Dominant White (DW): It was long thought that the white Thoroughbreds that cropped up in the breed were the result of the sabino gene, but the recent discovery of the dominant white gene has proved that most of the known white TB families are in fact dominant whites. Given that that color is the result of a randon mutation, its tendency to crop up from ordinary-looking parents finally makes sense. Several families of loudly marked TBs have been tested and confirmed as dominant whites, notably those of KY Colonel, Puchilingui, and Shirayukihime.

I haven't been able to turn up any photos of KY Colonel, but he was described as being a chestnut horse with high white stockings and a belly spot, a description that would fit a moderately expressed dominant white. He sired two dominant white horses, War Colors and White Beauty. War Colors was registered as a roan, and was therefore likely a white ticked, moderately expressed DW. War Colors died young and did not produce any offspring. White Beauty, on the other hand, was all white (or near enough) and is the well-spring of a number of well-known DW Thoroughbreds through her daughter World O'Beauty, her daughter Precious Beauty, and her daughter Patchen Beauty. PB has produced 5 DW foals to date, and several of them are following suit in the breeding shed already.

Puchilingui was a very popular sire of colorful Thoroughbreds, and his best known son Sato is carrying on his sire line. Puchi produced both moderately and maximally expressed DWs.

Shirayukihime, the daughter of the true black stallion Sunday Silence and a bay mare, Wave Wind, was quite a surprise to her breeders and is a terrific example of how dominant white can crop up from the most mundanely colored pairings. Five of her six foals (as of 2011) are white including the immensely popular Yukichan.

Though they have not all been DNA tested, it is likely that the other white Thoroughbreds are dominant whites rather than maximally expressed sabinos. Airdrie Apache's dam Not Quite White is almost certainly a dominant white, and it is likely that Airdrie Apache himself and his offspring are as well, such as Allamystique, Arctic White, Painting Freedom, etc. Other likely dominant whites include Our White Lady, The Bride, and The Opera House (pictured at left), among others.

As of 2011, there are less than 150 white Thoroughbreds registered worldwide, but with the explosion in popularity of colorful TBs in just the last 10 years, I imagine that number will climb steadily in years to come.

Frame Overo: Frame is known to exist in three TB families so far. If you know of more, please let me know! Unlike sabino, frame is very rare in TBs, and it's origins in the breed are not really known. The only frame overo TBs I know of have cropped up in the last 20 years. Because they are all the product of fairly obscure breeding, there is some thought that there may be a dose or two of non-TB blood lurking somewhere, but no one really knows. Regardless, they are papered TBs and the breed is definitely the richer for it.

The first frames I know of are Tri Chrome (1991) and Blue Eyed Streaker (1993), both by Blue Gazi, and the mare Fillipas Puzzle (1993) by Great Deal. All three horses exhibit classic frame overo patterns, and I suspect they all carry sabino as well. Tri Chrome unfortunately was euthanized at a young age after being kicked in the paddock before a race. Blue Eyed Streaker currently stands at Echo Hill Farm in KY. (I would love to see photos of Blue Gazi if anyone knows of any!) And Fillipas Puzzle is a puzzle without a doubt. Her whereabouts are unknown, and she has no known progeny.

Thankfully, the other frame overo TB family is more prolific. Oddly enough, it stems from the pairing of two "half" siblings, Pesty Axe and Torchy's Rainbow, both by Give Em The Axe. They produced two foals, Nite Spot in 1985 and Patchy Lassy in 1989. Nite Spot is a boldly marked bay frame overo + sabino, and his sister is a chestnut with a big blaze and tall stockings. Phenotypically, she only looks sabino, but clearly she is a minimally expressed frame as well.