Last time we started talking about sheep coat color genetics. We discussed the Agouti or pattern genes which are mainly responsible for your sheep’s coat color and markings. We also discussed the black brown genes and the extension genes that sometimes play a role in coat color. Today we are going to discuss our favorite genes, the spotting genes. The spotting genes are the highly unpredictable wild cards that make each one of our sheep a unique individual.
Spotting is the most poorly understood of the color genes in sheep. There is a lot we don’t yet know. What we do know is that there is a spotting gene locus. It is likely that there are a lot of other yet to be identified genes and or modifiers responsible for the myriad of spotting options we see in sheep.
Generally speaking spotting is recessive
As we learned last time, this means that your sheep needs two copies of the spotting gene to be spotted. It also means that a non-spotted sheep can be invisibly carrying spots that can manifest themselves in the lambs if crossed to another spotted individual. This means that once you have spotting, it is very easy to keep spotting. Spotted sheep crossed to spotted sheep must produce other spotted sheep.
This is where easy to predict ends. In our experience how much spotting, and what the actual markings are is near impossible to predict. However, we have seen a trend towards a percentage, maybe about 1/3 of the lambs, having markings almost identically mirroring their dams. You also might see commonly repeated markings appearing on lambs if you do the same ram/ewe cross repeatedly. This leads us to believe spotting is controlled by various genes and modifiers and it is not completely random as the single spotting gene theory would lead you to believe.
There is also a commonly held theory that if you keep breeding spotted to spotted you will eventually end up with a white sheep.
The idea being that spotting is additive, and that every generation would accumulate more spotting genes/modifiers and eventually have so many of them no pigmentation is left. This idea may have come about because there are a variety of sheep breeds out there carrying the Awt gene, but who are also one big white spot as evidenced by their pink skin and hooves. Wool from the spotting gene is whiter than wool from the Awt gene, so this was advantageous to select for historically.
We are not sure that this addition theory seems to hold true in our flock. We have some very spotted ewes that we breed to very spotted rams that consistently produce nicely marked lambs. We also have a mostly black ewe who when crossed to our spotted ram frustratingly always produces white lambs. These lambs are one huge white spot with pink skin. This leads us to believe that it might be a certain combination or combinations of genes rather than a straightforward additive effect that causes whole body white spots. This is something we are very interested to keep observing.
There are some spotting genes that we do know about and that have been named.
The most common one that most people agree on is the spotting gene, the HST gene, or the head, socks, tail gene. As you can probably guess from its imaginative name, sheep with this gene will have spotting on their head, hind legs and tail tips. Unlike most spotting, this gene is actually dominant. It is our belief based upon observation and research that one copy of this gene will produce head spots (sometimes no more than a small white patch on the top of the head). Two copies will produce the additional socks and tail markings.
This gene is thought to be the ‘gateway’ to allow other spotting to occur. It is always present if body spotting is present, but can be expressed in the absence of body spotting. As to whether it is modifier genes working on this gene that produces the additional spotting or separate genes that are restricted by this gene we aren’t sure.
It is believed that there is also a pigmented head locus with four allele options
They include no white markings, the persian allele, the turkish allele, and the afghan lethal allele. If you have ever seen a black headed Dorper, then you are familiar with the persian gene. When a sheep has two copies of this gene you get the black headed look. Where the color change line occurs varies depending on breed, and it can be as far back as around the middle of the animal. It is assumed that this variation is due to the accumulation of different modifying genes in different breeds.
You may also be familiar with the turkish allele. It is quite commonly seen in spotted Katahdins. The presence of 2 copies of this gene is responsible for the markings on sheep that have the colored circles around their eyes, with pigmented ears, muzzles, and legs. It is thought that if this gene is heterozygous, then it produces a diamond on the tip of the nose and top of the head.
Another neat marking pattern that is seen fairly commonly in some breeds is ticking.
There are just two allele options, ticking and no ticking. Ticking is thought to be dominant and is responsible for small colored spots to occur on white areas that are white due to the spotting gene. Ticking is unique in that lambs are not born with the spotting. The spotting develops later.
As we move on to other spotting patterns things become less clear. It is thought that modifiers of the spotting gene cause common markings like white bibs, neck and belly. Unfortunately, for many of the other body markings we are much less clear on how and why we get them.
One such example is the smut face or brockle look
It is a pretty common occurrence in the Katahdin breed, and a bit confusing so we thought it would be worth mentioning specifically. These are the white sheep with dark chocolate brown or black speckles on their faces and legs. Unfortunately the genetics behind this look is thus far not understood. What we do know is that these are black sheep who have the Awt gene. Some modifier or gene is allowing the sheep’s color to bleed through the Awt mask, but only on the face and legs. They are not actually ‘spotted’ sheep.
Something else confusing you might run across when breeding for spotting in hair sheep in particular is a brown or brown spotted sheep that isn’t a recessive brown. Sometimes Awt sheep can and do look like recessive brown spotted sheep. How?
To understand we have to go back to look at what Awt stands for. If you remember, it is the Agouti white and tan gene. The majority of Awt sheep are white, or fade to white, but not all. Tunnis sheep are a good example of the exception. Hair sheep are also capable of maintaining rich pigment from the Awt gene. This is another case where we have no idea why you get tan vs white. Nor do we know why it fades to white in most wool sheep but not the Tunnis. It is assumed to be yet to be identified modifier genes. Others theorize that it could be completely different Agouti genes.
Somehow in this scenario where the Awt is expressed as tan, spotting genes can affect the tan to produce a sheep that looks like a recessive brown spotted sheep.
It is thought that a sheep that has 1 copy of the spotting gene will produce a brown spotted sheep similar to the ewe pictured here. Two copies of the spotting gene will produce an animal that is pure white or almost pure white like her lamb.
Assuming this to be true, we are suspicious that these spotting genes/modifiers are different than the ones affecting the recessive spotting. This is because our single allele HST gene ram that is carrying whatever this spotting gene is fails to produce any spotted lambs when crossed to our recessively spotted ewes.
The amazing thing about spotting is that there is so much left to learn, and so much research left to be done. You might even be able to uncover some useful new information if you keep good pictures and records of your flock. If you are interested in reading in more depth, ‘The genetics of color in fat-tailed sheep: a review’ by Roger S. Lundie has some of the most comprehensive information on sheep color and spotting we have found.
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