[Jacob-list] Comments on horn genetics

Wed May 11 16:24:27 EDT 2005

I have inserted some comments in the various emails to try to expalin what 
may be troubling.  Emails do not allow for visuals and Q and A interaction and 
so these responses may suffer precision or clarity for some parties.  I have 
been cited several times for assault and battery on the precision of science and 
others for not being clearer. Bear with me as I try to cover some of these 
emails since I may be adding to the confusion.

a message dated 5/6/2005 4:35:31 AM Central Standard Time, nlgrose at yadtel.net 
writes:

> It is impossible for a sheep to be genetically different on one part of the 
> head from the other. 

I think Neal is alluding to the concept of bilateral symetry which is a 
rather sound principle but the term "impossible" might better be stated as "there 
is a low probability that a sheep is different on one side of its body than the 
other."  Else how to explain defects such as the extra mouth below the ear 
(otognathia), bent nose, etc as well as the occasional right blue eye, left 
brown eye.

There are 26 "unisex" sheep chromosomes; the ram has a XY which determines 
the sex, the ewe has an XX.  Together they total 27.  At fertilization, a zygote 
is formed representing half the 27 chromosomes of the am and ewe.  As it 
develpos it passes on the "genes" of ram and ewe; let me call this new life a 
gamete.  To estimate the number of possibie gametes in this probability game of 
"two parents", one simply takes 2 raised to the power of the number of 
chromosomes.  This big number is important to keep in mind. About 99.9% of each of the 
ram's and ewe's genes will be the same, but the .1% may carry slightly 
different information at the various locations on the chromosome.  All the various 
assortments of chromosomes, the random production of sperm and egg, the random 
recombination of chromosomes ... cannot change a gene.  You get the hand you're 
dealt.  The gamete later will be cqlled the neural crest and this is sort of 
where gene action can be first observed.

It may be useful to keep in mind that traits that are affected by only one 
gene are 
> rather rare. 

Most genes are little "job shops" that produce proteins for the next "job 
shop". And there are also major "distributrs" and "manufacturer/dealers" that 
produce the showroom model we see.  It is also helpful to remember that major 
genes have major effects; these are the "things we see" that set one breed apart 
from another.  A dominant gewe is always seen; a ressive gene lurks in the 
recesses until it "meets" another recessive gene. Thus we can genetically 
describe the Jacob as a dominant black, rcessive piebald polycerate.  This may be 
contrasted with another black breed with what geneticists call a dominant 
spotting gene ... which covers the whole sheep and we see a white sheep, or "horned 
in rams but notg in ewes" language.  Thus we can say the genotype (genes in the 
"job shop" produces a phenotype (what we see in the "showroom" with our 
senses).  

The phenotype does not always show us all the genes,  Not only the 95% of 
genes we have in common with sheep,.but the "unseen" recessive sheep genes that 
didn't pass "qualityy control" and can spring up later; e.g., ram and ewe are 
registered so why isn't this polled, hairy monster be registered as a Jacob? 
You need about six to eight generations to be sort of sure about the genotype 
and one "screwed up model" says there may be a problem in the "job shop".  

Some genes are pased to offspring in "bundles", some seem to be passed as 
hard-wired (absence of piebald on areas of the face and nape), others may be 
linked by being close to genes on "nearby" chromosomes (absence of piebald on some 
hocks, etc.), others random.  

For example: the size of an animal is affected by thousands of genes whose 
> effects are additive. This makes it relatively easy to predict the size of 
> an offspring based on the phenotype of the parents. This is a trait with high 
> heritability. Blue eyes vs. brown eyes in humans are also easy to predict 
> because the trait is influenced by one gene (you hazel eyed people don't exist, 
> go away). 
> 

Genes and their alleles can cross-over; daddies black hair and brown eyes, 
and mommy's blond hair and blue eyes produce :  ????  Is it predictable?

> 
> So, horns are influenced by different loci on a variety of chromosomes:
>  * Horn vs. dominate polled
>  * Horn vs. recessive polled (I don't know if the dominate and recessive 
> polled are at the same locus)
>  * Horn vs. scur, which may share a locus with polled
>  * Sex-linked hornless (Shetlands) which must be on the X or Y chromosomes
>  * Testosterone production and testosterone receptor genes 
> * A couple thousand "size" genes
>  * Physical blood flow impairment to the lateral horns due to the dominance 
> of the primary horns
>   
> Glad I could clear this up for you!
>   
> Neal 
> 
> >> ----- Original Message ----- 
>> From: Linda 
>> To: Jacobflock at aol.com ; buffgeese at yahoo.com ; Jacob-list at jacobsheep.com 
>> Sent: Thursday, May 05, 2005 7:04 PM
>>  Subject: Re: [Jacob-list] question on horns
>>  
>> 
>> So, is it possible for a sheep to be genetically both horned and scurred? 
>> Since they are on different chromosomes?   
> 

