Silver dapple gene

Last updated December 04, 2023

Black silver horse exhibiting strongly diluted long hair with darker roots and flat gray, dappled body color SilverIslaender.jpg — Black silver horse exhibiting strongly diluted long hair with darker roots and flat gray, dappled body color

The silver or silver dapple (Z) gene is a dilution gene that affects the black base coat color and is associated with Multiple Congenital Ocular Abnormalities. It will typically dilute a black mane and tail to a silvery gray or flaxen color, and a black body to a chocolaty brown, sometimes with dapples.^[1] It is responsible for a group of coat colors in horses called "silver dapple" in the west, or "taffy" in Australia. The most common colors in this category are black silver and bay silver, referring to the respective underlying coat color.^[2]

Mature bay silvers retain their reddish bodies, though the presence of small amounts of silver often gives them a chocolate appearance. The mane and tail are usually a sooty silver, darker at the roots, and the legs are usually a flat, brownish-grey mottled with silver. The hair around the eyes and muzzle may also show signs of silvering.

Silver dapple foals can be difficult to identify, but commonly have a pale, wheat-colored body coat, white eyelashes, and hooves with tapering vertical stripes. These characteristics fade over time.^[3]

Red-based horses, such as chestnuts and chestnuts with other dilution factors (such as palominos, and cremello) may carry the silver dapple gene, and may pass it on to their offspring, but will not express the gene in their own body color.^[1]

Silver mimics

The points of a bay are black, while those of a silver bay (shown here) are silver. SilverBein.jpg — The points of a bay are black, while those of a silver bay (shown here) are silver.

Gray: While gray horses grow progressively lighter, silver dapples often darken with age. A silver dapple masked by gray (i.e. carries both genes and, because gray is a dominant gene, becomes white with age) may be identified by use of DNA testing.
Liver chestnut: Silver dapples, especially bay silvers, are frequently misidentified as liver chestnuts or flaxen chestnuts. Flaxen manes and tails in chestnuts tend to have honey or red tones, while the pale manes and tails of silver dapples are soot-toned and darker at the roots. Liver chestnuts also lack the grey-brown dappling on the lower legs. The darkest liver chestnuts often have a kind of marbling on the lower legs, though this hair, too, should show red or yellow tones. Overall, chestnuts of all shades have red-yellow character to their coats, while silver bays recall grey-brown. Knowledge of the pedigree of the horse in question is often useful: two chestnut-based parents cannot produce a silver bay or silver black. DNA testing can be used in the most difficult cases.
"Sooty" palomino: Dark palominos may be hard to distinguish from silver dapples, particularly if the mane or tail of a palomino contain streaks of silver. A true palomino, with a red-based coat, will exhibit yellow or gold tones; a silver horse, in contrast, is by definition black-based and exhibits gray, black or brown undertones. A sooty palomino, like other creme dilutes, may have brown eyes a shade lighter than average,^[4] but this is not true of silver dapples. When actual color is in question, pedigree information or a DNA test can help clarify matters.

Prevalence

Silver dapple foal exhibiting typical wheat-colored coat and pale eyelashes SilverRockyMountainFohlen.jpg — Silver dapple foal exhibiting typical wheat-colored coat and pale eyelashes

Many breeds do not possess the silver dapple gene. The coat color is traditionally associated with the Rocky Mountain Horse and the Miniature Horse. Scandinavian breeds and their descendants such as the Icelandic horse, Nordland Pony, Shetland Pony, Welsh Mountain Pony, Welsh Pony, Swedish Warmblood and Finnhorse are also found in the silver dapple colors. American horse breeds known to have the silver gene include the Morgan, American Saddlebred, Missouri Foxtrotter, Tennessee Walking Horse, and the American Quarter Horse. European draft breeds such as the Comtois and Ardennais also occur in silver.^[2] Historically the color was found in the Dutch Groningen, but the breed was crossbred to produce the Dutch Warmblood and underwent subsequent genetic bottlenecking, so the gene may no longer be present.

Inheritance and expression

The front two Rocky Mountain Horses have the silver dapple dilution. SilverRockyMountain.jpg — The front two Rocky Mountain Horses have the silver dapple dilution.

