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Equine Coat Color Genetics
Equine coat color and/or pattern are strictly determined by genes present in each of these animals’ cells. With the exception of the reproductive cells, egg and sperm, each cell contain one pair of genes, of which one was passed on by the female(mare) and the other by the male(stallion). The composition of genes in pairs within one cell (internally) as described above is called Genotype, whereas external characteristics such as color and other observable structures are called Phenotype. Animals containing the same phenotype may have a different genotype which will have a decisive impact on their offspring’s’ outcome. Also, different versions of the same gene are called alleles.
Universally, the capitalized letter always represents the dominant allele and the recessive is always represented by lowercase. Organisms with two copies of the same allele of a given gene are called homozygous while organisms with two different alleles of a given gene are called heterozygous.
The Pure Blood Lusitanian (PBL) horses, has a range of colors that include; Grey, Black, Chestnut, Bay, Buckskin, Palomino and Isabel. Regarding horses, their genes might present characteristics either dominant or through inheritance which will be explained below.
Gray
This horse color reveals itself with time, meaning that the foal is born with one color and within a few days white hair begins to grow overall especially around their eyes and head. This is a continuous process until the horse grows all white/light gray.
The gene responsible for this coat color is represented by the dominant G so if there is at least one dominant allele present either from the male or female parent, the foal will be a Gray. Also, the gene G is dominant against every other gene ensuring that the smallest presence of this gene will make sure the horse will have this color coat with the possibility of being GG or Gg. Horses born gg will have other colors varying according to other genes and gray genes that do not emerge will illustrate the out coming color early.
A GG horse would produce a gray foal, whereas a Gg horse mating with another Gg horse may have the possibility of a different colored animal once both the genes passed on from the parents would be gg. In this case, the gg horse mating with another gg horse would also have a different colored offspring. As a result of this G dominancy, Grays have their genotypes’ always described as G_, indicating the indifference of the second gene.
Gray Horses: G_ _ _ _
Black, Bay and Chestnut
These colors originate from dominant genes B and A against b and a, recessive. The presence of gene B, notes the existence of black pigment in some part of the animals’ body whereas the gene A distributes this black pigment. Now, if the animal is B-recessive; bb, this horse will not contain the black pigment at all, therefore will be Chestnut colored. Chestnut color is an orange/reddish tone covering the entire body having mane and tail sometimes lighter than the rest of the horse. It is important to emphasize the fact that this color coat is B-recessive therefore no black pigment, so it will not have black even if gene A is present, as this gene is only responsible for the distribution of the black pigment at the animals’ extremities (mane, tail and legs towards the hoofs). Horses with genes B_A_, have black pigment both present and distributed, meaning that extremities can be black with the rest of the body orange/reddish. These horses are called Bay. For the horse to be completely Black, it must be A-recessive and have gene B present. This way, there would not be any distribution of the black pigment towards the extremities so the color would be the same throughout the entire horse. From my own experience it is possible to identify whether a horse is homozygous or heterozygous, for gene A, just by its phenotype. Bay heterozygous Aa horses commonly known as Zainos have darker patches throughout the body, especially on both sides of the neck and Bay homozygous AA horses present a reddish tone tending to a lighter tone equally around the body.
Black Horses : gg B_aa
Bay Horses : gg B_A_
Chestnut Horses : gg bb_ _
Palomino, Buckskin and Isabel
The Cr gene is responsible for these coats. It manifests itself according to quantity; one Cr gene is equal to one phenotype and two Cr genes equal a different one. This gene functions only to dilute the color of the fur coat of the animal. The presence of one Cr gene, will dilute every reddish part to a lighter yellowish color just like Palominos and Buckskins. For Buckskins, mane, tale, and extremities of legs keep their black color as there is only one Cr gene that will dilute only the reddish tone of the horse. As a result we have a light yellowish horse with dark extremities. Since Palominos have no Black tone at all their dilution occur completely throughout the body, and extremities lighten up tending to the color white. For a Isabel horse, the dominant genes are CrCr. As both dilutions occur (for reddish tone and black tone), the horse coat fades into a pearl white color, eyes are blue and mucosities, pink. Commonly, breeders call a horse Isabel any one with blue eyes and a pink nose. Actually, Gray can be mistaken for being Isabel as it might be hard to see white fur throughout the pearly grayish coat. The confirmation can only occur through genealogy or progeny.
