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Grullo or grulla definitions are in the rule books of many registries and associations.
You may see or hear of many variations or shades of the grullo color:
Genetically, grullo is caused by the dun dilution modifying black horses. It is not known exactly what causes the varying shades of grullo. Since the crème allele does not affect the black pigment it is not likely that a slate grullo is caused by the composite dilution of the dun allele being modified by the creme allele. Since a dun bred to a brown, seal brown or liver chestnut can produce a grullo it may be possible that the team of dun and creme dilutions acting on other modifiers may produce a slate grullo. However, this is merely speculation.
To have a grullo foal, the best bet is to breed grullo to grullo. However it is possible to produce a grullo by breeding any color horse to a grullo, if the foal is lucky enough to get one dun gene and one black gene from the parent. I have even seen palominos produce grullo foals. The chance of having a grullo foal even from two grullo parents has been reported to average less than 50% by reputable breeders. This can vary and depends a great deal upon whether the dun gene and/or the black gene are homozygous in the sire and/or the dam.
At birth grullo foals are often mistaken for dun foals, and even long-time breeders can be fooled. Grullo foals have even been born red dun, only to shed out grullo within the first year. I have also seen beautiful silver grullo horses with reddish tinted or sepia dun factor and dorsal stripe and a black mane and tail.
My curiosity as to what causes the grullo color has sparked an interest in Horse Color Genetics and prompted me to research and write this article. At the end of this article you will find a list of books and links that you may also find interesting. I hope you find this information helpful.
Basic Genetics of Horse Color and the Dilution Modifiers
Horse color classification by Distribution, Modification and Dilution of Black and Red Pigmented Hair.
Lets start with genetics and the definitions of gene and allele, this will help you understand the following information.
gene A hereditary unit that occupies a specific location on a chromosome and determines a particular characteristic in an organism. Genes exist in a number of different forms and can undergo mutation.
allele One member of a pair or series of genes that occupy a specific position on a specific chromosome.
Every cell contains a set of genes. Each set is derived from the single gene sets contributed by the sire and the dam at conception. The gene sets contain similar, but not necessarily identical, information. If both alleles are identical, then the animal is said to be homozygous at that gene; if the alleles are dissimilar, then the animal is said to be heterozygous at that gene. Information about the homozygosity or heterozygosity for various genes can be inferred from information about parents and/or progeny and can be used for predicting the outcome of matings. You can't tell by looking at an animal whether it is homozygous for any coat color gene, so homozygosity is not used for purposes of identification. Information about coat colors of the sire and dam may be used as an indication of homozygosity, heterozygosity or the presence of an invisible gene. Sire and dam coat colors may also be used as an indication of incorrect parentage or erroneous identification, for these reasons some familiarity with genetic relationships may be useful.
Both sets of genes function simultaneously in the cell. Often when the gene pair is heterozygous, one allele may be visibly expressed but the other is not. The expressed allele in a heterozygous pair is known as the dominant allele this is the gene that visibly affects the coat color or pattern, the unexpressed one as the recessive allele or "hidden recessive" and is sometimes referred to as an "invisible gene". The recessive gene can go from generation to generation even with selective breeding, and still may show up unexpectedly. The dominant genes, however; must be visible in one or both of the immediate sire or dam to be inherited by the foal.
Geneticists symbolize genes by letters such as E, aa, CC, D, gg, ww, toto, which is the genetic formula for grullo. A dominant allele of a gene can be symbolized by an italicized capital letter, and the recessive by an italicized lower case letter. At least 10 genes are responsible for creating the distinctive coat colors of the horse.
In any animal expressing a dominant (visible) allele of a gene, it cannot be determined by looking at the animal whether the second allele is a dominant or a recessive one. The presence of a recessive allele may be masked by a dominant allele, which leads to the expression "hidden recessive" or "invisible gene". Dominant alleles are never hidden by their related recessive alleles.
If an animal has no black pigmented hair, it has the genetic formula ee, which is simply red. An ee animal will be some shade of red ranging from liver chestnut to dark chestnut to chestnut, or sorrel. Manes and tails may be lighter to flaxen, darker (not black), or the same color as the body. These pigment variations of red cannot yet be explained by simple genetic schemes. And the shades of red are not consistently defined by breed associations or regions of the country, so I will not use specific terms for the shades of red to avoid confusion. Sorrel, chestnut and liver chestnut colors will be grouped together under the term red.