The silver dapple trait is caused by a missense mutation (labeled Z) in the PMEL17 gene on horse chromosome 6. It is transmitted by autosomal dominant inheritance (simple dominance). PMEL17 is active from quite early in embryonic development through to the mature cell's melanosome and is involved with the production of the black pigment eumelanin.^[2]

Melanins, which provide color in the eyes, skin, and hair, are found in two types: eumelanin, which produces black to brown pigment, and phaeomelanin, which produces red to yellow pigment. Most horses can produce both types; the brown appearance of a bay horse's coat is caused by alternating bands of eumelanin and phaeomelanin, for which the agouti gene is responsible. Eumelanin predominates in the legs, mane and tail of bay horses. By contrast, horses which lack a functional agouti gene cannot produce such alternating bands, and thus have wholly black coats with no visible phaeomelanin. Chestnut horses lack the ability to manufacture eumelanin altogether, and so have wholly red coats devoid of true black pigment.^[5]

Bay silvers retain their reddish body color with black points diluted to silver. SilverMorgan.jpg — Bay silvers retain their reddish body color with black points diluted to silver.

While the role of PMEL17 is not fully understood, the silver dapple gene exclusively produces dilution, or hypopigmentation, of eumelanin.^[2] The dilution changes black into various shades of platinum, silver and flat grey, though the original black-brown character of the color is usually preserved. The effects of the gene are more striking in the mane and tail. Horses with chestnut or chestnut-family coats - such as palomino, red roan, or red dun - are therefore unaffected by the gene and may silently carry it and pass it on to their offspring.

On the template of a black horse, which has a coat rich in eumelanin, the effect is that of complete conversion to varying shades of silver. Often the body remains quite dark while the mane and tail are strongly diluted. On the template of a bay horse, which has eumelanin largely restricted to the points, these points are converted to silver while the phaeomelanic body is mostly unaffected.

An undesirable effect associated with the gene is eye problems, notably multiple congenital ocular anomalies (MCOA). MCOA has multiple clinical signs including a bulbous bulging of the eye,^[6] cornea globosa, severe iridal hypoplasia, uveal cysts, cataracts, and in a few cases, retinal detachment.^[7] These conditions can be detected via ultrasound examination of the eye. In a 2013 study of Comtois horses and Rocky Mountain Horses, all animals carrying the mutated form of PMEL17 had some eye disorder, though milder problems in animals heterozygous for the allele versus those who were homozygous. None of the control horses of these breeds who lacked the mutated form of PMEL17 had any eye disorder.^[6]

Eye disorders in horses with the silver dapple gene SilverHorseEye.jpg — Eye disorders in horses with the silver dapple gene

Similar mutations in other species provide insight into the roles played by the PMEL17 gene. Relatively few such mutations are known, which suggests that the gene is involved in processes critical to survival. Several of the mutations known are related to pigmentation: premature silvering in mice, diluted and white plumage in chickens, and the widely known merle dilution in dogs. The merle coat in dogs is associated with auditory and ophthalmologic disorders, such as deafness and microphthalmia. In Rocky Mountain Horses, the silver dapple color is sometimes associated with Anterior Segment Dysgenesis (ASD) which affects the structures in the face and the front of the eye. Most often, the syndrome presents as benign lesions, though homozygotes may have impaired vision. As ASD is found in non-silver representatives of the breed, it is thought to be physically close to the silver gene and to have come from a silver dapple, ASD-affected foundation ancestor, thus is an example of the Founder effect.^[2]

Related Research Articles

Palomino is a genetic color in horses, consisting of a gold coat and white mane and tail; the degree of whiteness can vary from bright white to yellow. Genetically, the palomino color is created by a single allele of a dilution gene called the cream gene working on a "red" (chestnut) base coat. Palomino is created by a genetic mechanism of incomplete dominance, hence it is not considered true-breeding. However, most color breed registries that record palomino horses were founded before equine coat color genetics were understood as well as they are today, therefore the standard definition of a palomino is based on the visible coat color, not heritability nor the underlying presence of the dilution gene.

A dilution gene is any one of a number of genes that act to create a lighter coat color in living creatures. There are many examples of such genes:

<span class="mw-page-title-main">Sorrel (horse)</span> Coat color

Sorrel is a reddish coat color in a horse lacking any black. It is a term that is usually synonymous with chestnut and one of the most common coat colors in horses. Some regions and breed registries distinguish it from chestnut, defining sorrel as a light, coppery shade, and chestnut as a browner shade. However, in terms of equine coat color genetics there is no known difference between sorrel and chestnut. Solid reddish-brown color is a base color of horses, caused by the recessive e gene.

<span class="mw-page-title-main">Bay (horse)</span> Hair coat color of horses

Bay is a hair coat color of horses, characterized by a reddish-brown or brown body color with a black point coloration on the mane, tail, ear edges, and lower legs. Bay is one of the most common coat colors in many horse breeds.

At right is displayed the color traditionally called liver.

<span class="mw-page-title-main">Brindle</span> Coat coloring pattern in some animals

Brindle is a coat coloring pattern in animals, particularly dogs, cattle, guinea pigs, cats, and, rarely, horses. It is sometimes described as "tiger-striped", although the brindle pattern is more subtle than that of a tiger's coat.

