Dominant white/white spotting

"Dominant white" refers to a large group of genes that produce white spotting in various individuals and breeds.  It's actually one of the areas where they're doing a lot of research, so new dominant white genes are added to the list fairly often.  I'm not sure why everyone's saying suddenly, "Oh, it's white spotting, not dominant white," so I'm gonna stick with the name I learned it was for now.

The expression varies both from horse to horse and from gene to gene; some patterns look like sabino and others produce pure white horses.  But generally, the following things are true:

• Inheritance is dominant - one copy turned on and you can see the pattern.
• All of them are coming from the same areas of the DNA, so they're grouped together and it's assumed you can only inherit one dominant white gene from each parent.
• No horses have been found that have the same dominant white version turned on twice, so it's assumed that they are lethal at the embryo stage.  (There is an exception, which I'll call out.)
• New dominant white genes occur as random mutations, so they often appear suddenly and can actually happen in any breed.
• Eye color isn't usually affected, so unless there's another pattern or coat color acting, the horse's eyes will be normal brown.
• Skin is pink under the dominant white pattern.
• Some versions are a sabino-like pattern, and there's a lot of confusion between sabino and dominant white.
• In version where the horse is mostly white, but still has color, that color tends to be concentrated along the topline, especially the mane and ears.  There may also be roaning or interspersed spots in the white.
• Hooves tend to be light-colored.
• Mostly white horses can sometimes lose the non-white hair over time, eventually turning entirely white.

In the genetic test results, it's generally shown as w (off) or W1-W20 (on, for whichever one you've got).

Yeah.  20.  Maybe more.  I'm going to pull pictures where I can, although of course some of them are just going to be a white horse...

White-1 comes from the Franches-Montagne or Frieberger breed, and specifically from horses descended from the mare Cigale, who was born in 1957.

From the identifying paper, loaded on Wikipedia.

Horses are generally born almost white, and can lose what pigment they have as they age

White-2 is from Thoroughbreds.  They descend from a stallion named KY Colonel, born in 1946.  He's described as having high white stockings and a belly spot, much like a sabino, and passed those genes on to two of his foals.  One, War Colors, was registered as a roan but failed to produce any foals.  The other, White Beauty, passed down her white color to a line sometimes referred to as the "Patchen" line, after the mare Patchen Beauty.

White Beauty, borrowed from here.

According to The Blood Horse, one of the latest generation is Chief White Fox, and it looks like he has some cousins and half-siblings out there.  That said, it's still a small, rare line.

Some presumed W2 horses - War Colors being a great example - have been registered as roan or grey, but the "typical" presentation is all white, just like White Beauty.

White-3 comes from Arabians.  When I was talking about sabino in Arabians, this is one of the genes I was thinking of that isn't sabino, but (confusingly) is called sabino by the Arabian people in a lot of the information I've looked at.  It comes from the line of R Khasper, a stallion born in 1996.

R Khasper, from his owners' page on him here.  Looking good at 18 in the photo!

W3 shows up as high whites and belly spots - or more, all the way up to mostly or all white.  Looking at him and his foals, if there's color on the highly expressed horses, it's tending to congregate around the ears, the base of the tail, and in a downward-pointing triangle on the back between the haunches and withers.  There's a lesser degree of color along the neck, as well.  This is generally true of a lot of the sabino-type dominant whites, but that one triangle was just particularly striking to me!

White-4 is found in an entire breed, the Camarillo White Horse.  It all stems from a stallion named Sultan, foaled in 1912.  According to what I'm seeing, he was a Spanish Mustang, but was crossed to Morgans to form the beginnings of the breed.  Since the white coat is so characteristic of the breed, they do have an open studbook so they can cross out to non-white horses and maintain the color.  (Remember, W4W4 is believed to be lethal in utero.)

And they're pretty much just white in appearance.  At most, they might have a little color along the topline.

Borrowed from Wikipedia.

White-5 is from the Thoroughbreds again, this time descendants of Puchilingui (foaled in 1985).  W5 horses can range in expression from white blazes and high, irregular socks on up to white or near-white horses.

