Are loud overo markings due to overo plus W20?

Frame overo markings on horses include eye-catching white patches and often a white face accompanied by one or two blue eyes. The amount of white or 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. This case study is a very early look at how another gene, W20, appears to contribute to how much white pattern is present.

The 'frame' part of the frame overo name refers to the 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. Yes, it's a bit of a stretch. People get a bit poetic naming horse colours and markings.

Now, onto the genetics of frame overo, and how much white to expect. As I've said before in other places, the important things to remember when thinking about your horse's genes are:

  1. Your horse has two copies of each gene*, one inherited from either parent;
  2. 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
  3. 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.

*: except for genes on the X and Y chromosomes in male horses.

This case study looks at the third point: the interaction between two genes.

Here are the photographs I have of overo horses that do not carry the version of the KIT gene known as W20 (i.e. these horses have tested positive for frame overo and negative for W20). These examples are horses that are only overo that are negative for other white marking genes such as tobiano, splashed white or sabino.

Sierra Bell overo
KPM Sierra Bell, an overo Paint horse (added 5 Sep 2014)
Satin overo
Get The Picture ("Satin"), an overo Paint horse (added 5 Sep 2014, photo by ACC Photography)
Lacy overo
AC's Painted Lace, an overo Quarter Horse/Paint horse (added 5 Sep 2014, photo by ACC Photography)
Blacklead Champagne Cavalier overo
Blacklead Champagne Cavalier, a champagne overo miniature horse (added 5 Sep 2014)

These horses all have minimal overo markings.

Now, here are the photographs of horses that have tested positive to both W20 and frame overo (again excluding those that also have other white marking genes such as tobiano, splashed white or sabino).

Stevie overo plus W20
Stevie, an overo plus W20 Clydesdale x Paint x Quarter Horse (added 11 Sep 2014)
Peggy overo plus W20
KPM Serrano, an overo plus W20 Paint horse (added 5 Sep 2014)
Jive overo plus W20
Chalani Jive, an overo plus W20 Australian Stock Horse (added 5 Sep 2014, photo by In Motion Photography)

The W20 + overo horses appear to have louder markings, in some instances much louder.

Is this proof of an interaction between overo and W20? No, not yet. This is only a handful of examples, because there aren't many overo horses that have been tested for W20, especially when we limit it to horses with no other white marking genes. The next ones that we genotype or that are genotyped elsewhere might be exactly the opposite... but it sure looks interesting at this point.

Do you have another overo example that has been tested for W20? Contact me and I'll add it too.

So, if we assume for now that having W20 plus overo gives loud overo markings, what does this mean for overo breeders? Most Paint societies consider markings that are about 50% white as optimal, and a lot of people really like the look of horses with plenty of white. Breeding horses that are positive for both overo and W20 seems like it will be a good way to achieve this look.

Examples of breeding applications are (assuming that overo plus W20 reliably results in more white markings):

  • An overo stallion (or mare) who was also homozygous for W20 would produce plenty of white on all their overo foals
  • An overo stallion or mare mated to another horse that was homozygous for W20 would also produce plenty of white on all their overo foals
  • Any overo plus W20 horse has at least a 1 in 4 chance of producing another overo plus W20 foal
  • Any overo mating where neither horse has W20 is likely to produce only minimally marked overo progeny (at least when we limit this to horses with no other white marking genes)

Would you like to test your overo horse for W20? The order page is here.