Any horse with a mixture of white and dark hairs of any color may be called "roan." Such mixtures, which can appear superficially similar, are caused by a number of separate genetic factors. Many registries do not distinguish between these genetic differences. Identifiable types of roans include true or classic roan, varnish roan, and rabicano, though other currently unknown factors may be responsible for ambiguous "roaning." Gray horses, which undergo progressive silvering, may be confused with roans as well.
Classic or true roan is always present at birth, though it may be hard to see until after the foal coat sheds out. The summer coat may be lighter in the summer, but unlike grays, true roans do not progressively lighten. While the head of a true roan, barring any white markings, remains solid throughout its life, the first indication of "graying out" is often observed around the eyes and muzzle of foals.
A peculiar characteristic of the roan coat is that after the skin is broken or scraped, the coat grows back without any white hairs. These are called corn marks. Another useful characteristic is that the unaffected color on the legs forms a sharp, inverted "V" above the knee. This is not true of other roan-like coat patterns.
True roan is caused by a simple dominant gene that has been assigned to equine chromosome 21 (ECA21) in the KIT sequence. Other important coat color genes in the equine KIT sequence include tobiano spotting, extension (black), sabino-1 spotting, and at least four other dominant white-spotting patterns. These characteristics are part of equine linkage group II (LGII).
A study published in 1979 examined percentages of roan foals produced by roan parents found fewer than expected for a simple dominant trait. At the time, lethality of genetic conditions was based primarily on statistics, as modern methods of DNA testing and mapping had not yet been developed. Neither finding stillborn or short-lived foals from these roan parents, the researchers concluded that in the homozygous condition the roan gene was lethal to the embryo or fetus. Contemporary studies were beginning to identify conditions such as "dominant white" and "lethal white," and so roan was believed to follow a similar pattern. Genetic science in the 1970s could not provide a clear answer. "Lethal roan" has since been discredited by the identification of homozygous roans and the work of the late Dr. Ann T. Bowling, who found no evidence to suggest that homozygous roan was lethal.
The dominant inheritance of this trait means that two non-roan horses cannot produce a roan, although roan may occasionally be masked by gray or extensive white markings. True or classic roan is most common in European draft breeds like the Brabant, Ardennes, Trait Du Nord, Italian Heavy Draft and Rhenish-German Cold-Blood though it is found regularly in Quarter Horses, Paint Horses, Dales Ponies, Welsh Ponies, and Miniature horses and very rarely in Trakehners and American Saddlebreds. True or classic roan has not been satisfactorily identified in Arabians or Thoroughbreds. Despite this, Thoroughbreds and Arabians with roan-like coats may be registered as "roan."
Descriptions of roan coats warrants its own terminology:
Any coat color may be affected by roaning. Few combinations have the same unique terminology applied to the common roan colors, although palomino roans are sometimes called honey roans.
The varnish roan is not a true roan, it is actually one of the leopard complex coat patterns associated with Appaloosa, Knabstrupper, Noriker horse and related breeds. Varnish roans are not true roans and can be distinguished from true roans by the following:
Varnish roan is thought to occur due a single, simple dominant gene on equine chromosome 1 (ECA1). It also appears that specific white patterning genes produce the assorted blanket, leopard, and snowflake coat patterns. Without these white patterning factors, horses with one or two copies of the dominant Lp gene are "varnish roans." Varnish roans can be considered to have many, many very small leopard spots that reflect the mottling of the underlying skin.
A horse may be varnish roan as well as true roan and gray, but such combinations are extremely uncommon. The breeding of grays is discouraged by the Appaloosa Horse Club and not allowed by Knabstrupper breed registry.
Rabicano is a white pattern that falls into the category of roaning or scattered white hairs, the genetics of which are not yet fully understood. Sometimes called ticking, rabicano is common even in breeds that do not have true or classic roan, including Arabians and Thoroughbreds. This pattern usually takes the form of scattered white hairs around the junction of the stifle and flank, and peculiar rings of white hairs near the base of the tail. This trait is called a coon tail or skunk tail. Extensively-marked rabicanos sometimes exhibit striations in their pattern on the ribs, giving them a striped appearance. While rabicano itself does not produce white markings on the face and legs, it is often associated with any of the numerous sabino patterns, many of which have been mapped to the KIT gene. Other color patterns mapped to KIT include extension, tobiano, and true roan. This may explain the close association between rabicano and sabino, which are often observed in the same horse.
