White horses have captured people's attention for centuries. In some cultures they are loved, while others despise them. Many genes can be responsible for a complete white horse, but most are stable from foal to mature, with the horse staying approximately equally white during its lifespan. The grey gene is one of two exceptions.
Grey is progressive, meaning a horse is born with little or no white hairs, but gains them while he grows older. This process can be very fast, turning a colored horse into white within a few years, but it may also take a lifetime or even longer. This difference is at least partially hereditary, as the trait to lighten slow or fast tends to accumulate in breeds and/or lineages. But in every case, a horse will turn completely white eventually - the only exception is when it dies before reaching that stage.
Some older white horses actually regain some colored hairs. This variety of grey is called 'flea-bitten', as the horse seems to have many colored bugs across its otherwise white coat.
Grey is a dominant gene, meaning it technically should always show. In some cases, where a horse may lighten real slowly, it may however be overlooked, wrongly explained, or dismissed completely.
PhenotypeGrey is actually not a real colour, but a modifier, a gene that alters the appearance of hair colour. Every foal is born with its true coat colour, but has its coloured hairs replaced with white ones as it ages. Some breeds, like the lipizzan, tend to turn white very fast, while others take a lifetime. But even within breeds it may alter, depending on the genetics of the parents. This process will continue as long as it needs to turn the horse white, or until it dies beforehand.
Foals may be born with their true coat colour, but there are still some features that may give the presence of the grey gene away. One of the most obvious examples is the dark coat colour. Most foals are born with a neutral, quite light coat, that goes through different stages as the horse sheds out, but will usually end up slightly darker than shortly after birth. In grey horses, it seems as if the foal is born with its adult coat colour, except that the grey gene will actually come to lighten it. For example, black horses are usually born a more mouse-grey, only turning black after shedding out. Black foals going grey however, are born with their would-be adult coat colour, appearing almost black as night. The same happens with other coat colours, though not every coat has such a strong difference. This trait, to keep the coat as dark as possible before the hairs turn white, may be the source for some strikingly dappled horses.
Beside the dark coat, there is another tell-tale characteristic that will give away the grey gene: white hairs around the eyes. It does not appear on all horses, though in some more extreme cases a horse can appear strikingly goggled.
The first grey hairs announcing the start of the greying process appear on the head, from which they spread to the rest of the body, starting with the neck and moving towards the hindquarters. With each shedding out, the horse will turn lighter. In some horses, this patterns appear uniformly, mimicking a roan horse. In others, some patches lighten much slower, leaving dark dapples along the coat. The legs are usually the last part to turn white, especially in horses with a dark coat colour.
As the grey progresses, it will become harder and harder to recognise the horse's original coat colour. Some sheen of the old hairs can long be present, resulting in a steel or red glow, until there is no evidence left. It is therefore important that foal sketchers note down the foal's true colour, instead of simply writing 'grey', especially in breeds where some colours or patterns are well sought after.
While the coat turns ever more lighter, the pigment in the skin mostly remains unaltered. As most horses have a grey skin, they will keep this throughout their life, obvious in the dark muzzle or the darker areas around the eyes and private parts. If a horse has a blaze, or is spotted, the skin in those areas has been pink since birth, and will always remain such. The white hairs in those blazes will remain white as the horse turns lighter, but their contrast will lessen as more hairs in their surroundings turn white too, resulting in the visual disappearance of the original white pattern. For this reason, grey is usually selected against in breeds where specific colours or patterns are favoured. However, when such a greyed out horse is completely soaked in water, the skin colour will shine through the white hairs, resulting in a sudden reappearance of the before disappeared pattern.
In some cases, grey horses have skin depigmentation. Some grey skinned areas, most notably around the muzzle and eyes, will lose their pigment and turn pink.
While grey horses turn white as they age, other genes can cause horses to be born white. They may include several white mutations, overo lethal white (where a foal will die whithin 48 hours) and genes like cream, which can have a horse appear nearly white. The main difference between grey and the other variaties is the colour of the skin. The grey gene affects the pigment in the hairs, but leaves the pigment in the skin dark. White patterns and homozygous cream remove the pigment from the skin as well, leaving pink skin in its place. To determine whether a white horse is greyed out, or white due to some other effect, it is therefore best to look at the skin. Skin is best observed around the eyes and the muzzle, but always keep in mind these parts are also commpnly affected by white patterns.