A Note about Sugarbush Draft Horses

I see it over and over again, and no matter how many times it's said, it's still wrong. "Sugarbush Drafts are just an Appaloosa Draft Cross". Uh.... no. The Sugarbush Draft Horse was a breed created many years ago in Ohio. While the initial cross was made using Percherons to Appaloosas, in the many generations following, the breed has been solidified into a consistent type. Saying these horses are "just" a draft cross makes as much sense as saying that AQHA horses are "just" a Thoroughbred cross, American Cream Drafts are "just" a dilute Belgian, or that Morgans are "just" a grade.

Friday, July 16, 2010

Dun and Creme, the yellow genes

Buckskin, Dun, Palomino, Grullo.  All these diluted colors, it gets so confusing!

How often do you see ads for a "Buckskin" horse, that has a dorsal stripe and leg bars?  Or that lovely "Dun" who is really a bay?  For some reason, these dilutions are really hard for people to figure out.  Ok at least half of the problem is wishful thinking.

The first difference between these 2 genes is how they are inherited.

Dun is a simple dominance gene (if you have a dominant copy, then the gene is expressed, and there's no difference between DD and Dd).  Creme is also the "Palomino" gene, or the "Buckskin" gene.  It isn't nearly as cooperative for breeders, since Creme is an incomplete dominant gene.  They are on different parts of the DNA strand, so even if the phenotypes look similar, the genes are different.  Homozygous dun (DD) is not lethal as was once guessed.

Incomplete dominance simply means that there are 3 options, homozygous dominant (CRCR) heterozygous (CRcr) and homozygous recessive (crcr), and each one of these gentotypes (genetic code) has a different phenotype (appearance).  The appearance is dependant upon the horse's base color - Extension plus Agouti.

I only have one Dun horse, and sadly champagne makes it almost impossible to photograph her markings, so I'm going to use some images of other horses (i.e. not mine).  All images are linked back to their source.

The typical appearance of a dun horse is a tawny body with black points.  A buckskin is similar looking.  The difference between these 2 are the "primitive markings" that duns have.  This includes a dorsal stripe and leg barring, as well as the often hard to see dark shoulder patch.  If you look at a typical donkey, you see can see an example of primitive markings: the cross on their back.
Here you can see both the stripe on Maggie's shoulder, as well as the dorsal stripe down her back.  Now, horses don't have quite as definitive of a stripe on their shoulders.  Instead, dun horses have a sooty area, or just a darker area.  In some dun horses this can't really be seen at all.

Here is a good example.  This horse is offered for sale (is also listed as a buckskin - oops!), is not mine, and can be seen here.
Note the dark area on the shoulder? That's not a shadow, that's part of what the Dun gene does.  The stripe on this horse's back is very obvious in the second image.  Also, the light hairs seen on the horse's mane are called "Guard hairs" and are often, but not always, lighter on Duns.  The dorsal stripe though, is what many people use to distinguish a Dun from a Buckskin.  While a buckskin may have counter shading (a darker area on its spine) it will not be either as dark, nor as well defined as this horse's stripe.  Duns also have "zebra stripes" or "leg bars" on their gaskin and forearm.  These may be almost the same color as the body, or they may be much darker, or even black on some horses, depending upon their base coat color.

A Bucksin on the other hand, would look like this stallion, from this website:
You can see that this horse does not have a dorsal stripe on his back, but he is still a tawny color with dark points.

The main difference between Dun and Buckskin isn't the appearance, it's the method of inheritance.  In other words, what will I get when I breed this horse. 

Dun dilutes the body color towards the yellow, on ANY base coat color, and leaves the primitive markings. Granted, Dun does dilute red more then black, but it often turns black points a deep brown or even as light as red tones in some cases. Creme only affects red pigment, and is an incomplete dominance gene, so is harder to breed for.

Dun on black is a Grulla.  Dun on Bay is a Dun, or also called a Zebra Dun.  Dun on a chesnut/sorrel is a red dun.  Dun and the creme gene can also be inherited and exhibited at the same time, such as with Dunalinos, or buckskin duns (no nifty cool name for those that I know of yet).

Then you have the Creme gene.  If a horse is homozygous dominant for creme, then on a red base you would have a cremello, on a bay base you would have a Perlino, and on a black base you would have a smokey creme.  In its herterozygous form (CRcr), creme on bay results in a buckskin, creme on chestnur/sorrel results in a palomino, and creme on black results in a smokey black (very hard if not impossible in some cases to discern with the naked eye).  Creme can also be inherited on a brown base coat (also called seal bay). 

In both cases a horse that is homozygous recessive would show no alterations from it's base coat color from this gene.  Most people would say the horse didn't "inherit" the gene in question (slang).

