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.

Monday, July 26, 2010

Tobiano, the easy pinto gene

Ok, first off the reason I call these pinto genes, and not paint genes has nothing to do with breed.  Horses with large spots of color are commonly called pintos, regardless of their breed.  So, this includes Paint (APHA) Pinto (PtHA) as well as many grades, a few warmbloods, Tennessee Walking Horses, Missouri Foxtrotters, Shetland Ponies, Miniatures, and well, you get the idea.

Pinto genes are often refered to as genes that cause large white spots on the main body.  Now, as we discussed before, that's not all that they do.  Some pinto genes can cause only face and leg markings when in minimal form.  In fact, most of them can do this.

In other breeds that don't care for pinto type coloring (ApHC, AQHA of the past, and such) these genes are regularly allowed in the gene pool, so long as expression is kept to a minimum.  Let me tell you, appaloosas have all types of pinto genes in their pedigrees, and people love the chrome, until the right combination of genes occurs to express a full blown pinto pattern, and not just some pretty face and leg white.  Suddenly, a breeder is left with a foal that can not be registered.

Of course, I mention Appaloosa because that's the only color registry I'm personally familiar with.  I have to note here that the Sugarbush Draft Horse is a colored breed, but selection is not made by color.  Horses with excessive white are allowed, and the only gene selected against for color is.... Tobiano.

An interesting little tidbit I came across today:  Tobiano was named for the he rescue of Buenos Aires by Brazilian General Tobias during some military event in the 1800s.  I didn't know that and found it rather interesting.

As to how to define Tobiano in a horse?  Well, tobiano tends to cause white legs from the knees (Carpus and Hock) down, but leave "normal" face and leg markings on the heads.  Horses with Tobiano in its minimal form will often have excessively high leg white in comparison to their face markings.  If you see a horse with 4 knee high white socks, but only a small star, then Tobiano is often the cause.

Now, interestingly, the minimal tobiano horses still produce the same as a "typical" tobiano marked horse.  The typical Tobiano will have a "halo" outline to its spots.  Basically it's an area of dark skin that grows white hair, causing a thick outline to the spotted areas.  While a horse with tobiano patterning might have more or less white (amount of white is related to those enhancers and suppressors as well as other genes that aren't well understood yet) they all tend to have a commonality to the type of spotting and location of spots on their body.  Tobiano markings are happy to go across the back, while other pinto markings don't.  Tobiano horses are often seen with a "shield" or dark area on their chest, although horses with higher amounts of white might not have this trait.  Spots on tobiano horses tend to have an oval shape to them. I think of the spots as "blobs of color" on the horse, as opposed to other pinto markings that are more organic and irregular.
This foal is pretty typical of tobiano on a horse.  Note how the white on the neck and hip cross over the back, the foal has 4 white legs, and appears to have a solid head.  White is seen in the tail, which is common with tobiano.
The horse above has those blob type markings I was referring to.  While her blaze makes me suspicious of additional pinto markings, most of what is seen on her body is a typical tobiano appearance.  When compared to the foal above, there are similarities in placement and type of spotting, although the mare is higher on the level of expression then the foal (more white).

As for the genetics of this pattern, well Tobiano is "easy" because it's a simple dominant gene.  A horse has it, or they don't.  The arguement that this gene is incompletely dominant can be made because many homozygous tobiano horses show something called "paw prints" or smaller dots in their white areas.  This is a great example of paw prints.
 Tobinao is a mutation of one of the KIT genes.  That really doesn't mean much to most people, but it's the location of the gene on the DNA.  Now the KIT genes tend to be very very close on the DNA.  The closer genes are to each other, the less likely it is that random recombination will take place between them.  Head hurt again?  Let me put that in simple terms.

When sex cells are produced (sperm or eggs) the DNA is split.  Just before this splitting, the DNA twists up together, and sometimes will flip to the opposite strand.  This is what allows a stallion (or mare) to pass on both his (or her) maternal and paternal traits, while only giving 50% of his DNA.  If this didn't happen, a horse would only pass on EITHER their Sire's traits, or their Dam's traits, never both.

So, If a stallion had a chestnut mom, and a black sire, but he's bay, he would only be able to produce chestnut (with agouti) or black foals... never bay like himself (a combination of dad's black, and mom's agouti).

Anyway, the closer genes are, the harder it is to split them up.  Think of it like trying to cut between knots on a string.  You need a bit of open space to fit the scissors in. If the knots are tied too close, you'd end up cutting the knot, and not the string between it.  Genes are pretty much the same way.

The genes that are so close are tobiano, roan, extension, and a gene for blood type.  The first tobiano tests traced blood type, and judged a horse homozygous based upon the chances of recombination.  This was inaccurate, but the best we could do at the time.  Now, DNA tests are available for the tobiano gene, and a horse that is DNA tested is much more likely to be accurately identified by the test.

Now, because the linked genes are not at the same location, just close, they are still inherited separately.  It IS possible for recombination to occur, but it's rather unlikely.  In other words, the percentage of foals would be skewed to reflect the parent's entire sequence of DNA here, rather then each gene being inherited on its own.  If you have a roan tobiano stallion, (Ee Aa Tt RNrn)  He would likely pass E T RN as a group OR  e t rn. But he would rarely (although still possible) pass E t RN, E t rn, e t RN, and so on.

Also, because pattern genes do not share a location (loci) on the horse's DNA, a horse can have multiple patterns at once.  Roan, rabicano, tobiano, frame, splash, sabino, appaloose... a horse can actualy have them all.  And all of these patterns are built on top of the horse's base coat.

Of course, I'd like to remind you here that pattern genes are actually genes that repress pigment production.  Often when people try to reason out horse colors, they think of laying the white on top of the base coat.  It's actually the other way around.  The horse starts as a clean slate (white) and the pigment cells are spread throughout the body producing coat color.  The color goes on top, but like trying to paint over wax, some pattern genes prevent the color from sticking in certain spots.

Now, hopefully my pinto loving friends out there will correct me if I made any mistakes in this.

AND, time for your test.....


Give me the genome (string of letters) for this horse.  You can use question marks where genes would be unknown.  (Hint: this horse is seal bay)

4 comments:

  1. E? At? T?(probably Tt)--cute boy!
    One might almost think splash, except there's no facial white. Maddie and Jackson have a half brother (same stud) with almost the same markings (on a black base coat); APHA refused to give him regular registry status, even though he tested, through UCDavis, as homozygous Tobiano!

    ReplyDelete
  2. Yep, I think you got it EvenSong. He/She does look awfully splash, but the lack of face markings, as well as that funny extra tall hind sock (hind left) give it away.

    Sorry to hear about the half brother. You'd think with DNA proof they would have allowed him full papers. I suppose that's part of the reason why I'm so into the genetics of it, and love helping others to understand it all. The colored registry rules (not just APHA) all came about before we understood the genetics of horse colors.

    ReplyDelete
  3. E? At? T?(probably Tt)--cute boy!
    One might almost think splash, except there's no facial white. Maddie and Jackson have a half brother (same stud) with almost the same markings (on a black base coat); APHA refused to give him regular registry status, even though he tested, through UCDavis, as homozygous Tobiano!

    ReplyDelete
  4. My pony, Sky is a Hackney x Cob, she has 4 white socks and a big blaze would this be considered a Tobaino?

    ReplyDelete