polled animals. I have not seen these on Jacob sheep. Sex-linked 'hornless' are seen in Jacobs from time to time, though they are clearly undesirable.) There seems to be a gradient from one to another type, and the genes for one horn on a sheep's head have to be the same as for the others. I also THINK that the horns always occur GENETICALLY in multiples of two, though we have certainly had some with odd numbers expressed. The most frequently seen of these is a ram with three horns on one side and two and a "bump" between them on the other. 

What we are seeing here appears to be a single gene for polycerate. However, the most frequent genetic conditions are trait expressions that are affected by numerous genes. For example: polycerate expression may be strongly modified by a couple of dozen genes that determine head size and shape, horn bud placement,  and who knows what else. This would be an indication that horn shape and placement and so on has a relatively low heritability. There is little guarantee of what you will get from one generation to the other unless you look at an unusually large number of test crosses.

Neal
  ----- Original Message ----- 
  From: Jacobflock at aol.com 
  To: patchworkfibers at alltel.net ; jacob-list at jacobsheep.com 
  Sent: Tuesday, June 08, 2004 12:42 PM
  Subject: Re: [Jacob-list] More than four


  In a message dated 6/8/2004 6:22:30 AM Central Standard Time, patchworkfibers at alltel.net writes:


    Is there a difference, genetically, between a four horned sheep and a six
    >horned sheep?  or is the number past two
    >merely the result of random splitting? Is the propensity for more than four
    >horns heritable?



  The literature cites the following types: Polled (P), Hornless (Hohl) a sex linked gene which produces scurs and aberrant horns, Horns (presence of two true horns), Multiple horns greater than two (HMN).  I have not found anything in the literature other than these four types. 

  There is also a "rule of thumb" that goes something like "Major genes have major effects".  The question is whether the number of horns in excess of four is the effect of a major gene or a mutation of the true HMN gene.  One must also be cognizant of the nature of the "fifth" and "sixth" horns and determine they are true horns.  In one case of trying by positive associative breeding to produce horn number in excess of four, a six horn ram covered a five horn ewe and produced a four horn ewe. This breeding will be repeated to try to produce some meaningful results.  One key to understanding the phenomena is to examine the pedigrees and keep track of all progeny from HMN breedings.  I think Edie and Wayne VanValkenberg have had rather extensive experience in multiple horn breeding.    

  I am not sure that I generally accept that "Jacobs carry a horn splitting gene" or a "gene that randomly splits horns".  My sense is the H gene on Chr 10 and HMN are controlled by something like a hox gene and HMN in excess of four true horns (opposed to a scur/aberrant horn) is a mutation.

  Fred Horak



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