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HORSE COLOR GENETICS
by D. Phillip Sponenberg, DVM, PhD.
Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061
and submitted for publication in the Missouri Foxtotter News
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Horse color genetics is a complicated subject, and it is impossible to deny that the complexities can make it difficult to understand. By breaking down the subject into smaller pieces it is possible to begin to understand how the interactions of different factors result in the vast array of color in horses. A first step is to separate out white spots or hairs, and only concentrate on color. Following an appreciation of how the colors arise it is possible to add the white, and thereby understand the final combinations.
Color is the result of interaction of eleven or so different factors. Some of these are single genes, others are unknown genetically. Each unique combination results in a single color, so there are many, many possibilities. Some are common, some are rare, and some depend on the breed involved.
GENETIC CONTROL OF COLOR (IGNORE THE WHITE!)
The three main colors of horses are bay, chestnut, and black. All other colors can be built from these three, so understanding these three is the foundation for all of the others.
1. Chestnut (Ee) is recessive to nonchestnut (black or bay) (EE ). Chestnut horses are basically red, although many have "Bend Or" spots, which are dark or black spots up to a few inches across.
2. For the nonchestnut horses, bay (A+ ) is dominant to black (Aa ).
This relationship is tricky. Chestnut, though recessive, masks whether a horse would have been black or bay. A mating of chestnut and black horses can result in bay foals, even though both chestnut and black are recessive to bay.
3. "Shade" is a complicated trait that is probably controlled by several genes rather than a few. "Shade", for convenience, can be considered to make the base color dark, medium or light. It interacts with the basic coat colors to produce more designations:
color dark version medium version light version
bay blood bay, mahogany bay red bay, cherry bay sandy bay, gold bay
chestnut liver chestnut red chestnut gold or light chestn
black black black “summer” black
4. "Sooty" is a factor that adds black hairs into the base color. This changes the appearance from a clear color to a sooty color, and in some systems of color names will change names of some colors.
base color sooty variant
bay brown or mahogany bay (depending on naming system)
chestnut liver chestnut or black chestnut
black black
The interaction of sooty and shade provide many, many different nuances of color.
5. The mane and tail color on chestnut horses is variable. It ranges from a dark brown that is close to black, through bright reds, and on to the very pale flaxen colors that are nearly white. These changes are related to many, many different genes and are complicated.
6. Mealy is a single, dominant gene (Pa+ ) which causes lighter areas on the belly, muzzle, inner legs, and over the eyes. It is usually ignored in color description, except for changing black to seal brown, and changing chestnut to sorrel for breeders of draft horses.
base color mealy variant
black seal brown
bay mealy bay
chestnut sorrel
7. The cremello gene (Crcr ) in one dose dilutes red to yellow, and leaves black unaffected. With two doses it changes both red pigment and black pigment to cream, with pink skin and blue eyes.
base color one dose two doses
bay buckskin cream (technically perlino)
brown sooty buckskin cream (technically perlino)
chestnut/sorrel palomino cream (technically cremello)
liver chestnut sooty palomino cream (technically cremello)
black smoky cream (technically smoky cream)
8. The linebacked dun gene (Dn+ ) is dominant, and lightens body color, leaving a stripe down the back, bars on the legs, and frequently a stripe on the withers.
base color dun variant
bay zebra dun
brown coyote dun
buckskin gold dun
chestnut red dun
sorrel apricot dun
palomino linebacked palomino
black grullo
“sooty” black lobo dun
“light” black olive dun or olive grullo
cream linebacked cream
9. The silver dapple gene (ZZ ) is a dominant that acts to lighten black areas, and leaves red areas unchanged. It is rare, but increasing in frequency in some breeds. It results in colors that are frequently confused with chesnut, but that lack the redness of chestnut. It lightens manes, tails, and lower legs to flaxen, or can leave them relatively unchanged.
base color silver dapple variant
black chocolate silver, blue silver, silver dapple
brown brown silver
bay red silver
buckskin yellow silver
zebra dun linebacked yellow silver
chestnut chestnut (no visible evidence)
10. The champagne gene (ChC ) is a dominant that is rare. Lightens black areas to flat light chocolate, and red to light red or yellow. Skin to be pink or mottled, and eyes hazel.