A DNA diagnostic test is now available for the red factor that can be used to identify whether the black horse has an invisible red gene, that is whether it is EE or Ee. The test kit is available from UC Davis at: http://www.vgl.ucdavis.edu/horse/redtest.html It is important to understand that the red factor gene test is only informative for one part of the phenotype puzzle. The red factor test detects the recessive form of the extension gene. In particular, black breeders will need to understand that the distribution of black pigment is controlled by the A (agouti) gene. A gene test is presently being developed and will be available to help breeders screen horses for genetic variation at the agouti gene. Information will be posted here when the test is made available.
The gene that controls the distribution pattern of black hair is known as agouti and is expressed A or a. The allele A in combination with E will confine the black hair to the points to produce a bay. Many modifying genes produce various shades of bay horses like the red bay or blood bay. The sandy bay and mahogany bay are due to sooty, shade, pangare and other modifiers. These pigment variations of bay cannot yet be explained by simple genetic schemes either. But black point color - mane, legs, and tail - will always be prevalent in bays.
A=Agouti, Dominant Bay allele
Since each animal has two alleles at a locus, the genotypes at the Agouti locus can be shown as:AA= Homozygous for Dominant Bay
Aa=Heterozygous (one Dominant allele, one Recessive allele)
aa=Homozygous for Recessive Black
This is important to understanding Grullo because aa = recessive black, remember grullo is expressed E?, aa, CC, D, gg, ww. Grulla coloring is expressed in black horses (EE or Ee) with a D allele at the D locus and the aa allele combination at the A locus.
There are allele that affect the bay coat. Genes like the creme allele which when present in the heterozygous form produces the buckskin (this allele appears as palomino on a red horse) and when homozygous the gene produces the perlino. The creme allele only affects the red pigment, so the black manes and tails of bays will not be affected by heterozygous single copy of the "Cr" allele. The resulting color of buckskin will depend on the original bay coat and the modifiers. Typically deeper red bays will produce richer colored buckskins, lighter shades of bay will produce lighter shades of buckskins.
The creme allele causes dilution of the red pigment. When homozygous the creme allele produces the perlino or cremello. Heterozygous horses such as bay and sorrel have red pigment diluted to yellow but black pigment is not affected. A bay becomes a buckskin by dilution of the red color body to gold or yellow without affecting the black color of the mane and tail. A red horse, sorrel or chestnut, becomes a palomino by dilution of the red pigment in the body to gold or yellow with the mane and tail being further diluted to flaxen or white, these horses are known as palomino. A genetically black horse can carry the Ccr dilution allele without expressing it, since the creme allele does not affect the black pigment, it only affects red pigment. This means that a black horse can produce a buckskin or palomino when they pass on the creme allele. A black horse that carries an invisible Ccr can also produce a cremello or perlino when bred to a buckskin or palomino if the foal gets the Ccr allele from both parents.
It is stated that "The creme allele in a homozygous condition completely dilutes any coat color to a very pale cream with pink skin and blue eyes." Such horses are often called cremello, perlino or albino. Typically, such horses are the product of the mating of two dilute-colored animals such as palominos or buckskins, but can rarely occur when mating a black to a buckskin or a palomino because the black can carry the gene without expressing it. Cremello may be difficult to distinguish from white, the hair of cremello or perlino animals will be cream-colored, whereas the hair of white horses will be white. As with white and gray horses, with the cremello or perlino horses it is not possible to determine which alleles of other genes are present. And I personally don't understand how a buckskin can be diluted to perlino if the creme allele does not affect the black pigment, but somehow it does.
The second gene that can affect the bay coat is the dun. It is similar to the creme allele in that it dilutes the coat color but not the point color, it produces lineback duns that can look very similar to buckskins. Unlike the creme allele the dun allele affects both red and black pigments, this makes grullo (black) duns possible.
The dun or D allele determines a second kind of dilution of coat color and it's effects can be confused with those of creme or Ccr. However, there are several important differences of the effects of dun and creme on color. First, dun dilutes both black and red pigment on the body, but does not dilute either pigment in the points. Red body color is diluted to a pinky-red, yellowish-red or yellow; black body color is diluted to a mouse gray, slate gray or silver gray. Second, in addition to pigment dilution, a predominant characteristic of the allele D is the presence of a pattern which includes dark points, dorsal stripe, shoulder or transverse stripe and leg barring. Third, homozygosity for the dun allele does not produce extreme dilution to cremello or perlino like the creme allele does.
