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.
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.
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.
And this horse is a great example of the asymmetrical face markings typical of frame white horses.
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.