base color champagne variant
black champagne
bay amber champagne (gold or yellow with brown points)
chestnut gold champagne (mimics palomino or light sorrel)
palomino ivory champagne (very light, with dark blue eyes)
11. The final color interaction is brindle. This is rare indeed, and reorganizes the sooty countershading into vertical stripes.
base color brindle variant
brown red brindle
lobo dun grullo brindle
PATTERNS OF WHITE HAIRS AND WHITE PATCHES
The patterns of white hairs and patches do not interact as do the color factors. Instead, each is independent and as a result they can occur in any and all combinations.
1. Grey is a dominant gene (GG ) that causes white hairs to progressively grow into the coat. Grey foals are born colored, usually dark, and then lighten with age to white or nearly so, with pigmented skin. Subtypes of grey include dappled, iron (nondappled), fleabitten (small flecks of color), porcelain (white), and blood marked (large patches of color).
2. Roan is a dominant gene (RnRn ) that is lethal to homozygous embryos, so only heterozygotes exist. Roan is a mixture of white hairs into the body coat, but usually does not involve the mane, tail, lower legs, and head. The roan pattern changes black to blue roan, brown to purple roan, bay to red roan, dark chestnuts to lilac roan, lighter chestnut and sorrel to strawberry roan.
3. Frosty is similar to roan, only the mane and tail are also roaned.
4. Ticked (RaRa ) is probably a dominant gene. This pattern involves roan hairs in the flanks and at the base of the tail.
5. Paint or Pinto spotting is characterized as nonsymmetrical white areas on the body of the horse. Several distinct patterns are characteristic of this group.
A. Tobiano, (ToTo ) is a dominant that causes white on the legs, and vertical white spots on the body that cross the topline somewhere between ears and tail. The heads are usually colored, and eyes are usually dark.
B. Frame overo (OvFF ) is a dominant that causes horizontal white marks on the body and neck, lots of facial white as well as blue eyes, and usually leaves the feet colored.
C. Sabino may well be polygenic and causes abundant white on the legs, and then creeps up as belly spots and body spots that are usually flecked and roaned. Usually the head is largely white, as well.
D. Splashed white (SplSpl ) is a dominant that causes very crisp white areas. Usually the head, legs, and lower body of the horse are white, as if they had been dipped in white paint.
Subtypes of spotting patterns can occur with some of these genes. The Medicine Hat pattern is one of these, and the horses are largely white, with color remaining on the ears, tail base, chest, and flank. War Bonnets are whiter, with colored ears and very little else.
6, Leopard complex spotting is tyical of the Appaloosa, and many other, breeds worldwide. This is due to a dominant gene (LpOp ). Heterozygous horses, on average, have less white than do homozygotes. Patterns include mottled, snowflake, blanket, leopard, few spot leopard, varnish roan, and frost.
7. White (WW ) is a dominant gene that is lethal to homozygous embryos. True white horses are white with pink skin and dark eyes. Some have a few small spots of color in skin or hair, but most do not.
8. Birdcatcher spots are small white spots scattered randomly over the coat of some horses. These are not known to be genetic, and occur in many breeds.
9. White face and leg marks occur in most breeds. These are controlled by many, many individual genes all contributing a little bit to the final marks. The various marks do have different names:
star small, between eyes on forehead
strip small, thin, up on top of nasal bones
snip small, thin, below level of nostrils on top of nose or upper lip
chin spot lower lip
stripe narrow connected star, strip, snip
blaze wide connected star, strip, snip
bald face wide blaze over nostrils
apron face bald face extending along jaw to throat latch
paper face white head
coronet small spot on coronary band of hoof half pastern white up to pastern joint
pastern white up to bottom of fetlock joint fetlock white including fetlock joint sock white up to half of cannon bone 3/4 stocking white 3/4 up cannon bone
stocking white to bottom of knee or hock
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