Chetar Arabians ~ EQUINE COLOUR GENETICS ~ Grey ~

Grey is one of the most popular and most prolific of all horse colours. So much so that many breeders and owners actually consider Grey to be a Base Colour due to it being so very common. Genetically speaking, however, this is technically not correct and so is not considered a Base Colour for the purpose of this genetics section on my website.
In some breeds, such as the Percheron, this colour has been specifically chosen to the point of excluding most other colours. There are many people throughout the world that have an unparralled affinity for grey and will always prefer grey horses above all else, regardlee of the quality of said horses.

The genetic process behind grey is one of the easiest to understand of all of the equine coat colours. The characteristics of the process of greying out in horses is actually quite similar to the way the hair of some of us humans can turn grey with age, and is easy to understand because of this.

All Grey horses are born what is commonly termed a 'base colour', with the meaning of the term used in this frame being different to that of the "Base Colours" Chestnut, Bay and Black that I've previously explained in this section to my website - To avoid confusion I've referred to the base colour of a grey horse across this page as the "original" colour of the horse ie. the colour that the horse is born prior to said horse greying out [with 'base colour' being reserved solely for the colours Chestnut, Bay and Black as I've previously described on the Base Colours page.]

Grey can and will occur on any base colour, modified base colour, diluted base colour and/or superimposed white pattern on any of the formerly menitoned colours. Grey is dominant to all other colours and the genes responsible for them.

The Grey colour that we see phenotypically, while varying greatly between individuals, are all the result of a single process on the genetic level.
Remembering that all horse colours are achieved by the placement of pigmentation in the skin and hair shaft by melanocytes... The Grey colour is achieved by the gradual and ongoing depigmentation of the hair shaft. While all grey horses are born with pigmented skin and hair shafts, as they age the Grey Gene takes affect and gradually reduces the amount of pigment in the hair shafts that were pigmented during foetal development. The eventuality is that the affected hair shaft will have less to no pigmentation remaining in the hair shaft. The depigmented areas the hair shaft are transparent, though due to the pigment remaining in the skin underlying the hair shaft, we phenotypically see the Grey colouring in the coat of the horse due to the dark/black skin pigment reflecting through the hair shaft.

The Grey Gene is widely held to be the "strongest" of all genes that appear in a horses' genotype. This view has been reached by breeders across the globe due to the tendancy of certain greys that, genetically speaking, should have a 50% grey progeny inheritance rate, that actually have a 75% or higher grey progeny inheritance rate. Some families have an almost exclusive grey production rate even though bred to solid coloured mates and whilst being heterozygous for the gene themselves.

Grey is a Complete Dominant Gene, meaning that a] If a horse carries the Grey Gene, then it will most definitely be visible in the phenotype, and, b] the phenotype of the heterozygous form is identical to that of the homozygous form - there is no certain way to identify one from the other by assessing the phenotype alone. [More information on Complete Dominant Genes can be found on the Introduction to Genetics page]. The presence of the Grey Gene is dominant to its' absence. If a horse carries a Dominant Grey Gene, it will most definitely grey out with age.

Grey horses are born an original colour - be it a Base Colour and so Chestnut, Bay or Black - or a diluted colour, Palomino or Cremello etc - all Greys are born another colour completely different to Grey. Most 'going to be grey' foals, however, typcially indicate the colour that they will eventually become at the time of their birth. This indication most commonly comes in the form of "rings" of white or interspersed [with original colour] white hairs around the eyes of the foal at birth or soon thereafter. Other indicators include white hairs around the muzzle, on the legs and/or in the mane and/or tail.

The Shorthand representation of the Grey Locus is "G".
The shorthand representation of a Dominant Allele occurring at the Grey Locus is an uppercase "G" also, so therefore would look like this = GG
The shorthand representation of a Recessive Allele occurrin gat the Grey Locus is a lowercase "g", so would therefore look like this = Gg

Non-Grey ie. Homozygous Recessive = [Gg, Gg]
Heterozygous Grey = [GG, Gg]
Homozygous Dominant Grey = [GG, GG]

For various reasons, the Grey Gene is the one and only exception to the Extension Locus and Agouti Locus being represented first when writing the genotype for a horses' colour [when not using the alphabetical format]. These reasons include the fact that Grey is the only single Complete Dominant Gene that is in fact dominant over every one of the other colour genes. It is the one and only exception to the rule.

If a horse is Grey, the Grey Locus Alleles are respresented first, followed by the Extension Locus Alleles and then the Agouti Locus Alleles.
Any further colour genes that the horse may possess are represented thereafter.

If a horse is not grey then the Grey Locus Alleles - being [Gg, Gg] - are usually omitted from the written genotype completely.
It is safe to assume that any genotype that does not include the Grey Locus Allelic representations is indicative of a non-grey horse, and that the Grey Locus representation is in fact - [Gg, Gg].

There are two main "types" of Grey. They are characterized by different end result colours of grey, or the final amount of depigmentation of the hair shaft.
The first is characterized by the complete loss of all pigment in the hair shaft, and also sometimes in the skin [though this is relatively subjective]. This group of Grey horses are those that become completely white with age, that retain no pigment whatsoever in the hairs. These are the 'fairytale' horses with the flowing long white manes and tails..well, maybe not fairytale horses, but they all still have white manes and tails!
The second group of Greys will retain some pigmentation in the hair shaft. This retention is commonly on the lower extremeties and in the mane and tail, if not across the entire coat - this group includes the colours with various, further descriptive terms, such as "Dappled" Greys and "Fleabitten" Greys. The pigment retained in the hair shafts will be of the original colour of the horse ie. the colour said horse was born.

For a long time the two types of grey were commonly believed to be the result of homozygous dominant [completely white greys] vs heterozygous [greys with some retained pigmentation] allelic forms. This, however, has been proven to be incorrect upon dedicated genetic research. There is no link between how white a grey may/will become and the heterozygous/homozygous dominant nature of their genotype at the Grey Locus.
The differences between the various end shades of grey and various amounts of depigmentation has not been shown to be due to a genetic process at all. The rate at which an individual greys out, however, seems to be at least partially under the genetic control of independant modifiers.

More Information & Photos on the Various Greys Coming Soon!

The rate at which a horse will undergo the process of greying out varies between individuals. This rate seems, and is commonly held to be, somewhat subjective by breed. For example, Arabians appear to grey out at quite a rapid rate, whereas Percherons will take years upon years to grey out.

Regardless of the rate at which a horse greys out, most greys will follow a typical pattern of greying out. Typcially, the head and legs will grey out first [with these being the areas many grey foals are born with white hairs already present], followed by the body [commonly in patches at various fast and slow growth rates], and finally the mane and tail will go grey, if they go grey at all [see the Different Varieties of Grey below].