One of the myth most widespread in the horse world is the existence of the white horse. Although the expression is rooted in popular language, from a genetic point of view it is not correct. The white horse is actually a gray horse: an animal that was born with a dark coat and that, over the years, has progressively lost its hair color.
The person responsible for this process is the so-called progressive graying gene, known as gen Gray. It is a dominant gene that does not create a new layer, but rather acts on any base color. A gray foal can be born black, chestnut, bay or sorrel, but it is never born white.. It is the passage of time that transforms its appearance.
The mechanism is biological. The Gray gene accelerates the life cycle of melanocytes, the cells responsible for producing melanin, the pigment that gives hair color. As these melanocytes are depleted, new hair grows without pigment. At first, the coat shows a mixture of white and dark hairs; Over the years, white ends up almost completely predominating. The skin, however, remains pigmented, usually black or dark gray, and the eyes retain their natural color, a key difference compared to phenomena such as albinism or leucism.
This graying does not follow a single pattern. The speed and manner in which a horse loses pigment varies greatly from one individual to another, resulting in different appearances within the gray coat. Hence there are numerous terms to describe the different types of cowbirds.
One of the first stages is dark or slaty thrush. In this phase, black hairs still predominate, with some scattered white hairs, which gives the horse a very dark gray tone, almost a slate color. As the process progresses, problems may appear. robinsin which the dark hair is grouped together forming well-defined circles or rounds spread over the body.
Another variant is the wine thrushcharacterized by a wide mixture of white and reddish hairs, especially when the horse’s base coat was chestnut. From that same chestnut base arises the trout thrushin which small circles or spots of reddish hairs appear, reminiscent of the speckling of a trout. When the spots are dark, made up of black hairs, they are called black-eyed thrush.
Over time, many horses reach the stage of light or very light thrush. At this point, the white hair is clearly in the majority and there are hardly any dark hairs left, sometimes only visible on the mane or tail. It is in this phase that the horse is often mistakenly identified as white.
There are also less common variants, such as piebald thrushin which well-defined white spots appear, or the pork or patched thrushwhich presents irregularities in the color distribution, with lighter areas and darker others. All of these forms are not different layers, but visual expressions of the same genetic process at different times in the horse’s life.
From a genetic point of view, the Gray gene is epistaticwhich means that dominates over any other color gene. A horse can carry genes for black, sorrel coats or even dilution genes such as cream or silver, but if it has the Gray gene, its external appearance will end up being gray. That base color is hidden, although it is still present and can be transmitted to offspring.
This knowledge has great practical importance, especially for breeders. A gray horse can produce foals of very different coatssomething that today can be accurately predicted through genetic analysis. It also has health implications, since Gray horses have a greater predisposition to developing melanomasespecially in areas of pigmented skin such as the base of the tail or lips, which requires veterinary control throughout its life.
Understanding what a gray horse really is allows you to look beyond the apparent color. It is not a horse that changes its coat: it is a horse that loses the pigment in its hair over the years. Therefore, although it may seem like it at first glance, white horses do not exist. There are thrushes and each one of them tells the story of time and genetics in their mantle.