Rabicanos are not true roans and can be distinguished from true roans by the following:
Rabicano may occur on any base color and may occur in conjunction with any other white pattern, including true roan or gray.
Some forms of sabino, which is a pinto pattern, have roaning along the edges of other white spots or markings This pattern is often, though not always, associated with a broad blaze on the face, leg markings that are especially high on the hindlegs, and belly spots. The edges of the white markings are often ragged, irregular and frequently include intermingled scatterings of white hairs that resemble roan. Though at least one form of Sabino is also genetically linked to the KIT locus, it is not true roan, there are different genetics involved. True or classic roan is not associated with broad white markings of any kind.
The genetics behind roan dogs are still unclear, and at present candidate genes have been ruled out. There remains a great deal of ambiguity in terminology regarding mottled dogs, which are called roan, ticked, mottled and belton depending on the context. The roan or ticked color is described in many breeds of gundogs such as English Cocker Spaniels, American Cocker Spaniels, Field Spaniels and Brittanys, German Shorthaired Pointers, Spinoni Italiani, Lagotto Romagnolos and English Setters, as well as Border Collies and many other breeds.
In dogs, roan manifests itself only in unpigmented areas, the presence and shape of which are determined by other genes. This is in stark contrast to true roan horses and roan cattle, which are roan only in pigmented regions of their coat and may have white markings. Instead, dogs with roaning or ticking are born with clear, open white markings which begin to fill in with flecking in the subsequent weeks and continue to darken with age. Most breed standards use the terms "ticked" and "roan" interchangeably, with the former referring to clearly-defined flecks on a white background and the latter to flecks so closely-spaced that the mixture appears even. The terminology that relates the underlying coat color with the roan modifier is often breed-specific, but most standards call a black dog with roaning blue. In breeds that are characterized by roaning and ticking such as the Large Munsterlander, clear white-marked individuals may be called plated. The term belton is reserved for English Setters.
In 1957, Little suggested that roan and ticking were controlled separately, and postulated that roan may have been homologous to "silvered" coat in mice. This condition in mice is actually homologous to merle, which might be described by some as "roan." In 2007, the gene responsible for roan cattle (KITLG) was refuted as a possible cause of roan in dogs. Neither roan nor ticking, if they are independently-caused, appear to be recessive.
Breeds of cattle known for roans are the Belgian Blue and Shorthorn. Among the former, coat color may be solid black, solid white, or blue roan; the latter may be solid red, solid white, or red roan. Belgian Blues also typically exhibit spotting patterns, which are genetically separate from roan. As a result, most roan cows exhibit blotches of clearly colored and clearly white hair, with roan patches. Some "cryptic" roan cattle appear solid, but upon close inspection reveal a small roan patch. Roan cattle cannot "breed true" but breeding white cattle to a solid mate will always yield a roan calf. The white color typical of Charolais and White Park breeds is not related to roan.
Roan in Shorthorns and Belgian Blues is controlled by the mast cell growth factor (MGF) gene, also called the steel locus, on bovine chromosome 5. Part of the KIT ligand, this region is involved in many cell differentiation processes. Mast cell growth factor promotes pigment production by pigment cells , and without it, skin and hair cells lack pigment. With two functional MGF genes (homozygous dominant), cattle are fully-pigmented; without any functional MGF genes (homozygous recessive), they are white. MGF-controlled roan occurs when cattle possess one functional and one non-functional MGF gene (heterozygous), resulting in a roughly even mixture of white regions and colored regions.
The reproductive condition "White Heifer Disease," associated with the MGF gene, is characterized by homozygous MGF-white heifers with incomplete reproductive tracts.
About 25% of guinea pigs born to a mating of two roans are completely white, with a constellation of deformities called lethal white syndrome, although it has no relation to Overo Lethal White Syndrome in horses or double merle syndrome in dogs. These symptoms include:
It is worth noting that, unlike anophthalmic hamsters, lethal white guinea pigs are not sterile. Females may be unable to deliver live young.
Lethal white guinea pigs have a decreased lifespan in general, although individuals have reportedly lived to 6 or 7 years. Many die shortly after birth or at weaning age. With hand-feeding and regular dental care, lethals can live 2-3 years.