Have you ever wondered about the genetics behind sheep coat color? Or have you wanted to know how to skew your lamb crop towards being more colorful? Maybe you browsed around our website and realized that sheep come in some really flashy patterns not just white? Or perhaps you have an interest in producing naturally colored wool for hand spinning? Today we are going to start a series of articles looking at what we know about coat color genetics in sheep, and how you can start to predict and manipulate the coat color of your lamb crop.
It’s no secret that lot of people who purchase our sheep are doing so largely because they like the flashy color our flock produces. One of the wonderful things about Katahdin sheep is that their gene pool is full of coat color genetics. Because they are a shedding sheep with valueless wool and a very diverse gene pool, virtually any color is possible.
This can also make them more tricky to work with to produce a consistent result than many other breeds that have a limited number or color genes present in their gene pool. However, once you determine what genes your ewes and rams carry it becomes easier to make crosses that produce a consistent result. Today we are sharing what we have learned from or flock thus far so that you can learn alongside us.
Let’s get started with a basic genetics review
Without going all the way back through a high school biology course, here are a few genetics basics you need to remember as we move forward. Every gene in your sheep has 2 copies. They can be identical or different. There can be dominant genes that hide other genes and recessive genes that can be hidden. There can also be genes that are co-dominant that can both be expressed at the same time. Each copy of a gene is called an allele.
Additionally, there are several different gene locations that interact together to determine what your sheep’s wool actually looks like. To complicate matters more, there are various different gene combinations that can sometimes produce the same color sheep. However, for now we won’t worry about that. That is something we will discuss in a future blog article. We just wanted to mention it because it is something that may require you to do some research and critical thinking to determine what is going on if you have a lamb born an unexpected color.
In sheep like most mammals there are 2 pigments responsible for producing coat color.
They are eumelanin which produces blacks and browns, and pheomelanin which produces reds and yellows. White sheep lack either of these pigments in their wool fibers. In white sheep the expression of the pigment genes are suppressed. However, the genes for these pigments are still present, and all sheep are either a black sheep (eumelanin) or a brown sheep (pheomelanin). This detail will become important later.
There are a few different loci (or places on the DNA strand) mainly responsible for the coat colors in sheep. The first is the Agouti locus, the second is the black/brown locus. Then there is the spotting gene and finally the extension locus. In most cases the extension gene can be ignored. In many sheep breeds it is not a contributing variable. Therefore we will not spend much time on it in our series. However, if you’re working with a breed where the extension gene is common or things just aren’t adding up, its a good idea to consider if the extension locus might be affecting your coat colors.
First let’s talk about the Agouti locus
The Agouti locus is the ‘pattern’ locus. They are responsible for both the color and coat pattern of your sheep. The most dominant Agouti allele (copy of a gene) is the gene responsible for the majority of sheep being white. It is called the Agouti white and tan gene (Awt). It is also the most common agouti allele in sheep. If your sheep has one copy of the Awt allele, the sheep will be white or maybe tan (which typically fades to white in wool breeds). If your sheep is white, it probably safe to assume it has at least 1 copy of this gene.
Now there are also a whole bunch of other agouti genes in sheep, some of which we might not have even discovered and named yet. The agouti genes are responsible for all the other coat patterns seen in sheep (excluding spotting) such as badger face, blue, gray, black and tan etc. You can tell agouti genes from spotting because they produce distinct, symmetrical and repeatable coat patterns you see in some breeds like the Barbados Black Belly or Romney.
While describing each Agouti gene is outside of the scope of this article. It is important to note that Agouti alleles have a hierarchy of dominance. Some genes are dominant (Awt), some co-dominant, and some recessive (self color, Aa). 1 copy of each of two co-dominant genes results in them both being expressed in the animal’s coat sometimes creating unusual patterns.
If your animal has co-dominant genes you might have to do some detective work to sort out which two Agouti alleles they actually have. The American Romney Breeders Association has a good pictorial and written description of some of the most common agouti patterns seen in sheep. If you have a patterned sheep you are curious about, check out their descriptions.
We consider the majority of the Agouti genes to be our ‘enemy’ when breeding for our spotted sheep.
In our flock we don’t want white sheep, or patterned sheep. We are exclusively looking for spotted sheep. To simplify producing consistently spotted lambs, our preference is to have sheep with the most recessive Agouti gene, the self color gene (Aa). This gene is completely recessive and 2 copies of it are required for expression. It allows the natural expression of the black and brown gene we will be discussing next.
If your sheep has two of these Agouti alleles, it will either be a black sheep or a brown sheep. There will be no lurking pattern genes to pop up and surprise you. That’s what we want when we are trying to produce a consistent product. Keep in mind though that you CAN have sheep that are both spotted and Agouti patterned. They are far from uncommon. It’s just not our preference.
The black brown locus is very straightforward
At this locus there are two options. Black or brown. Black is dominant to brown. Either one or two copies of the black gene will produce a black sheep. 2 copies of the brown gene are required to produce a brown sheep. Two black sheep carrying an invisible brown gene could surprise you with a brown lamb. Two brown sheep will never produce a black lamb.
This gene affects skin color as well as coat color. A black sheep will have black skin, and a brown sheep will have brown skin. In the absence of spotting genes, or Agouti pattern genes interfering these animals will be either solid black or brown. Remember though, unless your sheep has two self-color Agouti alleles (or the correct extension genes), the Agouti genes will mask the expression of these color genes. However, every single sheep is technically either a black sheep or a brown sheep regardless of their wool color. This is evident in the pigmented nose leather, skin and hoof color on Awt sheep.
The third locus of interest is the extension gene
While this gene is not usually at play, it can cause seemingly impossible things to happen sometimes. Just like with the black and brown gene, the extension gene offers 2 options. They are the extension wild (Ew), and the extension dominant (Ed).
We said above that the Agouti locus is most dominant and it overrides everything else. White is always dominant to color. We learned that a self-colored sheep is the most recessive option. Therefore, it would seem that two colored sheep are not able to produce a white sheep because recessive genes absolutely cannot hide dominant genes for white. But then sometimes this very thing happens. Why?
What gives? How do two colored sheep produce a white lamb??
As it turns out when two colored sheep produce a white lamb the extension gene is sometimes to blame (the spotting gene discussed later can also be to blame). The Ew gene allows expression of whatever genes are present at the other 3 coat color loci. The Ed gene cancels the effects of the Agouti gene and allows expression of the black brown locus regardless of what Agouti genes are present.
The presence of the correct version of this gene suddenly makes the black and brown gene locus dominant over the Agouti locus. Meaning that a sheep can either be a recessive OR a dominant black. Inconvenient when you are trying to tease out the genetics of your flock. Fortunately, it is thought that most sheep are Ew, and are colored due to the self-color Agouti alleles.
So how do you get a white lamb from colored parents?
As you might expect from the names, the Ed gene is dominant to the Ew gene. As a result, two colored sheep with the genotype of EdEw can be bred together and produce an offspring with two copies of the Ew gene which suddenly allows expression of the hidden Agouti genes resulting in a surprise white or patterned lamb.
This Ed allele of the extension gene causes a complete upset of the dominance hierarchy we have been discussing. If you are dealing with a flock with some Ed alleles floating around the lamb surprises may be many. As you can now see determining the actual genes beneath a your sheep’s coat can get complicated fast, and there is still so much to learn.
Next time we are going to look at the other way two colored sheep can produce a white lamb–the spotting genes. These are the color genes we find most fascinating, and are probably the ones you are most interested in if you are looking to breed sheep that look like our flock.
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