B locus for pigment production
BB (Icelandic B1) sometimes written as B+ is black. This is dominant.
Bb  (Icelandic B2) is brown or moorit, and is recessive.

S locus for broken white markings or the Spotting Gene
SS sometimes written as S+ means NO SPOTS, or "unbroken color". It's dominant.
Ss  (Icelandic S2)is the "spotting" gene, giving "broken color" and it is recessive.

Janne Rosecrans reminded me once that a totally white sheep with a few dark spots on the face was probably not expressing the spotting gene. There are cases where a black sheep actually is dominant in the spotting locus to the nth degree and maybe has a small black spot showing. I've heard tell of it in icelandics. One way to tell is...did this white sheep come from black or moorit spotted parents?  If so and it's purebred, then you'll have the nth degree effect of those spotting genes.
   Face and leg spots are a still unresearched area...are they the spotting gene if they don't extend to the fleece? I'm not sure. I tend to think that unless the body is spotted, the spotting gene is not being expressed, but I cannot prove this, and perhaps a spotted face is the heterozygous expression of the spotting gene. Or, it may be a separate spotting locus.

E or Extension locus
Also known as dominant black. Not thought to exist in Icelandics/Shetlands. A homozygous white sheep which is bred to any other color will ALWAYS produce a white lamb. If a black lamb results from that mating, then black is dominant and again, something is askew in the lineage.


Every animal has inherits 2 genes at each locus, so the
genotype (actual written
nomenclature or "formula") for a certain grey sheep might be written:

Ag/Aa, BB/Bb, SS/Ss

Translated, this means the Ag/Aa  grey sheep also is carrying a solid color gene (Aa) you can't see because of the grey dominance. It has a black head and feet (BB) but is also carrying the brown (moorit) Bb which is hiding because black is dominant in the B locus. The sheep is not spotted due to the dominant SS but is also carrying the Ss spotting gene which is not expressed because of the dominance of the SS in the S locus.

So the grey sheep in the genotype above is also a half solid-color, half- moorit, half- spotted sheep-or at least has the propensity to throw one, because of the recessives. Sneaky little critters, aren't they?

Put simply, the sperm and egg have only one each of the genes necessary to make up
the chromosome. They donate them at random, depending on what their genotype is.
When breeding takes place, the genes line up and attach (maybe you've seen this on
public television) and then the new life forms, and it will become what the dominant genes
allow it to become.  Here's another word you'll see used:

Phenotype.   This is what the animal looks like with all the dominants expressed
in each locus.  However, as we see above, the genotype paints a much more accurate story
as to what is lurking in the sheep's background.

Incidentally, the proper way to write genotypes is to superscript the specific gene. Instead
of Awt, written in simple text form, the wt would be sitting to the upper right of the A
like in algebraic expression when is something is "squared" or "cubed". So if you see it written differently, it's OK for our purposes here.

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