Puchilingui, from here.

Probably the most high-profile example of the line is his son, Sato, but there are plenty of others!

White-6 was another Thoroughbred, but I can't find anything other than this:

From the original paper

According to what little I can find - and squinting at horse e that picture - it's a very roaned sabino-like type, and only one horse was tested at the time the paper was published in 2008.  Speculation points to a horse named Marumatsu Live as the horse in question, but I haven't seen anything but speculation.

White-7 is - stop me if you've heard this one - a Thoroughbred mutation as well.  Again, that one photo above is all I can find (horse f); squinting at it reveals another heavily roaned, sabino-like pattern.  The horse tested has not been identified publicly aside from stating that the dam and siblings were solid and the horse was foaled in 2005, so I've had no luck tracking down anything but speculation about who it is.

White-8 comes from Icelandics.  Equine Tapestry points to the Thokkadis family, although every other source says "unidentified Icelandic;" she does a lot of pedigree analysis to track colors, so while she has been wrong in the details of the genetics, she's generally right in the pedigree department.  Also seeing that noted on another, much more dubious source (it's a random forum, so... reader beware).  Unfortunately, searching Thokkadis and Icelandic doesn't turn up anything substantial other than a picture from the same dubious source as part of a much larger image that I cannot get to load.  Squinting at the photo strip above (horse g) reveals a white pattern that doesn't look all that different from W3 and W5, but the only other image I can find is about as big as the images in that strip and in an image that is horribly pixelated because it refuses to load.

White-9 is from the Holsteiners.  Again, unidentified single individual reported, and again about all I can do is point to that photo strip (horse h).  Similar to W8, I can see there's an alternative image in the picture that will not freaking load, but I can't say much about it.  It's white?

White-10 is much easier to find!  It's from Quarter Horses, specifically descendants of GQ Santana (foaled 2000).

GQ Santana, from here.

Looking at pictures of his foals, that dark eye is characteristic of the maximum expression; it's there in a whole lot of them.  Lesser expressions look more like sabino - high leg white, belly spots, etc.

White-11 comes from a draft breed called the South German Coldblood, and specifically seems to be from the bloodline of Schimmel.  The South German Coldblood is one I hadn't heard of before; it's similar to the Noriker, but has been shown to be genetically distinct.

I found one picture other than the lovely (if tiny) one in the photo strip above (horse l), and I'm not entirely sure where it belongs because it's on Pinterest.  It also shows what I know has to be a young draft, probably 2-3 years old and deep into the awkward stage. Poor baby.  lol

White-12 is yet another unidentified Thoroughbred, and I'll cycle back to the huge number of TBs in this list at the end, because it's interesting.  Suspect forum source says the tested horse is deceased, and I'm not spotting a picture other than the one that won't load.  (Argh, so frustrating!!)  It looks similar to W5.

White-13 is assumed to be from Quarter Horses; it was tested in a single individual or small family of QH-Peruvian Paso crosses.  Again, the image-that-won't-load is the only one I can find, and the horse it shows is... white.

White-14 is another Thoroughbred, but this one's identified!  It comes from a Japanese bloodline, descended from a mare named Shirayukihime.  Like W2, there's a lot of pure white going on here.

Yuki-chan, one of Shirayukihime's foals. From Wikipedia.

White-15 sends us back to the Arabians, and specifically to the Rhocky Rhoad / Khartoon Khlassic bloodline.

Khartoon Khlassic, from his website.

Rhocky Rhoad, from his owners' site.

Again we're looking at a very sabino type of spotting - face white, high white legs, and belly spots.  The most expressive of the foals I looked at had a huge belly spot that extended up onto her body, but most of them look closer to these two stallions.

White-16 was been found in the Oldenburg.  No word on where exactly it came from, but a horse named O' Wie Weiss went through the auctions and and is theorized to be W16 - and possibly her dam as well.

From here, linked off the CotH forum.

