Shetland Color Genetics

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In the beginning there was a solid white Shetland ewe and a solid black Shetland ram. They got together, and in the course of time was born a new ewe lamb. Only the lamb was quite different in color from the parents; she was a lovely medium brown with white spots all over! "How could this happen?!?" demanded the ram. "I don't know, my side of the family has always been all white!" snapped the ewe. The lamb didn't care, she scampered off and played with many half siblings of all different colors.

Note: If you have no background in genetics, it would be a good idea to review the Color Genetics Terms and the Sheep Color Genetics Primer before continuing on this page.

In basic Shetland (meaning most primitive breeds) color genetics, most wool color is determined by two pairs of genes: Agouti and Black/brown. An additional gene, Spotting, determines whether the animal will express irregular white spots on its body.

Introducing the Genes and Their Alleles

Agouti is the gene which determines the "pattern" of which fibers will have color, and which will be white. In the shorthand of genetics, these are the symbols used for each Agouti gene variation or allele:

Awt

stands for Agouti gene, white and tan coloring. It means that the animal is genetically all or nearly all white, suppressing all expression of the Black/Brown gene. It might have a few brownish fibers in its wool or on its legs, head, and tail. This gene is dominant to all other Agouti genes, and bosses over the black/brown gene besides. So mom's family in the example above really could have been all white for generations before a colored lamb, black or brown, might show up.

Agt

indicates Agouti gene, graying and mouflon pattern. In this case, the sheep will be colored on the upper parts of the body, and have white from under the face, over the chest, on the inside of the legs, on the belly, and up to under the tail, in the wild sheep or mouflon pattern. At the same time, all the colored parts will be a mix of white and pigmented fibers by the time the sheep is a year old. According to my understanding of Adalsteinsson's research, this is a single gene. He considers it dominant to all other Agouti genes except Awt.

Ag

means Agouti gene, with graying pattern. This means the animal may be born solid black or brown in color, but that more and more white fibers will mix in until by the time the sheep is one year old, it will clearly be salt and pepper 'gray' or white and brown mixed, which some Shetland breeders call musket or fawn. Ag is recessive to Awt. This means mom, in the example above, could have been Awt/Ag, and the breeder would not know that unless she test bred that ewe for whatever was hiding under the white wool. This gene is recessive to Awt and Agt, equal in strength to Ab and At, and dominant to Aa. When combined in a pair with Ab or At, this gene causes a mix of white and colored fibers in the colored areas of the sheep, while the white areas stay white.

Ab

represents Agouti gene, badgerface pattern. Badgerface is a special pattern that's the reverse of the natural wild coloration sheep have before domestication. A badgerface sheep's belly, on its chest, and the inside of its legs and tail are colored, along with specific markings on its face, while the rest of the body is white, or white/tan. Ag and At are equal in dominance. When Ag is combined with Ab, the colored ares of badgerface are a mix of white and pigmented fibers. When Ab is paired with At, a mostly-white sheep results, with unusual and distinctive markings of color at the borders of the two original patterns of badgerface and mouflon, and a specific pattern of color on its face.

At

is the wild pattern. Sheep which were never domesticated still show this pattern of colored upper bodies and white under parts. It's still often called 'reverse badgerface' because the coloration pattern is the opposite of badgerface. This gene is equal to Ag and Ab, with results described under those genes. It's recessive to Awt and Agt, and fully dominant to Aa.

Aa

symbolizes self-coloration, or solid colored sheep. It's recessive to all other Agouti genes. This means a pair of Aa genes is required for the sheep to be a solid color of black or brown.

Black/brown is probably the easiest of all genes, as it has only two alleles we know of. This gene regulates the production of the pigment eumelanin. It's an off/on situation. If black is on, brown is off. If brown is on, black is off.

BB

is fully dominant. If it's present in a sheep, that sheep is programmed for black color. Of course, that black may be affected by other genes.

Bb

is fully recessive. If it's present with BB, then it won't show up at all except by test breedings. If there's a pair of brown genes, though, Bb/Bb, then the animal's pigmented areas will be brown.

Because of the on/off nature of this gene, if you think you see brown and black together on the same animal, look again! Brown on a black animal is either due to an environmental factor such as sunbleaching (the tips of the fibers exposed to the sun become rusty brown) or else the red/tan pigment, pheomelanin, is showing up.

While black on a brown sheep is usually just an extra-dark brown. And apparently gray wool on a brown sheep is a mix of white and brown fibers. In case of doubt, view the fiber under magnification in direct sunlight. Black fibers will definitely be black, brown definitely brown.

Spotting on sheep is another on/off gene. The shapes of the spots are a different matter altogether! It's important to differentiate spotting from patterns. Spots are usually irregular in shape, are primarily seen on the body, and are not symmetrical. Patterns are usually symmetrical on the body and the face when the face is involved.

SS

represents no spotting and is fully dominant. If a sheep has just one SS gene, it will not express spots.

Ss

is fully recessive to SS. So it takes a pair of Ss genes to make a spotted sheep.

We don't know anything as yet about what makes the spots appear in particular places, or take particular sizes or shapes.

Genotyping Sheep

In the example at the top of the page, a white ewe and a black ram producing a brown, spotted lamb, the genotype of the lamb is surprisingly simple to figure out. It's basically a simple logic puzzle.

First, take a very close look at the lamb, in direct sunlight if at all possible. She's brown, and spotted. So that means she must be Bb/Bb in order to be brown. And she must be Ss/Ss to be spotted. In the areas on this lamb which are brown, look carefully to see if there are any stray white fibers on the lamb's head, tail, above her feet, and on the scrotum if it's a ram. Also look at the lamb's skin where you can see it, especially her lips and nostrils. Is the skin dark brown or a medium brown?

