Horse Color Genetics: The Silver Gene

Review: E/e=black or red, A/a=bay or black

Dilute genes are any genes that make the horse turn a lighter color, like dun, cream, or champagne. Silver is sometimes put in the same category, but I personally don't think of it as a dilute, but more of a modifier.

Silver, similarly to agouti, only works on horses with dominant extension (E/e, or E/E). A horse must have black pigment for silver to be 'active'. It won't show up on a horse with red pigment (e/e).

What exactly is silver? A horse with black pigment and the silver gene (abbreviated with a Z) will always have a significantly lightened mane and tail, which can range from a silver color (hence the name), to a blonde or cream color. Sometimes even a bizarre orange color, like the Morgan below.

All black pigment, whether on just the legs or over the whole body, depending on whether the horse is black or bay, is usually lightened to some degree. The normal color is sort of chocolate, but it can be much darker, but not quite pure black. The one below is on the darker end. If it is a bay horse, then the whole body will not be lightened, but the black on the legs will.

Silver is a simple dominant gene, with exceptions. One copy or two dominant copies won't make a difference, the horse will look the same. The exception is a horse with red pigment will not be silver, like I mentioned before. There is a catch, and that is that red horses, or horses with recessive extension, can still carry the gene. A red horse can have the silver gene and hide it, since there is no black pigment to lighten. The pony below could carry it, and when crossed with a black or bay horse could produce a silver baby.

When paired with a black horse, it is called 'silver dapple' or sometimes 'black silver', and with bay, 'silver bay'. Basically, add on silver to whatever the color is, with the exception of silver dapple. Silver buckskins are very pretty, although unusual. Not neccesarily rare, but silver is not very often paired with other modifiers, but it is possible. The first pony is a silver bay, and the one below is a silver buckskin.

Flaxen chestnut, which can be a very dark color, is sometimes confused with silver dapple. Because a red horse can range in color from bright red to a liver color, and then paired with a cream mane and tail, the darker flaxen chestnuts are sometimes easy to mistake as silver dapple, and vice versa. However, the silver gene usually causes the mane and tail to be not nearly as white as a flaxen chestnuts. The horse below is bay roan silver.

The color has been partially isolated in particular breeds such as Rocky Mountain, Morgan, Shetland, Miniature, and Icelandic. It can also be in Quarter horses or other breeds, but is more unusual.

Using Horse Colors to Know a Purebred

There are certain horse registries where only certain colors are allowed. Most registries have colors that are not neccesarily 'illegal', but just aren't present in a purebred. Using this information, you can have a clue as to the breed of the horse, and also to recognize a non-purebred when you see one. This could also be useful in being angry at model horse makers for using colors that are 'illegal' in certain breeds.

Let's begin with Arabians. Most Arabs are recognizable, yet the colors allowed are very limited. The coat colors allowed are:

Base colors (red, black, bay, brown)

Gray

Roan/Rabicano (it appears the AHA identifies rabicano as roan, although true roan is not apparent in the breed)

Sabino (Although not SB1, as yet unidentified)

Dominant White (careful when identifying, this can be confused for other forms of pinto)

Colors NOT allowed:

Cream (that includes palomino and buckskin)

Dun

Champagne

Appaloosa

Tobiano

Frame Overo

Splashed White

Roan

Silver

Here is a stallion that appears to be pure Arab, but his coat gives him away. The owner states that he is 7/8 pure Arabian, which is about as good as it gets in terms of color and breeding.

Most of the dominant white coats actually look more sabino compared to several of the other dominant white mutations, so they are a little more recognizable than other pinto patterns.

Quarter Horses:

The AQHA is very forgiving, and are annoyingly un-meticulous about the registry colors. There aren't really any rules. They can come in every color under the sun, but AQHA doesn't allow loud pinto patterns. Cropout-producers would be allowed, which honestly isn't fair because purebred Quarter horses could carry something like sabino and produce a loud sabino foal that couldn't be registered.

Thoroughbred:

The Jockey Club is a little unusual in that there are no genetics taken into account. Palomino is allowed, yet buckskin or any double dilutes aren't mentioned. The Jockey Club seems a bit indifferent to actual color. If another color is present, they wouldn't care about the color; rather that the foal would not be purebred.

Colors allowed:

All base colors

Palomino (am I the only one who wants to know why there aren't more of these?!?)

Gray

White (this may be double-dilute, or dominant white. Quite a few white patterning mutations are found in Thoroughbreds)

Colors Not Allowed (or simply not present)

Dun

Champagne

Rabicano

Pinto (including, I believe, SB1, meaning unusual markings wouldn't exist)

Appaloosa

They also mention roan, but it is listed with gray almost as though the process of graying could count as roan. Whether roan is actually apparent in Thoroughbreds I am unsure.

Mustangs:

Because Mustangs are allowed to just breed as they want, almost every color is allowed. Again, whether the actual gene is available is the question. I don't think any genetic studies have been done on Mustangs, other than assuming their color genes are the same because they originated from the other horses. The BLM is not really concerned about the genetics, because most people do not buy their Mustangs for breeding purposes. There aren't any rules either, so I am going to have to guess as to which colors are 'available'. Also some areas of the BLM have more color variation than others. The only color I think isn't present at all is champagne, which I believe is a domestic mutation.

