|
Equine Color Genetics  Colors may appear differently on different computers due to personal settings. This page is best viewed in Internet Explorer 4.0 and up (though it should work in most other browsers), with a resolution of either 800 x 600 or 1024 x 768. An Introduction to Genetics: Horses, like most animals, have two sets of chromosomes. The location of a gene on a particular chromosome is called a gene locus. Because there are two of each chromosome, there are two loci for every type of gene. This means a horse has two genes for each trait (two for eye color, etc.). There are multiple varieties, called alleles, of each gene, so a given horse may have two different genes for a particular trait. A horse with two different alleles for a trait is termed heterozygous for that trait, while a horse with identical alleles is termed homozygous. There are usually two possible alleles for a gene, though there can be more (for example, there are three alleles for human blood type). Generally one allele is dominant over the other, meaning that if an individual is heterozygous for a trait (has two different alleles), the dominant gene will be expressed and the recessive gene will not be expressed. When making genetic notations, a dominant allele is represented by a capital letter, and a recessive allele is represented by a lower-case letter. So a horse homozygous for a dominant allele would be noted "AA", a horse homozygous for a recessive allele would be noted "aa", and a heterozygous horse would be noted "Aa".
Surprisingly, there are only two possible "base colors" for horses - red (chestnut) and black. Every other color is the result of some modifier gene acting on top of one of these bases. Black is the dominant allele, and chestnut is the recessive. The gene that codes for coat color is usually represented by the letter E (because it is located at the extension locus on the chromosomes) - black is is noted as "E" and chestnut as "e". Because chestnut is recessive, the color is only expressed in horses that are homozygous for the recessive allele (if a dominant allele were present, it would override the chestnut and the horse would be black). Therefore, all chestnut horses are homozygous, or "ee". Breeding two chestnut horses together can result in nothing but a chestnut foal. Another common term for the color chestnut is "sorrel." Black horses can be either homozygous dominant (EE), or heterozygous (Ee), because only one copy of the dominant allele is necessary for the black color to be expressed. A heterozygous black horse does carry the recessive chestnut allele, however, and can produce chestnut foals. It is even possible to get a chestnut foal from two black parents, provided both are heterozygous. A homozygous black horse does not have the chestnut allele to pass on, and therefore can only produce black-based foals no matter what color the other parent is. Shades of Chestnut: The chestnut color can come in many different shades, which is often confusing. The Thoroughbred colt Point Given, pictured above, is a typical expression of chestnut. Below are some examples of variations.
Flaxen Chestnut: It should be noted that blond sorrel and flaxen chestnut are NOT palomino, which is caused by the cream gene and will be discussed further on. Flaxen manes and tails are thought to be caused by a recessive gene, usually denoted by the letter "f", which acts only on red-pigmented manes (i.e. a black-based horse carrying this gene would not be visually affected). Because the flaxen allele is recessive, it is only expressed when in its homozygous (ff) state. Heterozygotes (Ff) and dominant homozygotes (FF) will not have flaxen manes and tails.
Bay is actually not a true base color in and of itself. Rather, it is the result of the agouti gene acting on a black base. The agouti gene restricts black to the points (muzzle, ears, mane, tail, and lower legs). The agouti gene is generally represented by the letter A - an individual homozygous for agouti would be "AA", a horse with only one copy of the gene would be "Aa", and a horse without the gene (a black horse) would be "aa". It should be noted that chestnut horses can carry agouti, but since the gene affects only black pigment, there is no visual difference between a chestnut with agouti and a chestnut without. For this reason it is possible to breed a chestnut horse to a black horse and get a bay foal, though only if the chestnut carries agouti or if the "black" horse is actually a very dark bay that has been misidentified. Breeding two true black horses together will never result in a bay foal, since true blacks do not carry agouti. Shades of Bay: Bay, like chestnut, comes in many shades. Some examples can be seen below.
Genetically speaking, there's no such thing as a white horse. Most of the horses referred to as "white" are really greys, like the Thoroughbred gelding Gem Twist pictured to the left. (Other "white" horses will be discussed later, under pinto patterns.) Note that grey horses still have dark skin, which can be seen where the hair is thin on the muzzle, around the eyes, and inside the ears.
Stages / Shades of Grey: Keep in mind that, since grey is a progressive color, the darker individuals shown below have not completely greyed out and will change color as they age.
