Other Sheep Color Genetics
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Breeds of sheep which arose when shepherds were selecting for white wool are generally seen mostly in just that color, white. Occasionally you find a 'black sheep', and since that term is used as a euphemism for a member of the family that is not approved of, we have a clue as to how welcome colored sheep are in white flocks!
Today handspinners have a greater appreciation of naturally-colored wool. Environmentally, too, it's nice to have a variety of colors without using any potentially dangerous chemicals. So colored wool is making quite a come-back.
The color genetics of breeds that are normally mostly white wooled is a bit different from that of breeds that never had color bred out of them. All sheep retain the same basic color genes (see Color Genetics Terms and Sheep Color Genetics Primer for an overview of terminology and basic genetics). In addition to the agouti and black/brown, sheep have a gene called "extension".
The extension gene controls whether or not the agouti gene will have a chance to do its job in determining whether the sheep is white, gray, or colored in other patterns. Extension is known to come in two forms in sheep. The dominant form, called "dominant black", is written Ed. The recessive form is written E+.
Most sheep of both primitive and modern breeds carry the recessive pair E+/E+. This allows the full expression of agouti. So breeds that have been selected to be white will have genetics something like: Awt/Awt B?/B? E+/E+ meaning it will look white and you won't know what the black/brown gene is hiding without some crossbreeding. While breeds such as Shetland may be something like: Ag/Aa BB/Bb E+/E+ and (with this sample genotype) be salt and pepper gray.
If the Ed gene is substituted for the E+, though, a different tale is told in the wool. The white, modern breed, sheep in the example above is now going to be either black or brown, even though the agouti gene told it to be white, because 'dominant black' Ed bosses the agouti and tells it to be quiet. Shetlands are not believed to have Ed. If they did, Shetlands known to carry Awt, Ag, or any of the other agouti gene alleles, would not be white, gray, etc, after all, but be a solid black or brown color, depending on their black/brown genes.
We had a first-hand experience of this when we bred our black Border Leicester ewe to a brown Shetland ram. The resulting twins were white and black. The black I could understand, but white? After learning more about dominant black, I realized Mary's genotype must be along the lines of: Awt/A? BB/B? Ed/E+ Bred to Alladin, genotype Aa/Aa Bb/Bb E+/E+, the black lamb must have been A?/Aa BB/Bb Ed/E+ while the white lamb would have been Awt/Aa B?/Bb E+/E+. Mystery solved, although it's taken me a long time to fully understand it.
Other theories have been hypothesized regarding the extension gene, particularly in reference to controlling the expression of pheomelanin vice eumelanin. To date, none of these theories have been verified with test breedings well enough to consider here. I do feel that the 'apricot' lambs born in registered Shetland flocks are expressing much more pheomelanin than usual, and I wonder if it's not an effect of another extension gene, but not enough is known yet to explain it.
Many thanks to the members of Sheepfirstname.lastname@example.org for my new-found understanding of extension and how it works. Special thanks go to Dr Phil Sponenberg, who patiently explained why my whacky extension theories didn't work.