Why there’s no such thing as a white horse.

Black, brown, bay, chestnut, palomino – horses come in several colours. Except white. Dr Mac explains why.

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A white horse is actually grey – it’s a colouration that occurs when a gene causes the hair coat to gradually lose its colour.A horse may be born chestnut, black, or even palomino, but if its genetic makeup has a dominant grey gene, the coat will change over the years, turning dark grey when the horse is six to 12 months old and often pure “white” by the age of six.

Understanding the genetics
Each animal has a genotype (genetic makeup) and a phenotype (what you see when you look at the animal). Often, the two aren’t the same. For example, a mare can have a gene for pinto-colouring, but have hardly any white markings. Here, genotype isn’t “expressed” in the phenotype.

Genotype develops from the pairing of genes on paired chromosomes. One of these chromosomes comes from the horse’s sire and one from its dam.Although the genes are paired, they’re not identical and this results in “dominant” and “recessive” genes. An animal can have a pair of dominant genes, a pair of recessive genes or one of each. Every cell in the animal’s body has paired sets of chromosomes in the nucleus.

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These act as a template for the production of the proteins that keep our bodies running and cause the expression of our phenotype. Certain dominant genes, like the grey gene, will prevent the phenotypic expression of other genes. Dominant genes are reflected in capital letters, recessive genes in lower case letters. So “G” represents a dominant grey gene, and “g” a recessive non-grey gene.

Grey gene inheritance
Grey is the most dominant colour. If a horse has even one grey gene (Gg) it’ll be a grey horse, no matter what its birth colour. If a horse is grey, at least one of its parents was grey. You can’t have “hidden” grey genes. A grey horse can have one or two dominant grey genes. That is, its genotype could be Gg or GG. If it has two dominant genes (GG), it’s known as a “homozygous grey” and all the foals of this horse, whether it’s male or female, will be grey in colour.

But if you mate a homozygous grey horse (GG) to a horse of solid colour, like a bay or chestnut, all of its offspring will be Gg. And a Gg grey horse can have foals of solid colour. So if you mate a Gg horse to a chestnut horse (Gg X gg), there’s a 50% chance the foal won’t be grey. You can mate two grey horses and have a foal of solid colour. This happens when a
heterozygous grey horse (Gg) is mated to another heterozygous grey.

In this case, there’s a one in four chance of getting a non-grey, solid-coloured foal. There’s also a two in four chance of breeding a heterozygous grey (Gg) and a one in four chance of breeding a homozygous grey (GG).

Grey gene not breeders’ favourite
A grey gene can mask the Appaloosa and paint or pinto genes, as the mottled coat colour becomes a uniform grey or white with time. That’s why these breed registries discourage the use of grey horses in breeding programmes. But they can use a solid-colour horse bred out of two greys as it has no chance of breeding grey.Contact Dr Mac c/o [email protected].