Both of these colors are caused by the cream gene, generally denoted by the letter C ("Cc" for heterozygotes / single dilutes and "CC" for homozygotes / double dilutes). Palomino is cream acting on a chestnut base coat, and buckskin is on a bay base coat. Black horses with the cream gene are termed "smokey blacks," but there is little, if any, visually detectable difference between a non-dilute black and a smokey black. The cream gene dilutes only the red, or chestnut, pigmentation in a horse's coat, which is why buckskins retain their black mane and tail and why blacks are not affected at all. This gene is incompletely dominant, meaning it
Dun (and grulla - also spelled "grullo", which is dun on black) is also a diluting gene that acts mostly on red pigment (though black is slightly diluted, as can be seen with grullas). Unlike cream, however, dun is completely dominant (i.e. only one copy of the gene is necessary for full expression), and there is no difference in appearance between heterozygotes (Dd) and homozygotes (DD). Duns, especially bay duns, are often mistaken for cream dilutes (buckskins), as their coloring is similar (to make things ever more confusing, a horse can possess both the dun gene AND the cream gene - see examples below), but there are certain distinct traits that set duns apart. These are known either as "dun factors" or "primitive markings." The most easily recognizable is a distinct dorsal stripe. While any horse can have a dorsal stripe, the dorsal stripe of a dun tends to be very dark and have crisp, hard edges, giving it a "painted on" look. This can be seen in the lower picture to the left. A second dun factor, sometimes difficult to see depending on how much black there is on the legs, is leg barring. Duns have several horizontal stripes across each of their legs, usually ranging from slightly below to slightly above the knee area. The third dun factor is shoulder baring or shoulder "masking," which is not always present. Often a dun will have one or two short vertical "bars" radiating down from the dorsal stripe over the shoulders.
Unlike cream and dun, silver dapple is a gene that dilutes only black pigment in a horse's coat This gene is responsible for the color commonly known as "chocolate sorrel" (which is actually not sorrel at all). On bay horses this leaves the mane and tail a flaxen color and the legs / face usually take on the same color as the body, or slightly lighter. The main body coloring is not affected, since silver dapple does not act on red pigment. For this reason, bay silvers are often mistaken for dark flaxen chestnuts. If such a horse produces a bay or black foal from a chestnut-based mate, however, it is obvious that they are actually silver dapple, since chestnuts cannot supply the black coloration. Black silvers take on a very dark liver color with a flaxen mane and tail, and are quite striking. The Rocky Mountain Horse stallion Hillsdale's General Grant, pictured to the right, is a good example of this color. Chestnuts with the silver dapple gene, much like blacks with one cream gene, show no visual evidence that they carry it. Since silver dapple dilutes only black pigment, and chestnuts have none, they remain their normal chestnut color. The only way to tell for sure that a chestnut is a silver dapple is if they produce silver offspring from non-silver mates. Like dun, silver dapple is completely dominant and there is no visual difference between a homozygote (SdSd) and a heterozygote (Sdsd).
Champagne is the most recently identified of the four dilute genes (the others being cream, dun, and silver dapple). It is completely dominant, and there is no visual difference between a heterozygote (Chch) and a homozygote (ChCh). Champagne lightens the coat much like the other dilute genes, and is easy to confuse with cream in particular, but there are some tell-tale characteristics that set creams and champagnes apart. For one, champagne removes all true black pigment from a horse's coat, whereas cream (in it's heterozygous form) acts only on red pigment. The most striking feature of the champagne gene is the metallic "glow" that it adds to a horse's coat. However, it must be noted that this metallic coloring alone is NOT necessarily indicative of champagne (see the bit on Akhal Tekes further down). It can be difficult to identify a champagne horse at birth, as foals are born with darker coats that can look almost like normal chestnut, bay, or black, but which will lighten as the horse grows up. One way to tell the difference between a champagne and cream horse is to look at their skin color. A single dilute cream (heterozygote) such as a palomino or buckskin will retain dark skin, whereas all champagnes are born with pink skin. The skin remains pink, but will begin to take on a mottled look as the horse ages. This is easiest to see where the hair is thin, around the muzzles, eyes, ears, and genitals. It should be noted, however, that double cream dilutes (homozygotes / cremellos and perlinos) also have pink skin. Another feature of the champagne gene is that all champagne foals are born with bright blue or teal eyes that turn to a light hazel color as the horse ages. While double cream dilutes also have blue eyes (single cream dilutes retain dark eyes), the blue eyes of a cream horse will not change color as they age. The terms used to refer to the various forms of champagne (depending on the base color) can be confusing, particulary since the terminology has changed over the years. Champagne on chestnut is known as "gold champagne," champagne on bay is known as "amber champagne," and champagne on black is called "classic champagne." The confusion comes when you throw other dilute genes into the mix, since champagne can appear in conjunction with other diluting factors. Previously, all horses that carried both champagne and cream were termed "ivory champagne." This, however, does not tell us what the base color of the horse is. More recently, palomino champagnes are called "gold ivory," buckskins champagnes are called "amber ivory," and snmokey black champagnes are called "classic ivory," noting whether the horse has a chestnut, bay, or black basecoat. No distinction is made between single and double cream dilutes that also carry champagne.
