Truly blue horses do not naturally exist, unlike certain birds or insects that display structural blue coloration. While the concept of a blue horse appears in art or folklore, their coloration is governed by specific pigments and genetic mechanisms. The perceived “blue” in horses is an optical effect or a descriptive term for certain coat patterns, not a true blue pigment.
The Reality of Horse Coloration
All horse coat colors stem from variations and dilutions of two fundamental pigments: eumelanin and pheomelanin. Eumelanin creates black and brown hues, while pheomelanin produces red and yellow tones. These pigments, in varying amounts and distributions, create the wide spectrum of horse colors.
The base coat colors in horses are chestnut, bay, and black. Chestnut horses produce only pheomelanin, resulting in a reddish coat. Black horses produce eumelanin throughout their coat. Bay horses produce both pigments, with eumelanin restricted to the “points” like the mane, tail, and lower legs, while the body displays pheomelanin-based reddish-brown hues.
Understanding “Blue” in Horse Coats
The term “blue” when describing horse coats refers to specific patterns that create a bluish impression, notably “blue roan” and “blue dun” (also known as grullo). These are not true blue colors but optical illusions. A blue roan horse has a black base coat with intermingled white hairs throughout the body. This mix creates a steel-grey or bluish appearance, especially when viewed from a distance or in certain lighting. The individual hairs are either black or white, not blue.
A blue dun, or grullo, horse has a black base coat lightened by the dun dilution gene. This gene dilutes the body color to a smoky, mouse-grey or blue-grey, while leaving the mane, tail, and lower legs dark. Blue dun horses also exhibit “primitive markings,” such as a dark dorsal stripe down the back and sometimes horizontal striping on the legs. These visual effects are due to pigment dilution and distribution, not the presence of a blue pigment.
The Science Behind Horse Coat Colors
Horse coat color genetics involve interactions between multiple genes that control pigment production, dilution, and distribution. The Extension (MC1R) gene determines whether a horse produces black pigment, with the ‘e’ allele resulting in red (chestnut) and the ‘E’ allele allowing for black. The Agouti (ASIP) gene controls black pigment distribution; its dominant ‘A’ allele restricts black to the points, creating bay, while the recessive ‘a’ allele allows black to cover the entire body.
Various dilution genes act on these base colors to create lighter shades. The Cream gene, for example, dilutes red pigment to yellow in a single copy (palomino on chestnut, buckskin on bay) and to a pale cream in two copies (cremello, perlino). The Dun gene, which is dominant, lightens both red and black pigments across the body but leaves the points and primitive markings undiluted. Other dilution genes include Champagne, Silver Dapple, and Pearl, each affecting specific pigments or producing unique visual effects.
Pattern genes overlay these colors, creating diverse markings. The Roan gene, a dominant trait, causes white hairs to be intermixed with the base coat color across the body, while the head and lower legs remain solid. This mechanism explains the appearance of blue roans on a black base coat. The interplay of these genes leads to the wide variety of equine coat colors observed today.