[Jacob-list] Jacob color genetics

[Jacobflock at aol.com]

Fred Horak here with some information that might point to answers or shed 
light.  This may get me to over simplifying and "distort" the true science 
but without getting tooooo deep, I hope it may add to some understanding and 
the discussion.

First, consider the history of the breed.  The Jacob is clearly and 
definitively a developed breed and its origin sufficiently obscure that its 
foundation breeds are unknown but speculated.  This is known.  Three flocks 
of "Jacobs" are documented to 1750 but only the two oldest are known to be 
"four horn"; Charelecotte, a later flock, is known to be 'two horn'.  At this 
time, 1750-1850 it was known as the "Spanish sheep with four horns" or "the 
horned piebald". The name Jacob or Jacob's Flock was used to describe only 
two of the eight documented flocks in the 1850s.  By 1900 there seem to be 
about 35 documented flocks of "Park Sheep" which included "Spanish sheep with 
four horns", "horned piebald" sheep, four flocks of Jacob sheep, African 
sheep, Zulu sheep, Persian sheep, etc.  In general these "Jacobs" are 
described as black and white piebald with horns in both sexes.  There is one 
exception ... and only one documented source for "lilacs" I have seen.

The earliest references I have been able to find suggesting a source for 
"lilacs": (1) is that from 1933 from Whipsnade zoo in Dunstable, Bedfordshire 
... black and white piebald with four horns; then from the zoo in 1960s: the 
breed at Whipsnade is piebald, tri-colored, brown, tan, as well as black and 
white (from the zoo's director) and these were given to Assinbone Zoo 
(Winnipeg,Canada) in 1963 ... they were then known as "Scottish four horn"; 
not as Jacobs.  In the 30 years between 1933 and 1963, the black and white 
added a new color.

When the Jacob Sheep Society (1969, Great Britain) began to popularize the 
"Jacob" as a breed name the breed standard described the breed as a dominant 
black with a recessive piebald, horned in both sexes.  The lilac was 
considered as a Jacob cross bred because it was limited to a few flocks.  
Now, that did not preclude including Jacobs with known crossing to other 
breeds.  For example, the Castle Combe flock included crossbreds from African 
sheep imported in 1879.  The Castle Combe flock produced a lot of "black" 
lambs ... by contrast, the Whipsnade Zoo flock had some "brown", lilac, as 
well as black and white ... and by 1967 all the Whipsnade Jacobs were lilac 
... black confined to feet and leg markings and face patches.

So what we got in North America is the "unimproved" type both black and white 
and "lilac" and a more "improved" type that met with the JSS breed standard 
and as described by Todd Hescock.  What is known today is the British Jacob 
has been improved largely with the Dorset Horn but always breeding back to 
Jacobs to improve the Jacob.  (See also the article in the Spring JSS Journal 
which reflects the typical expression of pink nose, heavily streaked horns 
admixture; easy to put into the breed but is still being expressed thirty 
years later.)

A thought that was raised by a fellow Jacob breeder in MD last week: "We must 
do a better job of educating breeders on some basic color, piebald and horn 
genes or we will find the Jacob breed unable to consistently reproduce 
itself.  We can't just change the standard to accomodate what is in 
everybody's flock."

A history of the breed color sources, I think, is a helpful lead into 
understanding the genotype.  Now I know you want to look at a sheep and know 
its genotype but it is difficult to do.  The best example of how the 
"phentype" can lie about the "genotype" is the Hebridean, a homozygous 
recessive black.  About 5 years ago, it was discovered that every once in a 
while a "white" sheep popped out and would be culled ... and so they did a 
progeny test with 60 sheep and 75% of the "black" registered Hebrideans 
turned out to be dominant blacks ... and not true Hebrideans.  The progeny 
test to prove a pure Hebridean involves accounting for all lambs born and 
producing 14 white progeny from breedings to pure-bred white ewes and 0 black 
lambs ... which gives a 99% level of "certainty". 

An aside: On the phenotype issue related to a recent observation:  four 
'bumps" on the head does not mean a lamb is 4horn.  Be acutely aware of the 
hornless gene expressed in the 4horn ewe which is evidenced by aberrant 
horns, long scurs and short scurs. This is a "cross bred" trait ... and its 
source can be found in the AMBC Jacob listings.