The dominant pangare gene is responsible for horses with lighter pigmentation on the muzzle, flanks, underbelly, lower and inner legs (sometimes), and around the eyes. Because it is dominant, only one copy of the allele need be present for the coloration to be expressed. The Haflinger breed, like the gelding shown to the right is an excellent example of the pangare coloration. It is also commonly found among Belgian Drafts, Exmoor Ponies and Mongolian Wild Horses, but occurs quite frequently in many other breeds (for example, Icelandics - the mare to the left shows classic pangare patterning), as well as in donkeys and mules. The pangare coloration is also sometimes referred to as "mealy" (i.e. "mealy bay" etc.).
Not much is known about the genetics behind the "sooty" coloring, also referred to as "smutty." This factor causes a horse's coat to take on a dirty appearance, from where it gets its names. The coloration can be expressed over the entire body, or restricted to certain areas, such as the face.
The Pinto Patterns: There are actually four separate pinto patterns, caused by four separate, unrelated genes. Each will be discussed below.
Tobiano is, perhaps, the least confusing and most easily identified of the pinto patterns. Like all the pinto genes, tobiano is dominant (the recessive being "non-tobiano"). Only one copy of the gene is needed for the pattern to be expressed. Tobiano is characterized by high white on the legs, white crossing the back/topline, and a dark head with minimal face markings. If a tobiano has anything more than a moderate blaze, it is quite likely that they are also carrying at least one other pinto pattern (combination patterns will be addressed further on). There is a wide range of possible expressions of the pattern. In its most minimal form, a tobiano might have only four white legs and no body white at all, or only a small marking across the whithers. A minimal tobiano may even have one or more completely solid legs, though this is very rare. On the other extreme, a tobiano may have only a dark head and no other pigment. Most tobianos, however, fall somewhere in the middle. The colored areas tend to radiate out from the head, the chest, and the flank. Sometimes there will be color over the rump, around the tailhead, and the tail will be colored. Other times the rump will be white, leaving the tail white as well. Wherever white markings touch or cross the mane or tail, white mane and tail hairs will result. Though there is often little difference in appearance between heterozygous and homozygous tobianos, one tell-tale sign of a homozygote is the presence of small freckle-like pigmented spots within the white markings. These are known either as "cat tracks" or "ink spots," and are only present on tobianos in the homozygous form (though not all homozygous tobianos have them). The Saddlebred filly in the second of the two pictures above is a good example of this. Below can be seen examples of the various expressions of tobiano.
This is the pattern most commonly referred to as "overo," though there are, in fact, three different overo patterns, each caused by a different gene. Frame has long been thought to be "recessive to tobiano," but this is not the case. It is caused by a gene entirely unrelated to tobiano, and is actually dominant ("non-frame" being the recessive alternative). The frame pattern gets its name from the distribution of its markings. White areas are concentrated on the sides of the horse, with the topline, chest, and underbelly being dark, giving a "framed" appearance. White face markings tend to be extensive, with individual's often having a "bald" or "apron face." The legs are usually dark, or have only low white markings. Since frame is the only pinto gene that does not put white on the legs, a frame overo horse with more than moderate leg white is most likely carrying at least one other pinto pattern as well. As with tobiano, the frame pattern has a range of possible expressions, from a horse with barely any white markings at all (even just a bald face, with no other white!), to a horse whose sides are entirely white, with only the "outline" areas left dark. Unlike tobiano, however, frame overo cannot exist in a homozygous state. All adult frame overo horses are heterozygous. If two frame overos are bred together, and both parents pass on the frame gene (since both parents would be heterozygous, each parent has only a 50% chance of passing on the gene, and there is only a 25% chance that both will pass it on), the foal is born entirely white and dies within a few days of birth, due to an underdeveloped digestive tract which makes it impossible for them to obtain nutrition. This condition is known as "lethal white overo," and is best avoided by simply not breeding carriers of the frame gene together. There are genetic tests available to determine if a horse is a carrier of frame overo. Examples of the various expressions of frame overo are pictured below.