Tenessee Walking Horse:

The TWHBEA is relatively loose, and Tenessee Walkers generally come in a rainbow of colors, which can make for very interesting breeding options. 

The colors allowed: 

All base colors

Cream

Champagne

Tobiano

Sabino

Roan\

Dun

Silver

Grey

'White' (it states in the rules that white is rare; I'm going to guess that they are referring to some form of dominant white)

"Overo" 

There are several things I was surprised about; I have never heard of a dun TW, but upon Googling it, there are some, but not many.

Another possibly confusing term is 'overo'. I've seen many tobianos...

And some sabinos....

But did not know that 'overo' was a possibility. The question: which one? Sabino has already been listed, so it has to be either splash, or frame.

This one looks frame, and it turns out there are quite a few of them. I can't find anything on splashed white.

To wrap all that up, the only color 'out' is appaloosa, and even then there is a registry for spotted gaited horses, a 'Walkaloosa'. 

There might be a second post in the future, talking a little more about several registries with unusual color rules.

Why the "Strong Gene" Theory Doesn't Work

While looking at stallions available, I have heard many breeders make the mistake of assuming that a horse has 'strong genes' for a certain color.

For example: one breeder owned a APHA paint stallion. They said that 90% of his foals (he had quite a few) were paint-marked. Therefore, the paint genes were 'strong', but they couldn't guarantee future offspring would be marked as well.

By using what we know to be common sense in genetics, there are no such things as 'strong' genes. There are only heterozygotes or homozygotes, making either a 50% chance of a gene or 100% chance.

Now, 90% paint-marked foals from a heterozygous stallion may seem like a really large number, but you have to remember that there are other things that could contribute to this.

1. Breeding paint to paint could mean that whatever paint genes the foal inherited could have been from the mother. 

2. The stallion is truly homozygous, but some foals were so minimally marked they were recognized as solid. In which case, that foal could produce a crop-out paint and cause a lot of confusion.

Another common mistake among breeders is to assume a horse that has like-parents produce homozygous. For example: a black horse has two black parents. Therefore he is homozygous for black.

This is also not true. If one or both parents were heterozygous for black, then the resulting foal could be heterozygous or homozygous.

All this to say: if you choose to breed and color means something to you, don't believe anything the breeder says about the numbers. Unless you use common sense to figure it out (i.e. look at other offspring and parents), or he has done a genetic color test.

For more reading on the basics, go here: http://michaelashopeandhorses.blogspot.com/2014/11/horse-color-genetics-re-explaining.html

For more reading on crop-out paints and why they happen, go here: http://michaelashopeandhorses.blogspot.com/2014/10/horse-color-genetics-identifying-crop.html

An Interesting Bit on Albinism

My dad was recently talking to me about albinism because one of his insurance customers has a grown albino child. He was very interested to hear about albino, and also was fascinated when the man told him that everyone has the possibility to produce albino. This is not exactly true, but it is close.

Everyone has heard of various albino creatures, and most people know that it is a recessive gene. But does anyone actually know what that means?

Within people, there are several different kinds of albinism. The exact way of defining it is blurred by what we see in animals. According to NOAH (National Organization for Albinism and Hypopigmentation), most people with albinism do NOT have red eyes, which is often the most defining characteristic that we think of. What else is partially myth to albinism, and what actually causes it?

Albino is not a gene in itself, but what causes it is the gene that creates pigment. Similarly to any other genetics, when both genes are turned 'off', it creates no pigment. The reality of what no pigment means can actually vary widely within the individual. Some people do not even appear 'albino' by our standards. I am now hesitant to add any photos, unsure of what the truth about each individual case whether human or animal is.

The statement that 'all people carry the albino gene' is not quite true, because most people do not carry it. If everyone carried it, then one in four people would be albino. That is what makes it rare. The only time it can happen is when two people who are heterozygous (carriers of the recessive gene) have a baby, and only then there is a 25% chance of albino. 

Using the 'albino is recessive', that means that pigment genes are dominant. A heterozygote is a carrier for that recessive gene, but phenotypically they will appear like any other person. And, just because they are a carrier does not mean that every child of theirs will be albino.

Lastly, because this is a horse genetics blog, stay tune for Part Two: Albinism in Horses

Albinism in Horses

To follow up with the promised post after the last one, I of course have to talk about albinism in horses. To recap, albino is when there is no pigment in the skin; all genes that control color are off.

There are quite a few curiosities on the subject, including a simple fact that will surprise you: albinism has never ever been proven in horses.

While you may be able to Google quite a few images of horses that appear albino, there has never been a case in which they weren't able to prove it was something else. The horses above came up in the search, as well as the ones below.

As mentioned in the last post, red eyes only sometimes indicate albino, and there are several other things besides that would. However, with horses, there is no picture I can find of a horse with red eyes that doesn't look photo shopped, or when you look close up are actually blue eyes surrounded by pink skin.

According to various sources, blue eyes in horses is not exactly lack of pigment, and is as close to non-pigmented eyes as you can get, but again there isn't no pigment.