Even Black Horses, that technically should not be modified by the presence of a Cr gene, do become a lighter Black (Chocolate) if the gene mentioned is present, and despite the fact that this name is not official, I, personally do use it to differentiate from the original Black and believe that since genotype and phenotype are different, these fur coats should be given distinct nominations.
Buckskin horses: gg B_ A_ Crcr
Palomino horses: gg bb _ _ Crcr
(Chocolate) Black horses: gg B_aa Crcr
Isabel horses: gg _ _ _ _ CrCr
Yellow Buckskin, Lobuno and Clay
There is another coloration for coats that is called Yellow Buckskin, but is originally from another pair of genes. What determines this color is gene D. This gene is dominant, therefore it does not matter what is the second gene, and the horse will always be Yellow Buckskin. The fur coat goes through some sort of dilution, this time, for the color black as well as red. Head and extremities remain dark colored and a dorsal stripe is present and a slight resemblance of zebra coat close to the hoof. This coloration can be clearly seen on Sorraia horses also known as Lobunos for diluted Black, and Clay for diluted Chestnut.
Lobuno Horses: gg B_ aa crcr D_
Yellow Buckskin Horses: gg B_ A_ crcr D_
Clay Horses: gg bb _ _ crcr D_
Color Possibilities through mating:
Gray X Gray: With both horses heterozygous (Gg), every color is possible. If not, then the offspring can only be Gray.
Gray** X Black: Any coat color is possible.
Gray** X Bay: Any coat color is possible, with the exception of Isabel.
Gray** X Chestnut: Any coat color is possible, with the exception of Isabel.
Gray** X Palomino: Any coat color is possible.
Gray** X Buckskin: Any coat color is possible.
Gray** X Isabel: Gray, Palomino, Buckskin, Isabel and Black.
Black X Black: Black, Chestnut, Palomino, Isabel.
Black X Chestnut: Black, Chestnut, Palomino, Bay or Buckskin.
Black X Bay: Black, Chestnut, Bay, Palomino or Buckskin.
Black X Palomino: Black, Chestnut, Bay, Palomino, Buckskin or Isabel.
Black X Buckskin: Black, Chestnut, Bay, Palomino, Buckskin or Isabel.
Black X Isabel: Black, Palomino, Buckskin or Isabel.
Bay X Bay: Black, Chestnut or Bay.
Bay X Chestnut: Black, Chestnut or Bay.
Bay X Palomino: Black, Chestnut, Bay, Palomino or Buckskin.
Bay X Buckskin: Black, Chestnut, Bay, Palomino or Buckskin.
Bay X Isabel: Black, Palomino or Buckskin.
Chestnut X Chestnut: Chestnut only.
Chestnut X Palomino: Chestnut or Palomino.
Chestnut X Buckskin: Black, Chestnut, Bay, Palomino or Buckskin.
Chestnut X Isabel: Black, Palomino or Buckskin.
Palomino X Palomino: Palomino, Chestnut or Isabel.
Palomino X Buckskin: Black, Bay, Chestnut, Palomino, Buckskin or Isabel.
Palomino X Isabel: Black, Buckskin, Palomino or Isabel.
Buckskin X Buckskin: Black, Chestnut, Bay, Palomino, Buckskin or Isabel.
Buckskin X Isabel: Black, Buckskin, Palomino or Isabel.
Isabel X Isabel: Isabel Only.
-** Animals heterozygous with gene G (Gg), if not, only Gray.
Text by Flavio Rafael Monteiro
Translated terms
English -------- Portuguese
Gray ---------- Tordilho
Black --------- Preto
Bay --------- Castanho
Chestnut -------- Alazão
Buckskin -------- Baio
Palomino ------- Palomino
Isabel -------- Isabel
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