However, the actual line wasn't identified in the original papers, so this is only a tentative identification; the dam could easily belong to a known or suspected dominant white line (especially with warmblood lines, where they keep bringing in TBs), or this could instead be sabino-1. 

White-17 is from the Japanese Draft Horse - and again, we're sparse on information here.  It appears that there were two individuals located and tested in 2010, and the breed is known to have facial and leg white and the odd white individual, but... that's all I can find.

White-18 is from a Swiss Warmblood named Colorina Von Hof, born in 2009.  Again, there's a similarity to W5 - and again, the only freaking picture I can find is on Pinterest!  (I kind of hate Pinterest.  It's a neat concept, but it really robs all context from the images, and I'm trying to make sure that if I pull photos from somewhere other than my personal collection or Wikipedia, I can at least account for where it originally came from.)

White-19 is Arabians again, this time believed to come from a mare named Fantasia Vu.

Awestrukk, a descendant of Fantasia Vu. From his owners' site.

Again we've got the flashy sabino, but like the Khartoon Klassic lines, they don't really seem to come in white. Awestrukk there is a bit more flashy than the average, but nicely shows the pattern.  W19 horses sometimes have blue eyes, although it's not clear from what I'm seeing if that's because of the W19 or if there's a separate pattern going on.

There's also this guy, also W19:

HAAP Snowy River, from here. (Also found a source for the same photo that is... entirely in Russian... so we're gonna go for the one I know what it's saying.)

This presentation is atypical for W19, but is typical for this cross - Pandemonium River x  Fantasia Vu.  It's speculated that Pandemonium River carries an additional sabino or dominant white pattern that's adding to the white of W19.

And last but not least is White-20.  And it's the weird one that isn't lethal when two copies are turned on.

Where the others are flashy as hell all by themselves, W20 seems to be more related to simple white markings.  Copies of W20 have been found in horses that have nothing more than a blaze and some socks.

Like these two.  Borrowed from Equine Tapestry.

Looking at pictures, there are examples with even less white.

On the other hand - W20 is one of the only dominant white genes to be discovered in combination with any of the others.  When it does show up with another dominant white gene, it significantly increases the amount of white visible.

Shew of Gold GF (W5) with her white foal Supernatural (W5 and W20).  From Equine Tapestry.

There are more examples here.  It seems to be the oldest of the white genes, and has been discovered in a huge range of horses - Gypsies, Thoroughbreds, Icelandics, Minis...

Now, I said there's at least 20, and I said I'd come back to the Thoroughbreds!  There are the occasional mentions of additional W genes.  One site I looked at talked about W21 and W22, but didn't do much more than cite the name of the paper and a generic copy-pasted description of the paper itself. 

There are a couple of lines of TBs that look either dominant white or sabino, but researchers haven't identified what gene they have.  There's a mare named Turf Club who seems to have spontaneously mutated (speculation is that she's the W7 horse, but again nothing I found confirms this).

Another mare, Not Quite White, produced a stallion named Airdrie Apache that gets a lot of use as the sire of "pinto" Thoroughbreds; speculation is that she's a dominant white as well, with an as-yet-unidentified mutation.  Airdrie Apache definitely passes down his pattern, though!

Airdrie Apache, from here.

White Horse Productions calls out a few more that might be dominant white and might not.

There have been white Standardbreds that, because their parents were solid-colored, might be sabino (if the parents were minimally expressed) or might be new dominant white mutations.  And there's nothing that says there can't be other examples out there that have passed unnoticed.  Various sources I looked at call the gene where dominant white and sabino and some of the other white patterns sit "volatile" - it can and does mutate regularly.

There's also a color called manchado, which appears only in horses with origins in Argentina.  It's sometimes speculated to be environmental - although that does raise the question of why Argentina specifically and only - but it's got a wide enough distribution in enough breeds down there that some have questioned whether it's genetic, even though it can and has been reproduced down family lines.  If so, it seems to be recessive.  Equine Tapestry covers this in pretty good detail here.

From White Horse Productions.

So is manchado dominant white, despite being recessive?  Sabino?  Something else?  No one knows yet.