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Grey
Notation: G
Alleles: G, g
Dominance: G > g
Basic Function: Causes loss of pigment (colour) in the hair and parts of the skin over time.
Genotypes & Phenotypes:
GG = Grey
Gg = Grey or Flea Bitten Grey
gg = non-grey


Description:
Grey horses are born fully pigmented, but lose colour in their hair (and sometimes skin) as they age. How long this takes often varies by breed, as well as by whether the horse has one or two copies of the grey gene. Homozygous horses may turn grey faster than heterozygous horses. The colour may be called different things depending on the stage of greying and what colour the horse was before it started going grey.

Horses with the grey gene are prone to vitiligo (progressive loss of pigment from the skin) as well as melanomas (skin cancers). Homozygous horses are more prone to both than heterozygous horses.

Variations:

-Steel Grey: A very early stage of greying, usually on a black horse, where there is little dappling (not illustrated).
-Dappled Grey: Middle stages of greying, where dapples are apparent (Horse 1)
-Rose Grey: Early stages of greying on a bay or chestnut horse, where the reddish colour is still apparent, but looks sort of "washed out" (not illustrated).
-"White": The last stage of greying in most horses, especially homozygous horses, where most if not all of the hair is white (Horse 2). Not the correct name for this colour, as "white" should technically only be used for horses with the dominant white mutation.
-Flea Bitten Grey: A grey horse with tiny speckles of pigment, often the same colour as the base coat prior to greying. This coat colour pretty much only happens in horses that are heterozygous (have only one copy) for the grey gene (Horse 3).


Genetic Cause:
Greying in horses is different than in humans and most other mammals. It is caused by a mutation in a regulatory gene, STX17. Horses with the grey mutation over-express other genes, causing the grey coat colour, vitiligo, and the increased risk of melanoma.

Sources:

Pielberg, G.R. et al. (2008) A cis-acting regulatory mutation causes premature hair
graying and susceptibility to melanoma in the horse. Nature Genetics 40:1004-1009
https://www.nature.com/articles/ng.185