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).
|One of the gorgeous 2013 frame overo filly foals from Winning Colours Farm.|
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.)
|Gorgeous white markings on this mare and foal... but definitely not frame overo. Pic is wallpaper linked from here.|
- 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.
|A chestnut frame overo mare and her white foal. Foals with Lethal White Syndrome have non-functional intestines. Photo linked from article here, photo credit Stephanie Valberg.|
The inactive large intestine of Lethal White Syndrome foals is caused by a lack functioning nerve cells in the intestinal tissue.
|Australian stallion Overexposed, from Palamo Paint and Quarterhorse Stud. He has frame overo offspring, but his tobiano markings obscure the characteristic frame overo pattern.|
|Shadofax'A'Star - a beautiful white miniature horse stallion standing at Koriana Park. His white pattern genes include appaloosa.|
|"The Opera House", spectacular dominant white Thoroughbred mare who raced in Australia. She has normal brown eyes.|
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.