Dun pigment dilution pattern is called dun factor. In a red horse, the D allele produces a pinkish-red, peach or apricot colored horse with darkened points known as a red dun or claybank dun. In a bay or buckskin horse, the dun allele produces a caramel, yellow-red or yellow animal with black points known as a buckskin dun or buttermilk dun. A black or brown horse with the dun dilution will be a mouse gray, slate gray or silver gray color with black points known as a grullo.
The effect of the dun allele and creme gene can be easily confused in horses so care must be taken in identification. It is possible for a buckskin to have a line down it's back, but it will not have the shoulder stripe or leg barring if it does not have the dun allele. It is possible for an animal to have both the dun allele and creme allele dilutions, a situation which may be difficult to distinguish except by breeding tests and pedigree research.
The dun dilution is found only in a few breeds of horses. In the United States, it is most commonly seen in the American Quarter Horse, Kiger Mustangs, some other stock horse breeds, wild horses, as well as in some pony breeds.
There are two other dilutions that can affect the coat color of a horse, one is champagne the other is taffy. I will add more information on taffy as it becomes available, for now if you want to find out about the taffy gene, visit the Rocky Mountain Horse, they seem to have the most taffy colored or chocolate palomino colored horses.
Champagne - This is a fairly new term for colors that were once thought to be shades of palomino or dun, but now are understood to be the result of an entirely different gene or allele. This allele dilutes both the coat hairs and the skin pigmentation, unlike other dilution genes. The champagne gene causes red-pigmented hair to turn golden and black--pigmented hair to become chocolate. The skin looks brownish-pink, and the eyes will always be amber. Foals have blue eyes at birth that darken to amber. Red produces gold champagnes, and look like pink-skinned palominos with amber eyes. If a gold champagne is particularly light, it can be mistaken for a cremello, but the amber eyes are a indication that is a champagne, as is the skin, which has a slight brownish tint to it, unlike a true cremello. Champagnes that are genetically bay are amber champagnes, and like the gold champagne, will have the pink-brown skin, golden coat, and amber eyes, but the points will be chocolate-colored. Champagnes that are genetically black are usually just called 'champagne'; in the past, they were often called 'lilac dun' because this champagne often has a purplish cast to the coat. Like gold and amber champagnes, they have pink-brown skin and amber eyes, but the coat is a dark bronze color. Champagnes that also inherit the creme gene in addition to the champagne gene are often much lighter, and are called ivory champagnes. The color is dominant, a champagne horse must always have a champagne parent. Homozygous champagnes, probably aren't any lighter than a heterozygous champagne. You can visit the Champagne Horse website http://www.champagnehorses.com/ for more information on this dilution.
Combinations of the above colors, and others, exist and is referred to as gene layering. Many combinations can occur and make identification confusing. We have a dun roan mare, most of the year, she looks dun, but for a couple months of the year she is roan with a dorsal stripe and she had the most beautiful dun roan colt. Our dun roan mare is registered as a dun with the roan listed as a white marking. You may also see beautiful palomino horses with all the dun factor markings, this is sometimes called dunalino but is registered with AQHA as palomino or red dun with a flaxen mane and tail. There are now some references to dunskin, which is a buckskin (yellow rather than caramel color) horse with dark or black dun factor, horses of this color have been referred to as yellow dun, buttermilk buckskin dun or buckskin dun.
Here is a chart on the possible outcome if the base color is modified by one or more of the dilute allele. Keep in mind that the composite dilutes can be confusing to describe and may take some research from information about parents and/or progeny to decide what they really are.
I will add more on the composite dilutions as time goes on. The composite dilute horses are most beautiful to look at, but are more difficult to categorize and more confusing to try to explain and understand.
Just like in the genetics associated with the grey gene and the dun factor gene, it takes a roan to make a roan. One of the parents MUST have been roan in order for the foal to be roan.
True "roan" is described as: Where there is a mixture of white hair with the body color, lightening the general effect of the latter. Where the body color is black or black-brown, the mixture of white hairs gives a blue tinge to the torso, with head and limbs from the knees downward remaining the base color of black, resulting in a blue-roan. Where the body color is bay or brown, the mixture of white hairs gives a red tinge to the torso considered bay roan. Sorrel and chestnut with roan will be true red roan.
AQHA Recognized colors
There are a many foal colors that are difficult to determine whatcolor you should register the foal as with the American Quarter Horse Association. For instance a palomino dun should be registered as a red dun with a flaxen mane and tail and noted under remarks that the individual carries and expresses the creme gene. A dun roan could be registered dun and under remarks list that the individual carries and expresses the roan gene or registered as a roan and list the dun coloring under the remarks.
Color and Genetic Formulas
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