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.

Tuesday, July 13, 2010

The Basics of Horse Color Genetics

Ok, recently, I have had a few people ask me about all those letters I use when talking about what color the horses are.  Believe it or not, those letters mean something, and a string of them says exactly what color a horse is, and what color patterns its foals can inherit.

So, for all of my friends who think I've gone round the bend when I get excited about this stuff, I decided to put together a little tutorial.  I'll cover a gene a day.  Otherwise it would get WAY too long, and well, I'm already good enough at the super long posts! 

First, genetics follow some simple rules.  DNA is double stranded, so that means that each horse (living thing really) has two alleles (er, half of a gene) for each loci (location of the gene).  Take a horse like Spot for instance.....
Most people refer to his color as a black fewspot.  This leaves a lot of questions to a breeder who keeps color in mind.  Now, if I said that he's:

Ee aa gg dd chch LPLP PATN(large)/patn(large) PATN(Small)/patn(small) with sabino,

then you have a very good guess as to what color offspring he could have.  Each pair of letters represents one allele (some people use the term gene here, which is a slang use) that he can pass onto his offspring.  The capital letters sets, such as E, or PATN represent a dominant allele, while the smaller letters represent a recessive version.  Any time both letters are the same, Such as with aa, or LPLP, either capital (dominant) or small (recessive) then this is called "homozygous" from homo meaning same.  When the letters are different, like Ee, this is called "heterozygous" from hetero meaning different.

Most genes are a "simple dominant".  This means that if a horse is heterozygous (one big and one little) the horse will only show the traits from the big letter, or dominant allele.  Not all genes are like this though.  Common horse colors, such as palomino, or appaloosa are "incomplete dominant" genes.  This means that there are 3 "phenotypes" or expressions.  A homozygous dominant will have one appearance, such as a cremello (pale, almost white) or a fewspot/snowcap for appaloosas (solid areas of white with no polka dots).  When the horse is heterozygous (one copy of dominant, and one recessive) then it has a second apearance - Palomino, or a blanket with dots/leopard.  And the theird expression is a homozygous recessive - chestnut or solid.

Most genes are referred to by their dominant form, hence "the grey gene" means a horse will lose pigment over time.  A horse with "no grey" doesn't mean that the horse has a vacancy at this locus (gene location) it just means that the horse is homozygous for the recessive copies, which is what is considered "normal" in most cases. 

Also, some genes can be "hidden".  In this case, the gene in question requires a second gene to be active before the gene in question can express its self.  A good example of this is bay, or the Agouti gene.  Represented with A or a, this gene can only be seen on a horse that is black.  A chestnut can have a dominant bay gene, but you will never be able to see it, thus, the gene is hidden.

There are many MANY genes that go into the color of a horse.  Most people only list the applicable genes for their horse, and skip the rest.  Such as, in Spot's genetics above, I didn't list overo, tobiano, champagne, or a myriad of other things.  Some genes are not fully understood yet, so we only know that a horse shows the phenotype, but have no idea what genes (letters) are involved in it.  Such as how I mentioned that spot has sabino, but did not list a string of letters.  We know that there are multiple sabinos, and I have no way of positively knowing which he has, or in what state.

In the next few days I will cover the basic genes.  If you have one you're curious about, let me know, and I'll add it in, otherwise, I'll simply cover the genes found in appaloosa colored horses (i.e. no paint genes).

15 comments:

  1. I know that in humans, blue eyes are recessive. If two blue-eyed people mate, the resulting offspring must have blue eyes. But two brown eyed parents can also produce a blue-eyed child, if they both carry the recessive trait for blue eyes. Are blue eyes recessive in horses also? If so, would that mean that O carries the trait in order to throw blue-eyed Daltrey?

    ReplyDelete
  2. No, horses inherit blue eyes differently. In horses, light eyes are a result of pigment production, or limitations on pigment production (i.e. coat color, and white markings).

    So, Daltrey has blue eyes because he has the splash white gene. Where Splash limits pigment production (his blaze is up around his eyes) the pigment for eye color is also limited. That is why Soli has a half blue eye, because her blaze only is on the lower side of her eye.

    Arden has splash white as well, and it is heavier on the left side of her face, so only her left eye is blue. (yes, I have a lot of Splash white in my herd!).

    Other genes can also cause pigment limitation in eyes, including grey, amber, and even green eyes... but you have to wait for those!

    ReplyDelete
  3. Very interesting stuff - I'd love to learn more.

    We have a champagne pinto pony (Norman). My understanding is that champagnes always have amber eyes, and he does. How are the amber eyes determined genetically? (He's an overo with white over one eye, but both eyes are the same amber.)

    ReplyDelete
  4. Well, Champagnes don't always have Amber eyes, they can have grey, green, amber or a light brown eye, but always a diluted color. Mine has eyes somewhere between amber and brown.

    But this too is related to the pigment production. The champagne gene also dilutes the pigment in the skin to a pumpkin color. It's not quite pink, but not black either, and they have blue freckles.

    In other words, the pigment is diluted "half way" thus the horses eye color is also diluted "half way", resulting in a light brown (amberish) color. I'll go into this more when I cover the champagne gene, or I'll write a book here!

    ReplyDelete
  5. Genetic of color has always be fascinating to me. When I got into minis a whole new set of colors possibilities. I have a sliver buckskin right now and previous have a sliver bay. Two colors that even knowledgeable horsewomen look at me and say "what color is that!" lol

    I was actually just looking into what the difference is, genetically, between a flaxen chestnut and a palomino. My sister looks at all haffies and calls them palominos. Oh well. :)

    Thanks for the refresher!

    ReplyDelete
  6. Genetic of color has always be fascinating to me. When I got into minis a whole new set of colors possibilities. I have a sliver buckskin right now and previous have a sliver bay. Two colors that even knowledgeable horsewomen look at me and say "what color is that!" lol

    I was actually just looking into what the difference is, genetically, between a flaxen chestnut and a palomino. My sister looks at all haffies and calls them palominos. Oh well. :)

    Thanks for the refresher!

    ReplyDelete
  7. Very interesting stuff - I'd love to learn more.

    We have a champagne pinto pony (Norman). My understanding is that champagnes always have amber eyes, and he does. How are the amber eyes determined genetically? (He's an overo with white over one eye, but both eyes are the same amber.)

    ReplyDelete
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