Sooty is a term used to describe certain shades of horse coat color

Equine coat color

Horses exhibit a diverse array of coat colors and distinctive markings. A specialized vocabulary has evolved to describe them.While most horses remain the same color throughout life, a few, over the course of several years, will develop a different coat color from that with which they were born...

. Horses with the sooty trait have black or darker hairs mixed into their coats, typically concentrated along the top of the horse and less prevalent on the underparts. The precise genetic mechanism, or series of mechanisms, is not well-understood but the trait is presumed to be heritable.

In most cases, sooty coats exhibit pronounced countershading

Countershading

Countershading, or Thayer's Law, is a form of camouflage. Countershading, in which an animal’s pigmentation is darker dorsally, is often thought to have an adaptive effect of reducing conspicuous shadows cast on the ventral region of an animal’s body...

; the dorsal region is darker than the ventral region. However, some forms seem to produce darker lower parts. The "false dorsal" or "countershading dorsal" can mimic the dorsal stripe associated with dun

Dun gene

The dun gene is a dilution gene that affects both red and black pigments in the coat color of a horse. The dun gene has the ability to affect the appearance of all black, bay, or chestnut -based horses to some degree by lightening the base body coat and suppressing the underlying base color to the...

 horses and is associated with the sooty trait. The most extensive expression of sooty produces a dark, often-dappled cast oriented down from the topline. Many horses with the sooty trait have a darker mask on the bony parts of the face.

It was once thought that the sooty trait was responsible for turning chestnut

Chestnut (coat)

Chestnut is a hair coat color of horses consisting of a reddish-to-brown coat with a mane and tail the same or lighter in color than the coat. Genetically and visually, chestnut is characterized by the absolute absence of true black hairs...

 into liver chestnut, however it is not known to evenly darken the coat. The sooty trait is responsible for many dark bays and has a particularly pronounced effect on buckskins and palomino

Palomino

Palomino is a coat color in horses, consisting of a gold coat and white mane and tail. Genetically, the palomino color is created by a single allele of a dilution gene called the cream gene working on a "red" base coat...

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Although this trait has been called the "sooty gene", similar coat-darkening conditions studied in mice suggest that coat darkening is a polygenic trait. Just as in horses, the degree of sootiness in mice varies widely; some individuals have darker hairs that form a dorsal

Dorsum (biology)

In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

 line, while others have extensive sootiness throughout. A statistical analysis of 1369 offspring of five Franches-Montagnes

Freiberger

The Freiberger, also known as Franches-Montagnes, is a draft horse from Switzerland. It was widely used as draft and pack horse in the Swiss army. Breeding used to be heavily subsidized in Switzerland. It is the last representative of the light cold blood horse in Europe...

stallions indicated that darker shades of chestnut and bay might follow a recessive mode of inheritance.

Horses without any sooty effect are termed "clear-coated."