BUT it's never that easy is it.  While we have a good idea of the normal presentations of these colors, there are always exceptions!

Creme on Seal bay gives...well.. it's still called a buckskin, or occassionally a smokey brown.  A good example is talked about on this forum.
This horse would be E? At ? CRcr, or seal bay with creme.  Some might call this a smokey brown.



Also, there are countless examples of horses for sale called buckskins, but they are just sun faded bays.  For a horse to inherit one of these dilution genes, one of their parents MUST have it.  Even if their ancestors had it, if the horse does not inherit a gene, it can not pass it on.  Only recessive genes can be carried, and pass unexpectedly to offspring (such as with red Friesians).
 
So, if you have a chestnut mare (ee ?? crcr), who had both a buckskin(Ee A? CRcr) sire and dam, and you expect to get a buckskin foal... well....better breed to a Perlino (EE AA CRCR)!  Your mare doesn't have either the dominant Extension(E), nor the dominant creme gene(CR) to pass on.


Now, creme acts by lightening the red pigment, which is why a single creme gene on a black horse is almost undetectable - so little red pigment is visible.  Bays have their black pigment pushed to the extremities, leaving red pigment in their main body areas, and creme can then lighten up that red, turning it yellow, resulting in a buckskin.


Now, with a homozygous dominant creme horse, any pigment is lightened drastically.  This is because the proteins made by incomplete dominant genes have more impact with more production.  So, if a little bleach is good... a lot of bleach... bleaches a lot.  In the case of smokey cremes(E? aa CRCR), they retain the most pigment of the homozygous dominant creme horses.  This lovely horse is a smokey creme shown here.

See how dark his mane still is?  That's because black is harder to fade out for the creme protein.  Perlino horses tend to have less color then the smokey creme horse, and cremellos are the closest to white of the group.  All shades of horses with homozygous dominant creme are near white., though.  Again, this is due to the base color genetics.  Creme works on red pigment, so a red horse is easier to lighten then a bay (black based because it's E? A?).  All horses with 2 dominant copies of creme will have blue eyes and pink skin.
 
A homozygous dominant dun though looks no different then a heterozygous dun.  This is the nature of simple dominant genetics.  Also, because I'm not sure that I mentioned this specifically, Dun is denoted by D for the dominant form, and d for the recessive.
 
Often times these genetic codes change as we map the gene in question.  If the exact sequence of DNA was found previously in another species, then the gene inherits the name of the first discovery of it.  Agouti is a good example of this, as it was first found in mice, so we began using the same terminology and abbreviations as the mouse gene(A and a) in stead of calling it the Bay gene officially.
 
The color genetics of horses build on each other.  First you consider the horse's extension status (E or e) then it's agouti (A or a) and modifiers come after that.  

So, if you saw that a horse's genome was:

Ee AA crcr, what color horse would it be?

12 comments:

  1. Ooh! A test! I love tests! (and you thought you were a nerd!)

    Plain old bay - I think!

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  2. Eww, my formatting took a nose dive, and added in double spaces. ICK! And I can't seem to fix it.

    Ha, Kate... you sure that's the answer you wanna give? (I'll let others guess before I give the answer, check back tomorrow).

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  3. I agree with Kate, Bay (with the chance of a chestnut offspring with the little "e"--extra credit?).
    By the way, I've heard the term "dunskin" to describe the dun/buckskin combo.
    There used to be a really pretty stud colt in Arizona that was Dun, Buckskin, AND Champagne! He was VERY light in his body color and had chocolate points (like my amber Champagne filly had).
    My current claim to dilution fame is the dun overo in my avatar, "Canticle" aka Kate.

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  4. Small modification - red bay.

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  5. Ok, I was going to wait until midnight my time, but I don't want to forget....


    Yep, the answer is the standard red bay horse!


    Bonus points to Evensong for predicting potential offspring! =) Maybe I should keep a running tally?

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  6. Wow. So confusing, yet so fascinating. My Bella is a red dun. Her mane and tail are gorgeous. Sometimes they even have a purple tint to them and other just a bright red, like Carrot Top. lol!


    ~Lisa

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  7. I found this site by chance and love it. I used to work with a registered buckskin dun named Scooter. He has great examples of the black points. http://pinellashorse.ning.com/photo/1318184:Photo:31982?context=user

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  8. Sorry I can't get the link to work for ^above^ so just copy and paste.

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  9. Sorry I can't get the link to work for ^above^ so just copy and paste.

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  10. Fascinating! I love learning about colour genetics, I like the way you explain it, you don't talk down to people but you don't dumb it down too much either.. Thank you!

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