Copyright, 2007
The spotting gene that has been identified is relatively simple. Designated "S", it is either dominant SS (no spots) or recessive Ss (spots). Sheep with an SS/SS genotype will not produce spotted lambs. Sheep with the SS/Ss genotype can produce spots or not. Sheep with the Ss/Ss genotype are spotted. The amount and location of the spots appears to be controlled genetically, but the responsible loci have not been identified.
Gotland sheep were developed as pelt sheep. The wooled area of the sheep is severely downgraded if white spots are present. Even today, the grading criteria for Gotland sheep in Sweden and in the UK, places high value on even color through out the wool. Another factor that is highly desired is a clear, blue-grey color. This is associated with bright white, and not creamy white fibers. It is interesting to note that bright white has been associated with spotted, white sheep. What?... you say? How can a white sheep be spotted??? Well, white is a "pattern", inherited separately from "spotting". A white sheep can be genetically spotted, even though you don't see the spots. (Genetically: Awt/Aanythingelse Ss/Ss)
When colored sheep are produced from bright white flocks... (remember that "solid" recessive pattern, Aa? - white sheep can carry that recessive "solid" pattern for many generations, then pop out a colored sheep when both sire and dam carry the Aa gene)... those colored sheep are almost always spotted! Spots are carried at the "S" locus, and, as you may have guessed, visible spots are recessive.
A little more in depth (you can skip this paragraph if your eyes start rolling....) - remember the pigment phaeomelanin - responsible for tan color? It is thought that phaeomelanin is the culprit (or benefactor, depending on your point of view) behind the creamy white color. It has a modifier gene that controls its intensity (not covered here). It is thought that the spotting gene - S - affects how much the tan color is expressed. SS/SS sheep have a deeper tan color than Ss/Ss sheep. If you want blue-grey, you want as little phaeomelanin as possible. So, maybe you want spots!
There are spots and then there are spots. For pelt sheep, if spots are limited to the head, legs and tail tip, the pelt itself is unaffected. That would be okay. In fact, many Gotland sheep have these spots. The ewe below has a very clear blue-grey fleece - and a white spot on the top of her head. Her lambs have white on the top of their heads, and the one on the left has some white spotting on its muzzle. Very often these white top-of-the-head spots dimish in size to be almost unnoticeable in adult sheep- the sheep are still genetically Ss/Ss. Notice that the lambs' fleeces aren't yet obviously grey.
The spot on the neck and chest of the Finnsheep ewe below would certainly show up in the pelt and degrade the color quality of the pelt! There is yet another gene(s) that controls the extent, location and size of white spots. Suffice it to say that size and location of white spots is hereditary, as well as whether or not the sheep is spotted.
I'm going to slip another gene in here, since it's a short discussion. There is one gene that turns off the pattern gene. It is designated E, and is called "Extension". You might hear "dominant black" or "extension dominant". All this means is that whatever pattern is present in the sheep is not visible. As you might guess from the name, this "pattern off switch" is dominant. A sheep only needs one ED copy from one parent to turn off the patterns.
What does that mean? How can you turn off a pattern? Well, remember that every sheep is BLACK or BROWN. When a sheep is also Extension dominant, it will actually be black or brown - no white, no badgerface, no mouflon, NO GREY. Extension dominance has been identified in the gene pool of the longwools, in Shetlands and in Finnsheep. It is not common in Shetlands in North America.
Extension dominance does not affect spots, only patterns at the A locus. The Finnsheep ewe above is Extension dominant with spots. How do I know that? Well, she produced two white lambs when bred to a Gotland sire. Two black sheep producing two white lambs, hmmmmm. If you followed the earlier discussion, neither parent should have AWt in their genetic makeup. You might guess they'd be BB/BB Aa/Aa (solid black) and BB/BB Agg/Agg (Gotland Grey). We have assumed the Gotland is BB/BB Agg/Agg, so it must've been Mom's fault the lambs are white! She would have to be at least AWt/Aa, and might be AWt/AWt. The reason is that she's also ED - which turns off her (white) pattern gene. The good thing about her white offspring is that we know they're not extension dominant because the white pattern is turned on!
So what is the significance of ED in our Gotland sheep upbreeding program. Well, besides the fact that it covers up our desireable Agg, it can hide At, Ab, and even AWt. We want homozygous Agg/Agg! Further, there is a modification gene (another one of those pesky genes that is not identified but everyone knows it exists) that causes "age- greying", sometimes called "silvering" in longwools. Age greying looks a lot like Ag in adult sheep. The lambs tend not to have sugar lips, and it tends to proceed more slowly than Ag, and is sometimes less evenly distributed than Ag, but it is very easy to mistake an Age-greyed sheep for an Ag sheep. Well, darn.
The Blue-Faced Leicester x Border Leicester ewe below is Extension dominant with a silver/grey fleece due to age-greying. How do I know she's ED? Well, she produced a white lamb when bred to a Gotland ram! How do I know she's greyed because of age-greying and not the pattern Ag? Well, she's ED, so the pattern gene, including grey, is turned off. Doesn't her color look a lot like Gotland Grey? The good news is that her white offspring are not extension dominant.
Non-extension dominant sheep are designated E+. So a sheep showing extension dominance can be ED/ED or ED/E+, and any sheep with a visible pattern is E+/E+. Solid black, grey, brown or brown-grey sheep are actually unknown by looking at them. (If you are a geneticist, bothered by the E+ designation of a seemingly "recessive" allele, there is a third allele at E, the real recessive - Ee. It is not present in the Gotland upbreeding gene pool - whew!)