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 30, 2010

Frame Over and Overo Lethal White Syndrome (OLWS)


For those who have been keeping up, I apologize for the delay in this posting.  But, here it is, the basics of frame white overo! And actually, it's just the basics.

Frame white earns its name from the dark "frame" around the white areas of the horse, like on this stallion.  While many types of white markings are often thrown into the "overo" category, this is the main pattern associated with the name.  Frame white causes a horizontal type appearance to the body spots, and horses with this type of patterning often have asymmetrical face markings.

Horses showing frame white patterning are always heterozygous for the gene (Oo).  Horses who are homozygous recessive (oo) are solid (or at least do not show frame white) and horses that are homozygous dominant (OO) are non viable.  Homozygous dominant frame white horses exhibit what is called Overo Lethal White Syndrome, or OWLS.

To put it simply, homozygous dominant horses (OO) are born solid white, and have a non functioning gut.  How does a color gene affect the horse's intestines?  Well, nature likes to be efficient, and in reality color is often a bi-product of the genes real function, and not the main purpose of the gene.  We often relate the genes' functions to their most obvious result - coat color, just because that is what is easiest to see.  In the case of frame, it's not easy to see the intestines of a horse, but the eye catching white markings are hard to miss.

OLWS foals usually will die within 72 hours of birth, if not euthanized before.  The nerves that work the gastrointestinal system do not develop properly, and these poor foals are born with contracted and occasionally deformed intestines.  Surgery to remove or bypass the affected area has never been successful.

Word of warning here, not all solid white foals have OLWS.  Things like dominant white, maximum sabino, or even a surprise cremello foal could easily be misdiagnosed as a lethal white. 
The first vet I worked for had what he thought was a lethal white foal.  His overo mare, bred to an overo stallion, produced a solid white filly.  He was devastated, but the filly seemed to be thriving, and passed her meconium. Luckily he took the "wait and see" approach.  I have to note that while I adore Dr. Paul, he is not a geneticist, and he didn't know the difference between the overo patterns.  His mare what a sabino, and the stallion was a frame/sabino combination.  The resulting filly could not have been lethal white, the mare (oo) did not carry the genetics, but because it was an "overo to overo" breeding, lethal white was suspect.  Also, this was almost 15 years ago now, before the problem was well understood.

Frame is an incomplete dominant gene though.  There are 3 distinct phenotypes (appearances) associated with it.  Homozygous recessive horses are solid, heterozygotes show irregular patterning, and homozygous dominant foals die shortly after birth.  The gene is represented with O, for overo, but do not let this confuse you.  Overo is really a group of color patterns which includes frame, splash white, and sabino, as well as a few other more rare patterns like dominant white.

Sadly, I couldn't find much information on minimal frame markings.  It appears that frame can hide easily in other white patterns though.  Many horses believed to be tobaino/splash turn up carrying frame as well.  I know Evensong posted a picture of her mare that hides frame really well. 

This miniature filly is a minimal frame white, DNA tested Oo.  My problem is that she shows classic minimal splash white markings.  Since there is no DNA test for splash white (yet) it is hard to know if there is some interaction between these genes.  Could splash hide frame in certain situations?  Does minimal frame white mimic splash white face markings, but leave the legs dark?  It's hard to know.  Just goes to show that we have so much more to learn about coat color genetics!

Like other pinto genes, there are varying levels of expression for frame white horses.  Frame can be expressed with other pinto genes, such as splash, sabino, tobiano, and dominant white, which means that many horses are not "pure" frame white, and show traits of other pinto genes as well.

Here is a tovero stallion that shows both tobiano and frame traits.
 Note the irregularity of his otherwise typical tobiano blobs. His asymmetrical face marking could be caused by splash white (he also has 4 white legs) or frame.

And this horse is a great example of the asymmetrical face markings typical of frame white horses.
Notice the dark pigment on his topline?  This is where the gene earned its name, with the dark pigment "framing" the horse.  I have to admit here that frame is one of my favorite pinto pattern genes, even though I know so little about it.  While frame white horses are harder to breed for, due to the inability to breed frame to frame (with out risk of losing the foal), or the existence of homozygous frame horses, the results are truly stunning.

And everyone who thinks these horses are beautiful should read EvenSong's post about her experience learning about Lethal White.   EvenSong shares even more information about Frame Overo horses.  Definately worth the read.

9 comments:

  1. Wasn't it Evensong who has a blog post on lethal white? Who ever it was, if you give me the link, I'll edit it in. THANKS!

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  2. My favorite pattern as well. Are there other genetic defects associated with frame overo?

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  3. Our Norman is a champagne overo, I'm pretty sure - very asymmetrical face markings, although he has a lot less white than many of the example horses. There might be some other stuff in there - his white face has a dark circular patch in it.

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  4. Here's the link to my post of our experience with OLWS:
    http://mountaintrailmusic.blogspot.com/2010/01/on-wings-of-angel.html
    When my black and while mare Maddie was born (the year after Angel, same mare, different stud), APHA wanted to register her as straight Tobiano, but her apron face and blue eye told me different. When I got the DNA testing back she did indeed carry the O gene. I will NEVER breed her to an overo stud that hasn't been tested! And that same warning will go with her if I sell her. My dun overo Kate, got the gene from Angel's and her sire (different mare), so she, too, will not be bred to a frame.

    Thanks for the info, Heather. You got it pretty well right on.

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  5. Thanks for double checking me Evensong. I really wanted to make sure I got this one right. Overo patterned horses are SO lovely, but there's that downside of lethal whites. I would hate to bias any one against the horses, but also don't want to encourage someone with out explaining OLWS.

    I went hunting for your link, thinking to steal a bit of information, and for the life of me I couldn't find it.

    I have to say though, if I ever get a paint horse (that isn't here because it needs rehab) I definately want an overo! I'd steal Kate in a heartbeat, and I'm not even a fan of "yellow" horses. =)

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  6. I know what you mean about "yellow" horses...I've never been a fan of duns. But Kate got her mama's golden tones, and black points (she's probably EE A? Dd Oo) and on her, it looks pretty good, if I do say so myself (and I do)!

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  7. Thanks for double checking me Evensong. I really wanted to make sure I got this one right. Overo patterned horses are SO lovely, but there's that downside of lethal whites. I would hate to bias any one against the horses, but also don't want to encourage someone with out explaining OLWS.

    I went hunting for your link, thinking to steal a bit of information, and for the life of me I couldn't find it.

    I have to say though, if I ever get a paint horse (that isn't here because it needs rehab) I definately want an overo! I'd steal Kate in a heartbeat, and I'm not even a fan of "yellow" horses. =)

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  8. My favorite pattern as well. Are there other genetic defects associated with frame overo?

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    ReplyDelete