A gray horse has a coat color characterized by progressive depigmentation of the colored hairs of the coat. Most gray horses have black skin and dark eyes; unlike some equine dilution genes and some other genes that lead to depigmentation, gray does not affect skin or eye color. Gray horses may be born any base color, depending on other color genes present. White hairs begin to appear at or shortly after birth and become progressively more prevalent as the horse ages as white hairs become intermingled with hairs of other colors. Graying can occur at different rates—very quickly on one horse and very slowly on another. As adults, most gray horses eventually become completely white, though some retain intermixed light and dark hairs.

<span class="mw-page-title-main">Cream gene</span> Gene for several horse coat colors

The cream gene is responsible for a number of horse coat colors. Horses that have the cream gene in addition to a base coat color that is chestnut will become palomino if they are heterozygous, having one copy of the cream gene, or cremello, if they are homozygous. Similarly, horses with a bay base coat and the cream gene will be buckskin or perlino. A black base coat with the cream gene becomes the not-always-recognized smoky black or a smoky cream. Cream horses, even those with blue eyes, are not white horses. Dilution coloring is also not related to any of the white spotting patterns.

The champagne gene is a simple dominant allele responsible for a number of rare horse coat colors. The most distinctive traits of horses with the champagne gene are the hazel eyes and pinkish, freckled skin, which are bright blue and bright pink at birth, respectively. The coat color is also affected: any hairs that would have been red are gold, and any hairs that would have been black are chocolate brown. If a horse inherits the champagne gene from either or both parents, a coat that would otherwise be chestnut is instead gold champagne, with bay corresponding to amber champagne, seal brown to sable champagne, and black to classic champagne. A horse must have at least one champagne parent to inherit the champagne gene, for which there is now a DNA test.

Equine coat color genetics determine a horse's coat color. Many colors are possible, but all variations are produced by changes in only a few genes. Bay is the most common color of horse, followed by black and chestnut. A change at the agouti locus is capable of turning bay to black, while a mutation at the extension locus can turn bay or black to chestnut.

<span class="mw-page-title-main">Dun gene</span> Dilution 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 lightens most of the body while leaving the mane, tail, legs, and primitive markings the shade of the undiluted base coat color. A dun horse always has a dark dorsal stripe down the middle of its back, usually has a darker face and legs, and may have transverse striping across the shoulders or horizontal striping on the back of the forelegs. Body color depends on the underlying coat color genetics. A classic "bay dun" is a gray-gold or tan, characterized by a body color ranging from sandy yellow to reddish brown. Duns with a chestnut base may appear a light tan shade, and those with black base coloration are a smoky gray. Manes, tails, primitive markings, and other dark areas are usually the shade of the undiluted base coat color. The dun gene may interact with all other coat color alleles.

The Silver Brumby series is a collection of fiction children's books by Australian author Elyne Mitchell. They recount the life and adventures of Thowra, a magnificent palomino brumby stallion, and his descendants, and are set in the Snowy Mountains and southern regions of Australia.

<span class="mw-page-title-main">Grullo</span> Color of horses in the dun family

Grulla, also called black dun, gray dun or mouse dun, is a color of horses in the dun family, characterized by tan-gray or mouse-colored hairs on the body, often with shoulder and dorsal stripes and black barring on the lower legs. The genotype for grulla horses is a black base with dun dilution. In this coloration, each individual hair is mouse-colored, unlike a roan, which is composed of a mixture of dark and light hairs. The several shades of grulla are informally referred to with a variety of terms, including black dun, blue dun, slate grulla, silver grulla or light grulla, silver dun, or lobo dun. Silver grulla may also refer to a grulla horse with silver dapple, regardless of shade. In the Icelandic horse, the grulla color is called gray dun, in the Highland pony it is called mouse dun, and in the Norwegian Fjord horse, grå or gråblakk.

<span class="mw-page-title-main">Chestnut (horse color)</span> Horse coat color

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. Chestnut is characterized by the absolute absence of true black hairs. It is one of the most common horse coat colors, seen in almost every breed of horse.

Horses exhibit a diverse array of coat colors and distinctive markings. A specialized vocabulary has evolved to describe them.

Smoky black or just smoky is a hair coat color of horses which appears dark brown to black in color. Smoky black is produced by the action of a heterozygous cream gene on an underlying black coat color. Therefore, smoky black is a member of the cream family of coat color dilutions, and found in horse populations that have other cream-based colors such as palomino, buckskin, perlino, cremello and smoky cream. All smoky blacks must have at least one parent with the cream gene, and a smoky black can only be verified through DNA testing or parentage. Smoky black has been mistaken for faded black, dark bay or brown, grullo or even liver chestnut.

