Before discussing the specifics of each coat colour's genetic makeup, you must be familiar with some basic concepts and terminology. This is a simple glossary; I encourage you to do your own research in this area to better understand any term and ideas you may be having difficulties with.
CHROMOSOME : A chunk of the 'genetic code' that determines all traits, made up of strands of DNA. In horses, like in humans, chromosomes come in pairs.
GENE : A section of a chromosome that codes for the production of a certain protein.
LOCUS : The location of a gene on a chromosome. Because chromosomes are paired, there are two loci for each gene, one on each chromosome.
ALLELE : One of two or more forms, or 'options' of a gene. Alleles can be dominant, co-dominant, incompletely dominant, or recessive.
HOMOZYGOTE : An animal carrying the same allele on both loci for a specific gene. This means that the animal will ALWAYS pass this allele onto any offspring. A homozygote must have two parents carrying the allele it is homozygous for.
HETEROZYGOTE : An animal carrying two different alleles for the same gene. This animal could pass either one of these alleles onto its offspring.
DOMINANT : A dominant allele will always be expressed, even in a heterozygote. It will 'mask' the recessive gene. A horse expressing the dominant form of a gene must have had at least one parents also expressing it.
CO-DOMINANT : When a co-dominant allele is found in combination with another co-dominant allele, both will be expressed. This is not found in horses, but can be seen in people, for example, in the blood type 'AB' -- a combination of both type 'A' and type 'B'.
INCOMPLETE DOMINANT : When an individual is heterozygous for an incompletely dominant allele, a 'partial' effect will be observed. For example, a chestnut horse heterozygous for Cream will be a palomino, while a chestnut horse homozygous for cream will be a Cremello. It's the equivalent of crossing a red plant and white plant and getting pink plants as a result.
RECESSIVE : The allele that is hidden by the dominant allele. A recessive allele will only be expressed in its homozygous form.
GENOTYPE : The animal's genetic makeup.
PHENOTYPE : The animal's outward appearance.
All equine colours are based on the two base colours: chestnut (red) and black. Both of these are coded for on the E gene. If the dominant allele (E) is present, the horse will be phenotypically black. A horse carrying two dominant alleles will be homozygous for black (EE), and will not be capable of producing a red-based foal. In order for a horse to be phenotypically chestnut, it must carry two copies of the recessive allele (e). Therefore, all chestnuts are homozygotes (ee), and breeding two chestnut horses together will always result in a chestnut foal.
Breeding a chestnut to a non-homozygous black (Ee) will result in 75% homozygous chestnut foals and 25% heterozygous black foals. Breeding two heterozygous black horses together will result in homozygous black 25% of the time, heterozygous black 50% of the time, and homozygous chestnut 25% of the time. Note that a black horse cannot produce a bay foal unless bred to a bay or a chestnut who carries the Agouti gene [see next section].
The Agouti gene acts upon a genetically black [either homozygous or heterozygous] horse to restrict the production of black pigment on the body. As a result, a black horse carrying at least one copy of the dominant Agouti allele (EEA_) will appear bay/brown, with a red body and black points. It is interesting to note that a chestnut horse carrying a dominant Agouti allele (eeA_) will still appear chestnut, because there is no black pigmentation in the coat to act upon. A horse who carries two identical copies of the dominant allele will be homozygous for bay/brown (AA), and will not be capable of producing a black foal, only bays and chestnuts. Chestnuts can also be homozygous for Agouti, and these will only able to produce bays and chestnuts, depending on the colours of their mates.
Seal browns are Agouti (bay) based -- they are genetically identical to bay according to all currently available tests.
Light Blacks are black based -- they are genetically identical to black. Light Blacks have been reported as being born with blue eyes that change to gold as they age. This colour may be the result of a new dilution, but as very little information is available, no assumptions can be made.
Very little is known about seal browns and light blacks at this time, and for the remainder of this article, seal browns will be grouped with bay while light blacks will be grouped with blacks.
Chestnut horses with white/light manes and/or tails are known as 'flaxen chestnuts', a colouration thought to be caused by the F gene's modifying action on the red pigment in chestnut horses' mane and tail hair. This gene is thought to be expressed only in its homozygous recessive form (ff), but no conclusive results have been recorded yet. It is also suspected that the F gene causes the lightening of some chestnut horses' legs, usually seen in conjunction with flaxen manes/tails. Though flaxen occurs almost exclusively in chestnuts; very rarely, and almost always in the Arabian breed, a flaxen bay or black will be found. These horses usually only show streaks of white or silver through their manes/tails, and do not often have fully flaxen hair.
The Sooty modifying gene (STY/sty) can act on any colour, causing a darkening or dappling of the coat, usually more concentrated along the topline of the animal.