The most commonly mistaken color for albino is cremello, because of the very light-colored skin that often is pink with blue eyes. However, using the method of deciphering albino as in the last post, albino has no base color, and there is no pigment behind it. Cremello is a chestnut horse with two cream genes, so it would indeed have some pigment. Other double-cream horses (perlino, smoky cream) may be categorized among cremello in being mistaken for albino.
If you were to take the tiniest bit of red paint, and then add a dose of white, it makes pink. If you add even more white, it becomes light pink. Even if you keep adding white to the original red, the paint will never be truly white, no matter how much you put in.

One of the closest things to albino scientifically is dominant white. Dominant white is considered to be a white-spotting genes, along with patterns such as tobiano and frame overo. The main difference with dominant white is that it often shows up as one, huge spot, entirely covering the body and blotting out the under color. Did you notice that last sentence? 'The under color'. There is a color under there that they can pass on.

Dominant white can be expressed in many different ways, even within each specific mutation. It isn't always predictable, and because of that it isn't a fool-proof way to describe as albino. It seems like most of the time, it leaves the eyes their original brown color, although there are some cases of white. That would depend on the specific mutation.

Fully-expressed sabino is similar to dominant white, except that sabino never causes blue eyes, leaving the eyes dark.

So, I guess the underlying question is: why? Why has albino never been proven in horses? Why would horses be the exception?

Albino means there is no pigment whatsoever, and they never have any color to pass on. With all these genes, the horse also has a color underneath. The only way for a horse to be albino would be that somehow, the gene that causes pigment (extension) wouldn't be working.

Extension is the only gene that causes pigment. All the rest of the genes I speak of are simply instructions, or a way to lighten what was already there. Isn't that interesting?

For albino to happen, extension would have to find a way to not work. Because it is a dominant gene, one or two copies produces black pigment. In recessive form, it causes red pigment. That doesn't leave any option open for no pigment, see?

In the whole history of horses, we haven't found a way for it not work as of yet. Albino hasn't happened yet, that doesn't mean it can't. Extension would have to find a way to be missing entirely from the genome, but how when both parents have it boggles my mind. Colors, and the way they work, and the way we percieve them is constantly changing.

What Does Phenotype Mean?

On the Equine Genetics page on Facebook, there are a lot of people that use the term 'phenotype'. What does it mean?

It is a word that means 'the physical appearance of something', although a lot of people that use it have a way of, even on the internet, sounding snobby because a lot of people don't know what that means. And that is ok! I don't use it because it is just one more long word that confuses people.

In an example of how to use it: a black horse with one cream gene is called smoky black, although phenotypically, it may look the same as a normal black horse. Can you tell the difference in the horses below? Probably not. Phenotypically (or, the way they appear) they are the same.

Or: a chestnut that carries dominant agouti will have the same phenotype as one without.

See what I'm saying? Now that you know the word, you may want to be careful who you use it around.

Here is a post I did on several differences between horses who have the same genes, but different phenotypes: http://michaelashopeandhorses.blogspot.com/2014/11/horse-color-genetics-differentiating.html

What Exactly is the Job of Extension and Agouti?

Since my previous post on basic genetics, the one in which I talk about base colors, I explained that dominant extension makes a horse black, and recessive makes red, and if a horse carries dominant black and dominant agouti, it makes bay. In a sense, basically what I am saying is that a horse is either black based or red based, but that isn't true.

If you assume that what I said about the dominant form of extension makes black and added on top of that makes bay, then I am basically saying that agouti is a modifier. Agouti modifies extension. Read that statement again:  you will see the fault in it. If agouti is dominant, and is a modifier, then it would modify all extension, yet science dictates that that agouti only modifies dominant extension. In that case, it wouldn't be dominant. Somewhere, there is a breakdown that doesn't make sense.

Basically, erase everything I ever said about agouti being a modifier, and pay close attention: the order in which we mentally play out colors doesn't actually work the easy way. 

The truth is that extension does not create black pigment; extension is present in horses that are not black. That being said, what extension does is it creates black and red pigment. The dominant form of extension creates black, but when paired with agouti what exactly happens is a bit mixed in people's minds.

Think of agouti as instructions: it instructs the black pigment where to go. Even in recessive form, they are still instructions. Agouti is still telling the black where to go. 

So: DO NOT THINK OF EXTENSION AS THE BLACK GENE, AND DO NOT THINK OF AGOUTI AS THE BAY GENE. Because in truth, agouti makes both bay and black. Does that make sense? The absense (or recessive form) of agouti is actually what makes the horse solid black, and even if the horse was dominant for extension and agouti didn't exist, then maybe the horse wouldn't be black at all. 

Agouti does not add or dilute red pigment to make a black horse. This is not particularly important in actually understanding how colors work, and if you read this to a beginner it may make no sense whatsoever. The basic 'extension makes black, and agouti makes bay' will work up to a point, but understanding that agouti does not modify 'what is already there' is nice to know at some point. And it is science, and trying to make it simpler than what it is is fine, but NOT simplifying it into something not true.

For further reading, go to http://equinetapestry.com/2014/09/bay-is-not-a-modifier/