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Sheep Color Genetics Terms

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This is a list of terms that I'm defining as a layperson for amateur color geneticists. That is, I want these terms to be accurate but also understood. As always, email me, if you have questions or critiques. In general, these terms apply equally to all types of mammals and, in many cases, other living beings as well, but I am sticking to sheep for the most part. Terms in bold are defined in the list. I tried to put the terms in a fairly logical order so concepts build on each other.

Color Genetics:
The theories people have come up with to describe, explain, and predict the colors animals show. In this case, the wool and hair on sheep.

A tiny packet of information that tells an organism how to do something. In the case of color genetics, the genes tell an animal what colors to show, and how to show them. Each new creature receives one gene of each type from its mother and one gene from its father; therefore genes always consists of a pair. The pairs of genes then determine what that animal will look like. Note this means mom has two genes and gives one more or less randomly, and dad does the same thing. There are four main color genes I will discuss on the genetics pages. They are defined below.

This is one varient of a particular gene. Some genes may have only two variants, while others may have six or even more. In some cases, alleles may be proposed for one gene which turn out later to be the result of a different gene. Just part of what keeps color genetics interesting!

Usually there will be one gene which always dominates over its companion in a pair. Occasionally a gene is 'incompletely dominent', meaning that the second gene of the pair will have some influence over how the two are expressed.

It follows that if one gene is dominant and determines what the gene pair will do, the other is recessive. Although it may not be boss now, if given to the sheep's offspring, it may be dominant then, depending on what the offspring get from the other parent. Recessives do sometimes speak up and express themselves--when both of a pair is recessive, then they speak loudly indeed.

The description of how the sheep looks to the observer. It's important that the phenotype be very well described. Nuances as simple as whether the brown is a cool brown or a warm brown can make a big difference in our guess at the color genetics of that sheep. Another important clue is the appearance of a lamb at birth. Some colors fade for various reaons very quickly after birth. Yet another set of clues is the colors of the sheep's ancestors and offspring, in as much detail as possible.

The genetic explanation for what makes a creature look, grow, and to some extent, act (instinct), as it does. In this case we're just concerned with the genotype for the sheep's color. See our scrapie page for information on genotyping for scrapie resistance.

In color genetics, expression isn't how well the animal communicates emotion, but how its phenotype looks. If the animal is black, for instance, it is expressing eumelanin.

A chemical which migrates into hair and wool fibers and gives it color. Pigment production is turned on and off by hormones, which in turn are controlled by genes.

The pigment which makes animals black or a cool 'chocolate' brown. Important: If an animal looks like there is both brown and black in its fiber, and the cause is not sunbleaching, then the 'brown' is not caused by eumelanin. The cause is most likely a dark form of pheomelanin. Mammals cannot express both black and brown forms of eumelanin at the same time, even on different areas of their bodies.

The pigment which makes animals reddish brown through pale yellow--blonde in humans, 'tan' in sheep.

The gene that determines whether or not a particular fiber of wool will be colored. There are several variations on this gene. Each is considered a 'pattern' because it's essentially symmetrical in its expression on the sheep. The word 'agouti' comes from the name of a South American rodent that was the subject of some of the first serious color genetics work done.

When the pigment showing is eumelanin, the sheep will express either black color or a cool brown color. Because of the way eumelanin works, it can't show both black and brown from eumelanin at the same time. This gene is a simple 'on' for black and 'off' for brown or vice versa.

This gene may not exist in sheep, though it is common in some other animals. It acts to clump pigment so that some of the clear keratin of the fiber, reflecting white, shows in the spaces between the clumps. This makes the color appear 'dilute', or lighter than normal. Some people consider this gene to be called 'distribution' because of the clumping. Either way, a black sheep will appear 'blue' gray (not black and white mixed, but solid plain gray), while a brown sheep would appear a solid light brown (not brown and white mixed). The key to determining whether a particular sheep is expressing dilution is to check its 'points'. The fiber on its hair and legs should also be the 'dilute' color. If not, then dilution is not what is making a blue gray or light brown color.

Very little is known about the action of the extension gene in sheep. Modern sheep breeds such as Romney, if colored, are expressing a pair of 'dominant black' genes, which is an extension gene. I think extension may be doing more than just dominant black, but my theories remain theories until they can be thoroughly tested.

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Believe it or not, that's all the basic terminology needed to understand sheep color genetics. It's also helpful, though, to define the colors we'll be talking about. There are three ways of looking at color related to sheep color genetics:

  • The color we see when we examine the animal and its fiber
  • The name that color may have when we register the animal or talk about color with other breeders
  • The actual pigment, or combination of pigments, which produce that color

I've already defined eumelanin and pheomelanin. They are the actual pigments which produce the colors we see. And I've listed the registerable colors of Shetland sheep on another page, with photos of actual wool samples I've collected. So here I will discuss color words we use to describe fiber when we examine it, on or off the animal.

Eumelanin produces a lovely deep, dark, pitch black when the genes are right.

A cool, fairly dark brown results from the brown side of eumelanin.

Warm, fairly dark, reddish brown is called moorit from old Norwegian term meaning 'red as the moors'. Commonly today it's used to mean any brown. It's my opinion that a red brown comes from the interaction of eumelanin and pheomelanin pigments. This is not standard color genetics, but my theory about color in sheep.

This is one of the most commonly used and misused terms in color genetics. It can mean different things to different breeders. In my work, 'gray' is not enough by itself, unless used only as a registration color name. I use other descriptive terms to show exactly what type of gray is being discussed.

Salt and Pepper Gray:
A blend of white and black fibers, which looks gray from a short distance. Under close scrutiny it's obvious that the 'gray' is a mix of black and white.

Blue Gray:
A gray in which every fiber is uniformly colored the same solid gray color, even under magnification. There is no mix of white or black in the wool. This gray would be the result of the dilution gene, and the animal's points (head, legs, tail, which are generally covered with hair rather than wool) would be the same color, if this type of gray exists in sheep. This kind of gray is common in other animals, but I haven't seen any sheep's wool that matches this description.

Age-related Graying:
As a sheep grows older, it will get 'gray' just like older humans do. That is, more and more fibers will be white in color, making the overall color of the wool appear more 'gray'. Because of the confusion with the other 'grays', I specify age-related graying when this process is what I'm referring to. In sheep, it usually becomes evident after one year of age, and increases as the sheep gets older.

Another term that has different meanings for different people. Some feel any mix of white and brown fibers is a fawn color. Others define fawn as being the result of the dilution gene working on a brown base color. I haven't personally seen fawn that might be the result of dilution, and I don't use fawn to describe brown and white mixed, so fawn is not a term I use much.

This isn't so much a color as the action of sunlight on pigmented fibers. Generally the color changes to a lighter, warmer color than the original, at the tip of each lock of wool. Black sunbleaches to rust, cool brown sunbleaches to yellow, etc.

Another term that's not really a color in itself, and which has many meanings. What I call sunbleaching, for example, many people call fading. Many people also use the term fading to indicate the loss of pigmentation that occurs over the life of the sheep; ie, age-graying. I don't use fading at all because of these confusions.
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