It is possible for a Jacob ewe to express multiple horns, true horns, firml;y 
attached, fully loaded with the vein and artery systaem completely "plumbed" 
to the venus, and yet genetically carry the hornless gene (aberaant horns, 
short scurs, long scurs) or another recessive gene.  "What you see" in a genetic 
package is not always "what you get".  The hornless gene is sex-linked; that 
means the ewe must carry two copies of the Hhl gene on her XX chromosome.  As a 
recessive trait, both ewe and ram (XY chromosome) would carry the gene to 
produce the effect you see; the ewe is hl/hl or Sc if you prefer that term, the 
ram is hl - H.  You will never (never say never because of "mutations") see the 
hornless gene in the ram becasue the XX and XY sex chromosomes are of 
different length, genes, loci and alleles. Mothers Day notwithstanding, there are 
more female linked traits than male linked traits  .. simply beceause the female 
sex chromosome is longer. 

The reason I presently subscribe to the hornless gene (say, sex-linked ewe 
lateral aberrant horns, short scurs and long scurs) being on a different 
chromosome than the horn gene is becasue there is a certain conservation in nature 
that seems to support "similarity" among some species and we produced hornless 
ewes about 5 years ago and was never evidenced in rams for the same breeding 
the prior year.  Further, the "hornless gene" that produces scurs on cattle is 
on a different chromosome than the horn gene.  

What could you expect as a phenotype?  I'm confused about just what is behind 

> >> a four horned ewe with lateral scurs.  
> 

A four horned ewe with lateral scurs might be an oxymoronm.  On the othert 
hand, two true vertical horns and (either two aberrant horns, long scurs or 
short scurs) would represent an expression of the "hornless gene".  I understnad 
that MSWF attendees and the judge for the Jacob show observed several 
"hornless" black and white ewes with multiple aberrant horns both vertical and lateral; 
i.e., no true vertical horn attac hment to the skull.   (The judge was honest 
enuf to say he didn't know much more than they were black and white with 
horns and a rare breed should be evaluated for breeding purposes by a card 
system.)

Is this an expression of carrying both the horned and scurred gene or is it 
an 
> >> expression of carrying the hornless gene?  
>>  
>> If the hornless gene is expressed in the laterals on ewes, is it also 
>> expressed in some way on two horned ewes? 
> 

The hornless gene might be expreessed in a "two horn" ewe if it does not have 
two true horns and exhibits scurs or aberrant horns.  Sometimes these are 
casually or incorrectly refered to as polled.   Polled and hornless are not 
exactly the same.

The term "ewe" denotes sex and not age.  The ewe's horns appear in most cases 
to develop slower than the ram's and we expect a "lighter weight" horn in 
ewes.  But the definition of horn (heavy in the ram and light in the ewe) should 
be conserved.  There are some cases observed in ewe lambs that the "growth" is 
obviously not attached to the skull or does not have a veinous core.  If this 
unattached, wobbly lateral condition lasts until it is, say six months old, 
it probably isn't a true horn.  If, as a lamb or even a yearling for that 
mater, it breaks a lateral and there is no blood flow, it probably isn't a horn.  
If the ewe breaks off a lateral on one side, the other intact side (probably) 
will be a clue as to the nature of the "broken" side based on principles of 
symmetry.  True horn generally will grow back unless the horn core connection to 
the skull is severely injured or severed.

> >>  
>> It's a fascinating topic, but confusing (to me anyway)
>>   
>> Linda
>> 
>> 
>> 
>> On Tue, 3 May 2005 08:57:45 EDT, Jacobflock at aol.com wrote:
>> 
>> > The gene that controls polled or horned is located on chromosome
>> > 10.  The gene that controls multiple horns has not been reported
>> > out yet.  The hornless gene which produces scurs and aberrant horns
>> > is evidenced in the lateral horns of ewes.  The gene is known to
>> > exist but chromosome location is not known.
>> >
>> > The location of the hornless gene may well be on another chromosome
>> > IF the hornless gene (scurs included) in sheep is siimilar to the
>> > gene that produces scurs in cattle.  The poll/horn gene in cattle
>> > is on ch 1, the scur gene is on ch 17 or 19 .. I forget which.
>> >
>> > Fred Horak
>> 
>> 
> 

Fred Horak
St. Jude's Farm
Lucas, TX
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