This primer is the quick and dirty (and hopefully not too confusing) version of the Icelandic/Shetland color genetics theory.  You can cut and paste these next 5 pages onto a Word or similar text document for easy reference.


Locus: the regular lineal position occupied in a chromosome by any one gene or its allele. (So it's a location basically.)
Gene: A unit of heredity, generally considered to be located (i.e., locus) on a chromosome in the nucleus of a call.
Allele: one of two or more forms of a gene that occupies a particular locus on a chromosome. 
Homozygous: means the lamb has inherited two of the same genes. Sometimes if a
white sheep has two white genes in the A locus, for example, they will only throw a
white lamb, and folks might call this animal "dominant white."
Heterozygous: means the lamb has inherited two different genes then the rules of dominance come into play.

Some of  the above definitions came from an eleven pound scientific encyclopedia, not from my memory. Don't let these three terms scare you. Locus and gene I'll use quite a bit.  Icelandic/Shetland sheep colors and patterns are determined by two major color loci, locii, locuses, or whatever the plural is. Locations, OK?  Major ones are "A & B". Minor one is "S."

The "A" locus is the pattern locus. It's the major factor determining if your sheep will be white, grey or fawn, badgerface (katmoget in Shetlands), reverse badger or moufflon (gulmoget in Shetlands)  or solid black or brown (moorit). All the different expressions of pigment from pure white to zero white as with a black sheep are
patterns. Yep, grey is a pattern.

The "B" locus determines if your sheep will be black or brown factored. Even white sheep are genetically black or brown. White sheep can hide everything.

The "S" locus is the spotting locus, more properly "white markings". I like to think the A locus evenly distributes the amount of white fibers, while the S locus can make great big addition to the A locus, not instead of.


The A, or "agouti"  locus

In order of dominance, here's how they come through:
Awt (Icelandic A1) is the white (some call it white-tan for the tan pigmentation that sometimes appears which is not related to the B locus, below.)
Agt  (Icelandic A6) is grey moufflon (this is common is Icelandics, I don't think common at all
    in Shetlands, not sure if they even have this on Foula.)
Ag  (Icelandic A2) is grey, and there's quite a range of dark grey to light grey. Lighter grey appears dominant according the the dominance of pigment prohibition theory.
Ab  (Icelandic A3) is badgerface or Shetland katmoget
At  (Icelandic A4) is  reverse badger or black & tan also Shetland gulmoget.
Aa  (Icelandic A5) is solid black or brown, also called non-agouti. This is the most recessive!

Adalsteinnson says that in the A locus, the inhibition of pigment production dominates
over pigment production. I also read somewhere that in the Agt, Ag, Ab & At
combinations, pigment inhibition is dominant but some appear co-dominant. Don't
let that scare you. By the way, if you have two black or moorit Icelandic or Shetland or Finn sheep which produce a white lamb, something is  wrong or there has likely been a mutation. I'd guess there's another breed in the lineage.

I've looked through the Shetland flockbook and notice there are several instances of pigmented parents producing white offspring. My guess is that the "white" offspring produced is actually a very light grey as most folks don't determine sheep color by birth color when they register. Just because the grey fleece fades to white, that does not mean the sheep is genetically is actually a very light expression of grey.

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