Most people are familiar with the standard horse colors, e.g., black, chestnut, bay, palomino, etc. However, even experienced horse owners are surprised to find that these colors are the tip of the iceberg for horse color possibilities. Horses come in a fascinating array of colors, ranging from incredibly rare to extraordinarily common.
The rarest horse colors are:
- Dominant White: Heritable KIT gene mutation
- Mushroom: Heritable MSFD12 gene mutation
- Brindle: Heritable MPTPS2 gene mutation
- Chimera: Non-heritable embryonic activity
- Bird Catcher, Chubari, and Bend-Or Spots: Unknown heritable gene mutation
Some horse colors are so rare that few of us will have the opportunity of meeting one of these “unicorns” in real life. Their gorgeous coats make them standouts in a sea of chestnuts, bays, tobianos, and buckskins. Here is a list of the rarest horse colors ever documented.
What Are The Rarest Horse Colors?
There have been no studies comparing the statistical rarity of specific horse colors. Thus, no single horse color can definitively claim the number one spot for the rarest horse color.
Although the horse community cannot agree on which single horse color is the rarest, they agree that the following colors are some of the rarest horse colors in existence.
- Dominant White
- Brindle and reverse Brindle
- Bird Catcher, Chubari, and Bend-Or Spots
1. Dominant White
Dominant white refers to a KIT gene mutation that causes white spotting patterns that range from white stockings and loud face markings to a completely white horse.
There are currently 32 known genes causing dominant white in horses. The name dominant white refers to the fact that most genes responsible for dominant white are dominant, i.e., the horse only needs one copy of the gene for the horse’s color to be affected.
Genetic tests are available for W5, W10, W20, and W22. While the various genes responsible for dominant white are not rare, W5 and W22 maximally expressed thoroughbreds are considered rare.
W5 and W22 have only been found in thoroughbreds and thoroughbred crosses and are thought to be lethal in the homozygous form. Painted Patchen and White Vessel are dominant white thoroughbreds who made the news for their unusual coloring.
Unless you are a fan of Shetland ponies, you are unlikely to have heard of the rare horse color, mushroom. The mushroom gene is due to a recessive mutation of MSFD12, i.e., only Shetlands, who have two copies of the gene, show the mushroom color. Horses with one mushroom gene are carriers.
The mushroom gene dilutes the red pigment, phaeomelanin; the red-yellow pigment is responsible for the red coloration in chestnuts. The mushroom genes fade the deep red of phaeomelanin to a sepia-toned light yellow-brown. The dilution is most noticeably on the flanks, head, and neck.
The mushroom gene has a slight effect on bay horses and no effect on the eumelanin found in black horses. Mushroom-colored horses have light, almost white, manes and tails.
The mushroom horse color is extraordinarily rare and is only found in the UK bred Shetlands, although some scientists believe that the gene also exists in UK miniature horses.
3. Brindle And Reverse Brindle
Brindle is a relatively common color amongst dogs but remains a rare color in equines. Brindle horses are characterized by vertical stripes running from their topline down their body. The stripes of a brindle horse have a different texture and may be accompanied by different pigmentation.
Brindle is due to a mutation in the intron of MPTPS2, an X-linked semi-dominant gene. The X and Y chromosomes are the sex chromosomes that determine gender, i.e., females are XX and males are XY.
Thus, a male horse can only ever be heterozygous for brindle, while a female can be heterozygous or homozygous.
The semi-dominance of the gene means that the phenotype of homozygous females is different from heterozygous female horses.
Females with one copy of the brindle gene have the typical brindle phenotype, i.e., visible stripes and altered hair texture. Males with one copy and females with two copies have sparse manes and tails with no visible brindle hair texture pattern.
Brindles can be any color, but the most common variant is a light-colored horse with dark stripes. Reverse brindles refer to horses with white stripes.
Chimerism refers to a horse that possesses two distinct sets of DNA. There are two possible chimera variants, blood chimerism, and true chimerism.
Blood chimerism occurs when fraternal (non-identical) twins exchange hematopoietic blood cells. While blood chimeras feature two sets of DNA, they do not have the flashy coat of a true chimera.
A true chimera occurs when two non-identical zygotes (fertilized eggs) fuse to create a single zygote during the embryonic stage.
Because chimerism is due to embryonic and not genetic activity, the two-toned colors seen in a chimera are not heritable.
Blood chimerism occurs at a rate less than 0.011%, and true chimerism is even rarer, making a horse with chimera coloring one of the rarest horse colors.
5. Bird Catcher, Chubari, and Bend-Or Spots
Three types of spotting patterns create unique and incredibly rare markings on breeds that are not traditional spotted horse breeds:
- Bird Catcher Spots
- Chubari Spots
- Bend-Or Spots
Bird Catcher spots are white spots on a dark-colored horse. These spots may be apparent at birth or appear as the horse ages. Birdcatcher spots are named after an influential Irish Thoroughbred bred in 1833.
Birdcatcher was a prepotent sire and outstanding racehorse. In addition to his impressive physical prowess, he was famous for the white spots that covered him from head to toe.
Like Birdcatcher spots, Chubari spots are large white circular spots that can occur anywhere on a horse’s body. However, unlike Birdcatcher spots, Chubari spots only occur in grey horses. Eventually, the spots disappear as the horse greys out.
Chubari spots are occasionally called Tetrach spots due to a loudly marked thoroughbred called Tetrach. Tetrach was nicknamed the “Spotted Wonder” for his impressive track performance and flashy color.
Bend-Or spots are the last in a line of rare spotting genes. Bend-Or spots are dark spots found in chestnut and palomino horses. Like the other two spotting patterns, Bend-Or spots are named after a famous chestnut thoroughbred bearing distinctive black “oil” marks.
Scientists have not yet identified which genes are responsible for these unusual spotted horse colors. However, the cause of these spotted anomalies is almost undoubtedly genetic due to the heritability of the patterns, i.e., the spots occur in related horses.
Almost all rare horse colors are caused by mutations of the wild-type allele, e.g., KIT, MPTPS2, and MSFD12 gene mutations gave rise to the incredibly rare dominant white, brindle, and mushroom colors.
There are a few exceptions to the heritability of rare horse colors. A fascinating cause of non-heritable rare horse colors is true chimerism.
These chimeras are truly irreplaceable as the chimera-induced colors cannot be passed onto the horse’s foals. The unusual two-toned coloring seen in Chimeras is caused by embryonic activity and not a novel genetic mutation.