Part two draws on COGNOSAG.  The 'color' genotype of the Jacob has always 
been dominant black.  (Shetlands has a dominant gene that causes brown to 
appear beige or (if the sheep is black instead of brown) causes black to 
appear gray .. or lilac. and this dominant gene also appears to be 
temperature sensitive.  But the basic pigmentation henotypes are well worked 
out .. it is either eumelanin (black to brown) or pheomelanin (say yellow to 
brown). These MAY be subject to 'dilution' but the word dilution may have two 
meanings: one refering to the GENE, the other being how we DESCRIBE THE 
COLOR.  We don't do a good job of distinguishing between the two describing 
the color and the transmission genetics.)  With a dilution gene theory the 
myeocine transporter of eumelanin from the melanocyte would be weak but this 
would also involve a pattern gene ... accounting for black and lilac.

Remeber the brief history of the lilac Jacob?  There is a possible "lilac" 
connection with the lilac/gray Wenslydale (this is a recessive black breed) 
and the Portugese Horned sheep in which the lilac/gray is recessive to the 
black ... the fleece contains pigmented and white fiber ... 

The Dominant Black is caused by an allele (form of a gene) at the EXTENSION 
locus.  The Extension locus (always a capital E) controls fiber color and 
skin color.  (Yes, there is an argument to be made as to whether the 
extension locus really exists.  How many e sheep have you seen?)  Jacob black 
is formed in the neural crest (from conception but someone will say it is day 
60 before you see black fiber in black skin and that is probably true).  
Genes do something real ... in this case the gene sets up a receptor (like an 
outlet for a plug) that "plugs in" black melanocytes (MC1R); the "holes" for 
black.  Tyrosine is the "active ingredient" that produces the eumelanin ... 
black and this has been labeled TRP1.   There is another locus and receptor 
that may explain the "lilac" called DCT which produces tyrosine but this is a 
"gray" color and is labeled TRP2.  Black is a capital B, brown is a little 
b.  Brown is recessive to black and both are dominant to white.  So the Jacob 
starts out as a completely black sheep.

Now how does one get spots?  Consider two kinds of spots from the "spotting 
gene".  A dominant S producing a spot so big that it blocks out all color and 
leaves white ... this might be described as the S gene.  The spotting gene 
stops melanoblasts from being formed.  The receptors for "spotting" put up 
road blocks for eumelanin transport.  The other "spotting gene" is called the 
piebald effect and is the recessive form of the "spotting" gene.  In order to 
consistently produce (say 100% of the time) a "piebald" lamb, the parents 
must be a homozygous recessive. 

The dominant black and recessive piebald may not be as simple as assumed.  We 
must understand that the Jacob genotype has not been expanded 
"scientifically" since about 1926.  

Why do some lines have leg markings, others not?  Why do some have the 
"center of the face piebald" (reverse badger face) and others have patches?  
Which are breed specific and which are random?  What if there were a 
"pattern" gene? Some insight into these questions may be coming sooner rather 
than never.  

The Mouflon question and considerations.  The Jacob is a Mouflon type.  Most 
"modern breeds" are Mouflon mutations and crossbreedings.  Mutations from 
Mouflon to Jacob include (1) a dominant black gene, (2) piebald, (3) face 
patches and spotted legs, (4) badger face (5) multiple horns, (6) 
lilac/grey.  

The true Dorset Horn is and has been the historic "Jacob" cross; the Dorset 
Horn carrying an Ho horn gene and a S spotting gene with Awh(Agouti white).  
The Dorset Horn (and even the Dorset) expresses a weak Spotting gene when it 
expresses black pigment in fiber, hooves, facial flesh, etc.  On the other 
hand, an excessive s piebald gene eraes black and does so with a 'process' 
... that begins in the rear and moves to the front.  The last evidence of the 
recessive piebald s is the disappearance of the black "neck cape".

A final note on color and conclusions.  The true color of a sheep may not be 
accurate because we are limited in our description of color or, assume type, 
breed, or offspring described by onesies and twosies is conclusive.  Until 
one gets ten progeny from known breedings, and results verified, it is 
helpful information but not conclusive.  This is not to discourage cross 
breed tests for color or other traits; but to carefully measure the result 
and temper conclusions.  

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