[Jacob-list] Scurs and Horns

[Jacobflock at aol.com]

In a message dated 8/8/02 11:39:50 AM Central Daylight Time, SharHill at aol.com 
writes:


> I have a question about scurs.  I have a yearling 4 horn ewe who broke off 
> all 4 horns  when she was a few months old.  The top 2 grew back nicely and 
> 
> are quite solidly attached, though a bit funny looking with the blunt tips. 
>  
> However, the laterals grew back as little nubs that move around in the 
> skin, 
> as if they are not really attached to the skull.  Those are scurs, right?  
> (This is my first experience with this and have never seen them before.)


"The EWE broke 4 HORNS".  First, the diminutive nature of the ewe horn should 
be considered; smaller, perhaps more fragile the first year.  Second, as 
mentioned in Neal Grose's post, environment may have an effect. These and 
other factors may make it more susceptible to breaking than a ram's horns.   
The horn may have broken all or substantially all of the horn core at the 
skull.  The REGROWTH of the broken horn may result in a full horn with a 
core, a blunt horn or scur-like structure with a core, a keratin structure 
that does not have a core.  

If the ewe was a four horn before breaking all four horns it is still a four 
horn. The phenotype (the regrowth of the broken horn that might look like a 
scur) does not change its genotype (its assumed true "horned" genetic 
identity).  As noted in Gordon Johnston's  post ... "scurs are not broken 
horns".  The determination of phenotype and genotype is what is present and 
develops from birth.

> 
> I was just wondering about the genetics of it.   I will admit right now 
> that 
> I don't know much about genetics,  so forgive any "dumb" questions or 
> statements I might make. 

The following is known: the Horn locus is on Chromosome 10.  There is 
compelling evidence that the presence of horns is controlled by a single 
autosomal locus Ho.  This gene can have three forms or alleles: (1) Ho+ which 
produces horns in rams and ewes, (2) Hohl which produces horns in rams and 
"hornless" ewes, and (3) HoP which is the polled form.  

The "hornless" allele is sex limited (ewes) and interacts with the Ho+ allele 
to produce three conditions: aberrant horns, short scurs and long scurs.  A 
Horn is different than aberrant horns, long scur and short scur.  A horn has 
a core and substance, aberant horns, long scur and short scur are weakly 
"anchored"  to the skull often by a "dimple".  The aberrant horn is very weak 
and very small diameter core and the core does not run the length of the 
horn.  The long scur is "attached" by a "dimple" to the skull and has a 
"nubby" appearance.   The short scur is characterized by its short length 
(about an inch or less and may have a "plasticized appearance).

The presence of MULTIPLE HORNS (more than two) is another phenotype and it is 
a working hypothesis that is subject to another locus HNM and an incomplete 
dominant.  HNM would seem to have epistatic effects, e.g., SUED.




> The mother is a 2 horned ewe with very nice solid horns.  The father is a 4 
> horned ram, also with very solid horns.  I assume there is a gene that 
> causes 
> scurs to appear?  Do both parents have to have it for the scurs to show up 
> in 
> the offspring?  If they both have the gene, how great is the likelihood 
> that 
> scurs would show up?  I would be interested to hear how this works. 
> 
The literature says to consider the hornless allele as the expression of 
scurs and aberrant horns.  The literature is silent as to the expression of 
the hornless allele in multiple horned breeds.  It is my opinion that they 
might work in a similar manner, i.e., HNM interacting with Hohl.

> I was also wondering if anyone keeps track of, or knows how widespread the 
> defect is in the breed.  Also, do scurs ever occur on 2 horned animals, and 
> 
> do they ever show up on the top horns of 4 horned sheep?  Do they occur 
> more 
> frequently in ewes or rams or does anyone even know?
> 

The prevalence of the "hornless" allele will probably have to wait until the 
multiple horn genotype is understood.  All parents and their lambs must be 
accounted for to build any Medelian ratio for heritability.  However, 
collectively we must be able to identify a horn and correctly count horns.  
Counting horns is not easy in some cases, particularly immature Jacobs.

The basic genetics of horns suggests that the ewe is the prime candidate for 
observing the expression of the "hornless" allele (the "hornless" allele is 
limited to ewe expression).  By definition, the two horn ram carrying the 
"hornless" allele will have horns .. think of the ram as a "carrier" of the 
hornless allele.  Do scurs occur on the progeny of two-horn breedings? Yes, 
and the parent(s) and progeny would, by definition, not be Jacobs but a 
cross-bred.  (The definition of the Jacob says "horned or polycerate in both 
sexes").  

Does the "hornless" allele appear on four horn Jacobs?  Yes, but the 
expression seems to be that of true top horns and possible lateral aberant 
horns or scurs on ewes.  This seems to support the case for a HNM locus and, 
perhaps like Ho, subject to influence the Hohl allele. If the ram reported by 
Mary Ellen Hansson has lateral scurs ( from birth - not traumatized horns) 
this would be interesting.

Scurs are not the funny pencil thin, floppy "growths" near horns.  I think we 
must keep in mind the keratinous material that seems to produce growths that 
are neither horns nor aberant horns ... these sometimes just fall off by 
themselves. Here I think it is important to keep the definition of a "horn" 
close at hand; a core rooted to the skull.              

> The sire of this scurred ewe also produced several lambs with split eyelids. 
>  
> Is this in any way connected to the scur issue?
> 
Split upper eyelid defect seems to be directly related to the multiple horn 
condition rather than scurs per se.  It is observed as a notch in the eyelid 
(palpebrae), the upper skin (tarsal plate) is malformed and the lash (ciliary 
portion) may extend to the interior surface of the eyelid membrane (tarsus).  
It often seems to be associated with a noticeable notch in the occipital 
orbit of the skull (a notch in the occipital orbit is not itself SUED ... 
many species and breeds have notched orbits).   SUED is congenital, it is 
inherited, probably as a recessive.  It is present in other polycerate (more 
than two horns) breeds.   

There are several articles on horns and horn genetics.  Authors include, 
Montgomery, Dolling, Alderson, Partridge.  Perhaps there will be more 
information on horn inheritance this coming Spring following the outcome of 
some "controlled" breedings.

Meantime, reviewing the heritibility and pedigrees of one's flock may offer 
some insight but be sure to include all parents and lambs ... 

Fred Horak 
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