The Pangare` (P/p) gene causes the 'mealy muzzle' and light underbelly seen in many native pony breeds (Exmoor, Dartmoor, etc).
When an animal carries two copies of the dominant allele (CRCR), it will be expressed as a double dilution of the base colour. A genetically chestnut horse homozygous for cream will phenotypically appear cremello, or blue eyed, pink skinned, cream. A genetic bay will appear perlino, with slightly darker points than a cremello. A black horse affected by a double cream dilution will appear smoky perlino, slightly darker, and often 'bluer' than a normal perlino.
Cream homozygotes will always produce dilute (single or double, depending on the second parent) offspring.
A double dilute bred to a non-dilute will produce only single dilutes.
Double dilutes bred to single dilutes will produce 50% double dilutes and 50% single dilutes.
Double dilutes bred to double dilutes will always produce double dilutes.
Single dilutes bred to non-dilutes will produce 75% non-dilute and 25% single dilute foals.
Single dilutes bred to single dilutes will produce 25% double dilutes, 50% single dilutes, and 25% non-dilutes.
Base colours of all above crosses can be determined by disregarding the CR gene and looking for the presence/absence of E, e, A, and a alleles.
Like the Cream gene, the Dun gene (D/d) causes a dilution of red pigmentation, though it does affect black as well. Unlike the Cream gene, however, the Dun gene is completely dominant, does not have a double dilute form, and horses homozygous for the gene will be no different in appearance from heterozygotes.
The major, and defining difference between Dun and all other diluting modifiers is the presence of 'primitive markings' or 'dun factor markings', which are necessary in order to prove the presence of the Dun gene. These markings include dorsal stripes, transverse shoulder striping, leg barring and zebra stripes, facial cobwebbing/masking, dark ear tips, guard hairs (often white), and countershading/mottling on the body. Dorsal stripes and leg barring are present in almost every dun horse, while the expression of other markings varies between individuals.
When Dun interacts with a red (chestnut) horse, it produces red dun (Ddee) (also called chestnut dun). Dun and black produces grulla/grullo (DdEe)(also called black dun). The Dun gene, when interacting with the bay horse's genetic makeup, produces the standard 'Dun' colour (DdAaEe) also known as Zebra Dun, Yellow Dun, or Bay dun.
Breeding heterozygous duns will result in homozygous dun foals in 25% of foals, heterozygous duns in 50%, and homozygous non-duns in 25%. A homozygote will always pass on the dominant D allele to all offspring, meaning that all offspring will express the dun dilution.
Base colours of dun-diluted horses' offspring can be determined by ignoring the D gene's effects and looking at the interactions of the E, e, A, and a alleles.
The Taffy colours are caused by the Taffy gene's (Z/z) action on black pigment in the horse's coat, mane, and tail. Like duns, it is completely dominant, with only one dominant allele needing to be present in order for the dilution to be expressed. Homozygotes are no different in appearance from heterozygotes.
When the taffy gene is found in combination with red (chestnut), the horse does not usually express a difference in phenotype from a regular chestnut horse, because a chestnut horse has no black pigment to dilute, and the gene will often go unnoticed until the horse is bred and produces a taffy foal from a non-taffy mate. These taffy 'carriers' are known as Chestnut Taffies (Zzee). Taffy in combination with a black horse will result in either a Silver Dapple Taffy or a Chocolate Taffy (both ZzEe), depending on the presence or absence of dappling through the coat. These horses will be a chocolately brown colour with a silver or flaxen mane and tail, and in the case of the Silver Dapple, will express lighter dappling throughout the coat, sometimes making the horse appear slightly pinkish. A Red/Bay Taffy (ZzAaEe) is produced when Taffy is found in combination with Bay. Horses expressing this genotype will be a reddish colour with chocolate points; and flaxen, chocolate, or a combination of both, manes and tails.
Horses can be homozygous for the Taffy gene, and these horses will always produce taffy offspring.
Heterozygotes bred to other heterozygotes will produce homozygous taffies 25% of the time, heterozygous taffies 50% of the time, and non-taffies 25% of the time.
Heterozygotes bred to non-taffies will produce heterozygous taffies 50% of the time, and non-taffies 50% of the time.
