[Jacob-list] Hornless and polled

[Linda]

Thank you, Fred (and Neal),

I was hoping you would respond. 

Linda
 
www.patchworkfibers.com
Registered Jacob Sheep, Angora Rabbits, Handspun Yarn


> 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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