Horses and ponies have a really diverse range of white coat colour markings. Frame overo markings include eye-catching white patches and often a white face accompanied by one or two blue eyes. The amount of white on a frame overo horse or pony varies widely, ranging from a very discrete patch of white on the belly all the way up to majority white coat colours.
The ‘frame’ part of the frame overo name refers to fact that markings on the main body rarely if ever cross the spine, so if you squint a bit you can almost imagine the body colour making a darker “frame” around the white markings when viewed from side on. Yeah, it’s kind of a stretch I know. People get a bit poetic naming horse colours (champagne, silver and pearl, I’m looking at you too).
Frame overo white markings have clean edges. This contrasts with the lacy or fuzzy edges you see in sabino white markings, such as the traditional Clydesdale markings. (One or more articles on sabino will be coming up. There are many gene variants, all in the KIT gene, known to cause sabino white marking patterns, and more keep getting found.)
Now, onto the genetics of frame overo. As introduced in the piece on chestnut, black and bay or brown coat colours, there are three things to remember when discussing genes:
- Your horse has two copies of each gene*, one inherited from either parent;
- There are many different versions of each gene in the horse population, some of which can produce identical or nearly identical appearance and biological function, while other versions can produce a different coat colour or cause disease; and
- Each physical characteristic of your horse, including coat colour, results from the combined effects of the two copies of each gene, AND the interactions between a large number of different genes.
One version of the endothelin-B receptor gene – gene acronym EDNRB – causes frame overo markings. One and only one copy of the frame overo version of EDNRB is required for frame overo markings to occur. When a foal is born with two copies of the frame overo version of the EDNRB gene, it suffers from Overo Lethal White Syndrome, a truly tragic disorder which is fatal within days of birth.
Foals born with Lethal White Syndrome are usually completely white and have two blue eyes. Sometimes they can have small dark markings. While at first they appear normal, they will soon start to show signs of colic. Their intestines cannot move food along, an action which is normally automatic and is required for normal digestion (and normal bowel movements). As a result of this defect, their meconium is impacted high up in their intestines. Surgery does not work in treating this disorder, and if the foal is not euthenased it will suffer a painful death from colic within a few days.
The inactive large intestine of Lethal White Syndrome foals is caused by a lack functioning nerve cells in the intestinal tissue.
The frame overo version of the EDNRB gene and other white pattern marking genes occur together in the same horse. When this happens, your horse will have markings that result from the combined effect of all its white marking genes. For example, a horse could carry either one or two copies of a version of the KIT gene that causes sabino markings, as well as the frame overo version of the EDNRB gene. (The same applies to tobiano, splash, and appaloosa, and all the combinations.) This can make it next to impossible to detect the frame overo gene just from looking at a horse.
Inheritance of multiple white pattern genes can also produce completely white horses. Sometimes people think that these are Lethal White Syndrome foals that have somehow survived, but unfortunately equine genetic testing laboratories have never found two copies of the frame overo version of the EDNRB gene in an adult horse. There is such a thing as a completely white ‘dominant white’ horse too, although these horses have brown eyes instead of the blue eyes that are often seen when a horse has multiple white pattern genes.
The version of the EDNRB gene that causes both frame overo markings and Lethal White Syndrome was characterised independently by two teams of researchers. One team comprised primarily Australian medical doctors (go Aussie!) while the other research team was based in the USA. The article from the Australian group is available free of charge, the other is pay-walled. Both groups used the clinical parallels between Hirschsprung’s Disease and Lethal White Syndrome as a basis for their investigation of the EDNRB gene. Thank-you for your excellent and thorough work both groups.
* Except for genes on the X and Y chromosomes in male horses, where there will be only one copy of each gene.