If the lamb has no stray white fibers in the brown areas, and if the skin is dark brown, she's almost certainly a solid colored animal like her dad. So the Agouti gene is Aa/Aa. White fibers or lighter skin may indicate the lamb will 'go gray' in a matter of weeks. In this type of graying, caused by the Agouti Ag allele, white fibers begin to show up among the brown. If these white fibers begin to show, we'll know the lamb must have the Ag gene instead of Aa/Aa.

We now have a tentative genotype for this lamb's coloring: Aa/Aa Bb/Bb Ss/Ss, or, possibly, Ag/A? Bb/Bb Ss/Ss.

But how could the white ewe and black ram produce this lamb? And what if the lamb starts 'graying' in a few weeks?

Let's take a look at mom, assuming we know nothing about her parents or her other offspring.

All we know at first is that she's solid white. That gives us a genotype full of unknowns: Awt/A? B?/B? S?/S?

Now we look at the lamb again: Aa/Aa Bb/Bb Ss/Ss, or, possibly, Ag/A? Bb/Bb Ss/Ss. Note that baby definitely has pairs of recessives in the Black/Brown and the Spotting genes. One of each pair had to come from mom. So that fills in some blanks on mom's genotype. We now know she is: Awt/A? B?/Bb S?/Ss. Note that known dominant genes go before the slash /, while recessives go after the slash /.

We can do the same thing with dad's genotype. We know he's solid and black. OK, so there's a few white fibers sprinkled in his wool--any other dads out there who don't have a few gray hairs with youngsters running around? He has age-related graying, which typically starts after at least the first couple months of life. So those white fibers don't count on his genotype unless he starts graying out very soon after birth. We happen to know he was solid black until he was a year old, so Dad definitely isn't Ag.

This tells us he is truly a solid-colored animal, so he has a genotype of Aa/Aa BB/B? SS/S?. Note we could fill in more holes with him than with mom. Like the lamb, he has a pair of recessives. But because he's black, which is fully dominant to brown, we don't know if he carries a Bb or not. We can tell by looking at him that he's not spotted, so that gives him a SS gene for sure. Because mom is white, we can't tell what she has for spotting.

Now we can look at the lamb's genotype again and we find it again fills some holes in her father's. Remember the little one's genotype? Aa/Aa Bb/Bb Ss/Ss, or, possibly, Ag/A? Bb/Bb Ss/Ss.

Those recessives of Bb/Bb Ss/Ss tell us dad gave his daughter a Bb and a Ss. So dad's genotype rounds out to: Aa/Aa BB/Bb SS/Ss and he's all figured out.

It's time to revisit mom's last known genotype: Awt/A? B?/Bb S?/Ss. If we find her daughter is Aa/Aa, then that plugs one hole on mom's genotype--her Agouti gene pair is now known to be Awt/Aa because her lamb must get the Aa allele from both parents to be expressed. On the other hand, if the lamb's genotype at Agouti turns out to be Ag/Aa because she 'goes gray' early in her first year, then we know, again, that mom contributed the Ag since dad had none to give. In that case, mom's Agouti pair is Awt/Ag. Time and close observation of her offspring will tell which is correct.

To summarize what we've figured out:

animal	genotype
lamb	A?/Aa Bb/Bb Ss/Ss
ram	Aa/Aa BB/Bb SS/Ss
ewe	Awt/A? B?/Bb S?/Ss

Shetland Colors and Genotypes

These methods can be applied to many, but not all colors, in a Shetland or Icelandic flock. Some common fleece color names, with descriptions and possible genotypes are:

Note I haven't made an effort to classify degrees of graying, or the colors mioget, fawn, or dark brown. I have some ideas regarding how these colors are formed, but until I can either breed some of these in my flock, or talk with other breeders of these colors, I can't be certain what genotypes fit these colors, particularly since the colors themselves are not well defined. Then there are colors such as apricot, which show up quite frequently in Shetland flocks, but are not official colors. Test breeding could give us more information about how these colors are genotyped.

Test Breeding

Looking at our white ewe again, with a genotype of Awt/A? B?/Bb S?/Ss, how can we fill in the blanks on her genotype? We can arrange test breedings over a few years, and look at her offspring.

In this case, we want to know if she's got Ag or Aa hiding behind the white Awt. If she's bred to a solid colored ram Aa/Aa, but the offspring is not white or solid but gray (colored and white fibers mixed throughout the wool), then the ewe is Awt/Ag. If the offspring of these matings stay solid colored (other than age-related graying), then the ewe is Awt/Aa.

What about the black/brown gene? Is she carrying black, or only brown? If she is bred to brown or musket rams, and produces a black lamb at any time, then she is BB/Bb. If she always has brown lambs, that's a good argument that she's Bb/Bb, but you can never be 100% sure. Theoretically, if she is BB/Bb, then 50% of her lambs will be black when bred to a brown ram. But since the chance of a given color would be 50-50 every time she has a lamb, the odds could be skewed toward brown, just as tossing a penny can come up all tails for quite a while occasionally.

Figuring out her spotting genes is about the same as figuring out her brown/black genes. We know she is S?/Ss because she had a spotted lamb. If, when bred to a spotted Ss/Ss ram, she produces a lamb without spots, then she would be SS/Ss. If all she produces are spotted lambs when bred to a spotted ram, then she's probably Ss/Ss, but you can never be 100% certain.

Conclusion

If you love logic puzzles, color genetics is a fun, if sometimes frustating, hobby. Careful observation of the lamb at birth, as it grows, at one year of age, and older are essential clues as to the genetics behind its coloring. Careful recordkeeping are important, to keep track of these clues. It's always rewarding to be able to track backwards from a given lamb to its grandparents or even greatgrandparents and pinpoint which ancestor contributed a particular gene.