[Jacob-list] Hornless gene and breed conservation

[Jacobflock at aol.com]

In a message dated 5/19/2006 9:21:56 PM Central Standard Time, 
patchworkfibers at alltel.net writes:

>  
> I believe most breeders are selecting against the trait, but we have to do 
> so in a way that will not seriously reduce our rather limited gene pool. 
> 

If you can't get to sleep, do something that will put you to sleep.

The USDA National Germplasm study of one population of 2,400 Jacobs which was 
adversely selected for the hornless (sex linked scurs and aberrant horns) 
gene, i.e., unacceptable, population inbreeding for the Jacob breed rose from 2% 
to 5% over a fifteen year period and then went back down to 2%.  The higher 
rate of exchange of rams between breeders in which higher inbred animals were 
bred to unrelated inbred animals produced non inbred lambs.  The inbreeding of 
this population was determined to be "not high" but should be monitored such 
that it remains at the 2%-3% level or lower.

The gene pool for this population went from 1% to 5% in 15 years and retraced 
that increase in about 5 years.  The Jacob breed, based on this review, does 
not have as limited a gene pool today vs. its passing thru the genetic 
bottleneck of 25 years ago or even 5 years ago.  However, some Jacob flocks may be 
significantly inbred or a shepherd may breed the same sheep repeatedly which, 
itself, adds little to genetic diversity.  (USDA NAGP Study, 2004)

To conserve CRITICAL populations, some associations with a few members who 
hold the critical remainder stock allow a brief period of cross breeding of 
known pure bred males to selected females, one or two breedings for numbers and 
conforming type and then stop the "cross breeding".  These offspring are kept in 
a record but not registered.  The offspring are bred and placed on a pre-set 
program to "grade-up".  The first generation is assumed to be 50%, second 75%, 
3rd is 87.5%, etc. and at the 15/16 or 31/32 generation (96.875%) is 
"considered" the same as pure (but not the same as pure) and may be registered as 
upgraded stock.  At each generation, an animal that fails a test for phenotype or 
performance is generally excluded from the breeding program.  Inbreeding 
coefficients may increase during this time but at some point (certainly when 
depression is suspected) breeding to a pure-bred ram or comparable stock to reduce 
inbreeding.   (ALBC, Conservation Breeding Handbook)

While the use of pure-bred males in a grade-up program can lead to breed 
recovery from a critical population (CRITICAL population being significantly 
closer to extinction than a RARE breed), attention to the female offsping is 
crucial as this is the "bank" for the next generation.  Is it successful? Yes, some 
species and breeds have recovered critical populations. (Guld Coast, Black 
Belly, Pygmy Goat Assoc.)  

Some have had failures and recovery  ... the Hebridean, a polycerate 
recessive black rare breed was "graded-up" and some were identified as dominant black; 
ie., the "up-grades" look like recessive blacks but are genetically dominant 
blacks.  To check rams for the correct genotype, the registry would register a 
ram after it produced 14 white offspring and zero blacks from a pure-bred 
white breed ... thus proving it was not a dominant black (Parfait and Shipley, 
sp?).

So what does this have to do with the recessive hornless gene?   If the breed 
is at critical numbers (Y/N) and inbreeding is over 10% (Y/N) it makes sense 
to have a documented (Y/N) up-grade program.  These up-grades are being 
identified up to the 31/32 level (Y/N) so the sheep can eventually be registered and 
breed genotype conserved (Y/N) ... and individual shepherds know how to 
identify Hohl stock for their flock up-ghrade program and the genotype being 
conserved (Y/N).  Thus at some point one could prove the Jacob is a dominant black, 
recessive piebald, polycerate; conversely proving it was not a sex linked Hohl 
breed (Y/N)

With the numbers of HMN rams and ewes available and the complexities of 
up-grading, why fiddle with a known sex-linked recessive?  What does it contribute 
to the breed?  Are pink noses, white nape, scurs and aberant horns different?
Good night.  Fred Horak
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