[Jacob-list] Hornless and polled

[Neal and Louise Grose]

I am indebted to Fred Horak for sending me an article on the genetics on polled and hornless in Jacob's Sheep. With his permission, I am excerpting part of the article for the rest of the list.  

The following is from and article from the Jacob Sheep Conservancy Newsletter Vol.XIII, No. 3

Jacob horns: multiple genes, one "splitting" gene, a hornless gene

By: Fred Horak, St. Jude's Farm, Lucas, TX 75002


The horn chromosome of interest to the Jacob breeder is Chromosome 10.  This is the chromosome that carries the horn gene.  The fact that the horn gene is on chromosome 10 is rather unique.  Cattle and goats have their horn gene on Chromosome 1.  Further, the horn gene is autosomal; it is not on a "sex" chromosome.  However, the "sex" chromosome may influence the development of the horn.  

 

Horn genes and alleles: For an inherited characteristic, such as horns, the Jacob has at least two (perhaps four or more) genes on the chromosome which are inherited, one half from each parent.

.......



When the two genes of a pair are different, one is expressed (you see it), the other is hidden (it's still there but you don't see it).  The gene you see is called the dominant gene; the gene that is still there but you don't see is called the recessive gene.

 

Horn gene and its three alleles:  The presence of horns (two horns, four horns is another issue), is probably controlled by a single autosomal locus called Horns, or, in genetic shorthand Ho.  The Ho locus can have three alleles or forms: (1) Ho+ which produces horns in rams and ewes and would be the allele of the Jacob, (2) Ho*hl allele which is the hornless allele causing horns in rams and hornless or scurred ewes, and (3) the Ho*P allele which produces polled sheep.

.....

Multiple horns and the scur locus:  The presence of multiple horns, that is, more than two horns, is subject to the presence of a locus called HNM and is considered an incomplete dominant.  The HNM locus is also postulated as the cause of the congenital defect called Split Upper Eyelid Defect (SUED), the upper eyelid flesh and the bone of the occipital orbit being 'extensions' of the splitting of the horn core bud.

 

The hornless allele, Ho*hl is sex limited and interacts with the Ho allele producing three observations: aberrant horns, long scurs and short scurs. 

.......

The Hornless gene (Ho*hl) is expressed in three ways: (1) the aberrant horn which has a keratinous sheath but has a minimal core and is weakly attached to the skull, (2) the long scur and (3) the short scur.  Correct identification of the hornless gene in the ewe can be very difficult prior to four to six months.  What is considered as the more feminine, dainty, horn on the ewe may actually be an expression of the hornless gene.

 

The aberrant horn or scur appears:  The hornless gene (Ho*hl) is apparently recessive.  The hornless gene in the ram is not seen; the ram appears to carry normal horns but the ram can be a carrier.  The Ho*hl carrier ram when mated to a Ho*hl ewe produces aberrant horn and scurred ewes.  Like the Ho*hl ram, an apparently horned ewe can be a carrier of the hornless gene.

 

If one wanted to test for the hornless gene, one should concentrate on breeding a 'test ram' to a known heterozygous aberrant horned or scurred ewe.   If the ram produces an aberrant horn or scurred ewe, he is a carrier if even one out of six ewes is aberrant horned or scurred.  Ram lambs will always appear with normal horns and cannot be included in the six lamb progeny test.   





Editorial comment by Neal Grose:

The range of variations in "hornless" expression from smooth head to long scur presents a problem. To me, it would seem reasonable that it is due to incomplete dominance or multiple genes. This could be as simple as an interaction with testosterone or receptors governed by the X and Y chromosomes.



It is also possible that hornless is a "native" (though undesirable) part of the Jacob genome. There is probably overlap in the observation of less robust horned animals and what might actually be a scur. 



As Fred has pointed out, testing for this condition is difficult because of the stigma associated with reporting hornless sheep to the registry. For those curious enough, a test cross to a hornless breed such as Shetland might reveal more. And they are tasty.
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