Another one of the "overo" genes, sabino is characterized by high leg white (on the hinds legs, particularly) with markings that are pointy up the front, and facial white that extends on to the lower lip and chin. The range of expression is very large, and the most minimal sabinos are often not recognized as pintos as all. Expression can be as minimal as one or two socks and a small blaze (the key here appears to be that sabinos will have at least a little bit of white on the lower lip and/or chin; however, it must be noted that sabino is not the only pinto gene that will produce lip/chin white). In the most extreme cases, sabino can be so extensive that the entire horse is white (or nearly white, with small "freckles" of color throughout the coat). For example, sabino is responsible for the so-called "white" Thoroughbreds that seem to pop up out of nowhere. But these horses are still genetically chestnut, bay, or black underneath and can produce "solid" foals. It must be noted that these "white sabinos" are also different from the earlier described "white greys." A grey horse is born dark, and retains dark skin even after the coat has greyed out. An extreme white sabino, however, is born white with pink skin and remains so throughout its life. Usually sabino horses fall somewhere between the minimal and extreme expressions, with white markings extending upward from the legs, often resulting in a belly spot. The edges of the white markings tend to be very lacey, unlike the crisply defined outlines that result from the other pinto genes. The sabino gene appears to be dominant, but it does not behave the same way that the other dominant pinto genes do. It seems instead to follow the pattern of polygenetic inheritance (i.e. the trait is controlled by multiple gene loci, as opposed to just one), much like human skin color, where the more white the parents have, the more white the foal will have (though this is not always the case - note the white Thoroughbreds that sometimes spring up from two nearly-solid parents). Different expressions of the sabino gene are pictured below.
Splash whites are perhaps the most striking among pintos. White markings extend upward from the lower half of the body, with very crisply defined edges, giving the horse a "dipped in paint" look. In addition, splash whites will have extensive facial white (i.e. "bald faces"). Expression can vary from four white legs and a bald face to a horse with color restricted to a small amount along the topline. The tell-tale characteristic of a spash white horse is blue eyes. While horses of other patterns can have one or two blue eyes without carrying the splash gene, it appears that all horses carrying splash possess two blue eyes, no matter how minimal the markings. As with the other pinto patterns, splash white is a dominant gene, though it appears to be incompletely dominant, with homozygotes displaying more white than heterozygotes. In general, any splash white horse with white markings covering more than 50% of its body can be safely assumed to be homozygous (though looking at the parents would be useful to ensure this). The various expressions of splash white can be seen below.
Birdcatcher spots were named after the Thoroughbred stallion Birdcatcher, who exhibited the markings, and are also known as Bend Or spots (for the Thoroughbred stallion, Bend Or, who had them) or Chubari spots (usually referring to spots of a larger, often egg-shaped appearance, such as those displayed by Thoroughbred stallion The Tetrarch, and his more recent descendent Laissezaller). Horses with these markings are
The term "Bend Or spots" is sometimes also applied to the condition of having random dark spots throughout the coat. The Thoroughbred stallion Northfields was known to have such markings, and perhaps Bend Or did as well (in addition to his white spots). Again, it is not known what causes this phenomenon, since there does not seem to be a genetic component.
A unique characteristic of the Akhal-Teke breed is the metallic sheen often displayed on their coats. It should be noted that these horses are NOT champagne - they do not possess any of the skin and eye color characteristics that are evidence of the champagne gene.
When multiple color and/or pattern modifier genes are combined, there can be some striking results!
A Last Note on Horse Colors: Not all color genes are found in all breeds of horse. For example, true roan does not exist in Thoroughbreds, and Arabians don't come in palomino. Tobiano is not found in Spanish Mustangs. There are also breeds that only come in one particular color. Cleveland Bays are always bay. Friesians are nearly always black, though occasional chestnuts do occur. Norwegian Fjords are usually some flavor of dun. There are also breeds where certain colors are genetically possible, but cannot be registered. For example, until recently, the AQHA refused to register double cream dilutes (cremellos and perlinos). Note that palominos and buckskins were registerable, so it was entirely possible to get a double dilute from two registered parents. As of March 2003, the AQHA did away with the double dilute rule. Recommended Reading: Links / Sources Cited: http://members.aol.com/brindlehos/ http://www.champagnehorses.net/ http://www.champagnehorses.org/ http://members.aol.com/braebell/horse.html http://www.icelandichorse.is/colours.html http://doubledilute.com/ http://equinecolor.com/ http://www.ichregistry.com/ http://colormorgans.tripod.com/colorful.htm http://colormorgans.tripod.com/silvermorgans.htm http://thoroughbredchampions.com/sites/mumtaz/tbcolor.html |
