IN MY OPINION....
MOST of the white facial markings on otherwise SOLID horses are caused by Splash and Sabino and possibly Dominate White
. A good percentage I think are also caused by Overo, but not as commonly as Splash or Sabino. Tobiano
is also the cause, though quite rarely, of white on the legs but no white on the face. Usually these horses are OBVIOUSLY Tobiano, however.
In SPOTTED horses, most Overo's aren't really 100% Overo, but Overo + Sabino. IMO, Overo doesn't produce stockings, Sabino does. Overo doesn't produce milk chins. Sabino does.
I'll find a few examples of what I'm talking about here. I hope this is clear :) All photos are copyright of their respective owners, and are used here as example/educational purposes only Quarter Horses
From left to right: Splash, Overo, Sabino ???
Splash ??? - Probably a QH
Splash Draft or Gaited
Sabino (note also the milk chin) Quarter Horse
100% Overo, he MAY carry Sabino/Splash/Tobiano but he only SHOWS Overo. Note how the white markings are limited to the middle of the horse, do not cross the topline, and his blaze does not extend down past the chin. Quarter Horse
Overo + Sabino. Note the high, unusually shaped stockings (very common of Sabino's, I can bring several examples of this in MINIMALLY marked Sabino's) and the milk chin. He is mostly
Overo but Sabino got a little boogey-woogey in there too. My reasoning
In it's minimal form, small or slightly unusual stripes, stars and snips. Blue eyes may or may not be present. In more loud form, the face head is often all white and the markings are semi-crisp.
Larger blazes that do not extend down to the lower lip
. Remember that we are are talking about solid colored horses here. Blue eyes may or may not be present, though probably not horses that are not loudly expressed.
Large to medium blazes or bald-faces that extend to the lower lip, aka, milk chin.
DOMINATE WHITE -
Very, very tricky. These gene probably
mimics Sabino. Sabino White and Dominate White are easily confused because they are so alike.
Dominate Whites have pink skin and brown eyes, but some alleles produce a colt that mimics Sabino.
To quote, W5 is found in Thoroughbreds descending from Puchilingui, a 1984 stallion with sabino-like white spotting and roaning. Horses with the W5 allele exhibit a huge range in white phenotype: a few have been pure white or near-white, while others have sabino-like spotting limited to high, irregular stockings and blazes that covered the face. Twenty-two members of this family were studied, and the 12 with some degree of dominant white spotting were found to have a deletion in exon 15
I love genetics :) Don't even get me started on the Vienna gene in rabbits, which is VERY interesting. This is obviously just the surface of Overo, Sabino, Splash, and Dominate White. There is much more involved-ESPECIALLY in Splash.