Champagne colours are the result of the Champagne gene's (Ch/ch) interaction with the chestnut, black, and bay base colours. Again, it is a completely dominant gene, and there is no difference in appearance between a homozygote and a heterozygote. The Champagne gene acts on all pigment, turning red to gold, and black to a red, silver, or a chocolate colour. It also results in pink skin and striking white or blue eyes at birth that eventually darken to a teal, green, or amber colour. A metallic sheen to the coat is also very common within this colour group. When Champagne is found in combination with the chestnut gene, the Gold Champagne (Zzee) results. These horses look identical to palominos, but express the pink or orangey skin and teal eyes typical of champagnes. It is possible for horses of this colour to show manes and tails that are not pure white, but may be a gold or reddish colour. Champagne together with black produces the Black/Classic Champagne (ZzEE), which appears as a smoky or silvery brown, or occasionally a lavender colour, with darker, often chocolately, points and the typical pink skin and teal eyes. This colour is often mistaken for a form of dun, but can be easily distinguished by the lack of dun factor markings. When Champagne combines with the Bay genes, an Amber Champagne (ZzAaEe) is produced. These horses are similar in appearance to Golden Champagnes, but they display darker, chocolatey points, manes, and tails. When Champagne combines with Seal Brown, a Sable Champagne is produced. It is genetically identical to an Amber Champagne, and is almost indistinguishable in it's phenotype. Little is known about Sable Champagnes at this time.
When a homozygous Champagne horse is bred to any colour, it will produce only Champagne offspring, which may be homozygous or heterozygous, depending on if the other parents carries and passes on the Ch gene.
Heterozygotes bred together will produce homozygotes in 25% of breedings, heterozygotes in 50% of breedings, and non-champagnes in 25% of breedings.
Heterozygotes bred to non-champagnes will produce heterozygotes 50% of the time, and non-champagnes 50% of the time.
When a single Cream dilution is combined with Champagne, the colours produced are named by combining the base champagne colour and 'cream'. Gold Creams (gold champagne + cream) are a more pronounced cream colour, and display greenish eyes. Amber Creams (amber champagne + cream) and Sable Creams (sable champagne + cream)are similar to Gold Creams, but display darker points. Theoretically, a Classic (Black) combined with a single cream dilution (or conversely, a smoky black with a Champagne dilution), should be very similar or identical to normal black Champagnes, as single Cream dilutions do not affect black pigmentation. These would be called Classic Creams or Black Creams. Champagnes in combination with double cream dilutions have yet to be identified, to my knowledge, but theoretically, they should be very similar in appearance to non-champagne double cream dilutes.
Champagne and dun in combination would produce pink-skinned, teal-eyes duns with diluted boat colours. These are referred to as Amber Dun, Golden Dun, and Black/Classic Dun, depending on the base colour.
Champagne and Taffy have never been identified in the same horse. It's difficult to predict how they would interact, but it is likely that this combination would produce pink skin, teal eyes, and lighter body colours in these horses, as well.
Taffy combined with single cream dilutions results in a lightening of the body colour. Red/Bay Taffies are diluted to a yellowish colour with darker 'seal' points, called Lemonsilla or Cactus Chestnut. This colour is similar to dun, but no primitive markings are seen. Lemonsillas are also often described as palominos with non-white manes and tails. They can vary in shade from a very light golden to a very dark, almost black, colour with yellow highlights.
Chestnut taffies, on their own, are usually indistinguishable from non-taffy chestnuts, but when a single cream dilution is added, the 'Fawn' Palomino is produced. These horses show a pinkish body colour with white/light manes and tails. Silver Dapple and Chocolate Taffies diluted by a single cream gene appear similar to Fawn Palominos, but are usually slightly darker, often with a bluish cast to the coat, and are called Fawn Taffies. Double cream dilution acting on Taffy has, to my knowledge, never been identified. In theory, these horses should not appear very different from normal double cream dilutes, but perlinos and smoky perlinos may display lightening of the manes, tails, and points.
Taffy interacting with Dun produces Taffy Duns, whose shades will depend on the base colour, and will be named for the undiluted Taffy colour that has been diluted (ie Red Taffy Dun, Chocolate Taffy Dun, Silver Dapple Dun).
Pearl (PRL/prl), found only in Quarter Horses/Paints decended from My Tontime/Barlink Macho Man, Andalusians, Lusitanos, and Paso Finos at this time.
Royal Hanoverian, perhaps misidentified Champagnes? Champagne with a cream double dilute?
Roan horses display white hairs mixed in with the base coat colour over the body, while the head, legs, mane, and tail remain solid. Roans are named from their base colour, ie Bay Roan, Palomino Roan, Black Roan, etc. An important distinguishing feature of true roan from gray is that it is not progressive -- a roan will remain the same shade of roan through its entire life, but may become 'darker' (ie closer to its base colour), whereas greys will ALWAYS become lighter with age. Roan is caused by a dominant gene, expressed only when in its heterozygous form (Rnrn). There is no such thing as a homozygous roan; some believe that the homozygous form of the Rn gene causes fetal resorption or abortion early in pregnancy, but this theory has yet to be proven either true or false. Regardless, no horse has been reported, to my knowledge, that has produced 100% roan foals.
Home | Guests | Forum | Establishments | Resources | My Account | Contact
Copyright 2008 Maya Kenedy. All rights reserved.
Please read our Terms Of Use.