Seal brown is a hair coat color of horses characterized by a near-black body color; with black points, the mane, tail and legs; but also reddish or tan areas around the eyes, muzzle, behind the elbow and in front of the stifle. The term is not to be confused with "brown", which is used by some breed registries to refer to either a seal brown horse or to a dark bay without the additional characteristics of seal brown.

Roan is a horse coat color pattern characterized by an even mixture of colored and white hairs on the body, while the head and "points"—lower legs, mane, and tail—are mostly solid-colored. Horses with roan coats have white hairs evenly intermingled throughout any other color. The head, legs, mane, and tail have fewer scattered white hairs or none at all. The roan pattern is dominantly inherited, and is found in many horse breeds. While the specific mutation responsible for roan has not been exactly identified, a DNA test can determine zygosity for roan in several breeds. True roan is always present at birth, though it may be hard to see until after the foal coat sheds out. The coat may lighten or darken from winter to summer, but unlike the gray coat color, which also begins with intermixed white and colored hairs, roans do not become progressively lighter in color as they age. The silvering effect of mixed white and colored hairs can create coats that look bluish or pinkish.

Flaxen is a genetic trait in which the mane and tail of chestnut-colored horses are noticeably lighter than the body coat color, often a golden blonde shade. Manes and tails can also be a mixture of darker and lighter hairs. Certain horse breeds such as the Haflinger carry flaxen chestnut coloration as a breed trait. It is seen in chestnut-colored animals of other horse breeds that may not be exclusively chestnut.

Dogs have a wide range of coat colors, patterns, textures and lengths. Dog coat color is governed by how genes are passed from dogs to their puppies and how those genes are expressed in each dog. Dogs have about 19,000 genes in their genome but only a handful affect the physical variations in their coats. Most genes come in pairs, one being from the dog's mother and one being from its father. Genes of interest have more than one expression of an allele. Usually only one, or a small number of alleles exist for each gene. In any one gene locus a dog will either be homozygous where the gene is made of two identical alleles or heterozygous where the gene is made of two different alleles.

References

1 2 "Horse coat color tests -Silver Dilution" from the UC Davis Veterinary Genetics Lab
1 2 3 4 5 Brunberg, Emma; Andersson, Leif; Cothran, Gus; Sandberg, Kaj; Mikko, Sofia; Lindgren, Gabriella (2006). "A missense mutation in PMEL17 is associated with the Silver coat color in the horse". BMC Genetics. 7: 46. doi: 10.1186/1471-2156-7-46 . PMC 1617113 . PMID 17029645.
↑ Brunberg, Emma (2006). "Mapping of the silver coat colour locus in the horse" (PDF). Institutionen för husdjursgenetik. Retrieved 2008-02-17.^{[ permanent dead link ]}
↑ Locke, M. M.; Ruth, L. S.; Millon, L. V.; Penedo, M. C. T.; Murray, J. D.; Bowling, A. T. (2001). "The cream dilution gene, responsible for the palomino and buckskin coat colours, maps to horse chromosome 21". Animal Genetics. 32 (6): 340–3. doi:10.1046/j.1365-2052.2001.00806.x. PMID 11736803.
↑ Castle, W. E. (1954). "Coat Color Inheritance in Horses and in Other Mammals". Genetics. 39 (1): 35–44. doi:10.1093/genetics/39.1.35. PMC 1209634 . PMID 17247465.
1 2 "Examining Eye Issues in Silver Dapple Horses". thehorse.com. May 13, 2014.
↑ Ségard, E. M.; Depecker, M. C.; Lang, J; Gemperli, A; Cadoré, J. L. (2013). "Ultrasonographic features of PMEL17 (Silver) mutant gene-associated multiple congenital ocular anomalies (MCOA) in Comtois and Rocky Mountain horses". Veterinary Ophthalmology. 16 (6): 429–35. doi:10.1111/vop.12021. PMID 23278951.

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[Davis-1] 1 2 "Horse coat color tests -Silver Dilution" from the UC Davis Veterinary Genetics Lab

[brunberg-2] 1 2 3 4 5 Brunberg, Emma; Andersson, Leif; Cothran, Gus; Sandberg, Kaj; Mikko, Sofia; Lindgren, Gabriella (2006). "A missense mutation in PMEL17 is associated with the Silver coat color in the horse". BMC Genetics. 7: 46. doi: 10.1186/1471-2156-7-46 . PMC 1617113 . PMID 17029645.

[mapping-3] Brunberg, Emma (2006). "Mapping of the silver coat colour locus in the horse" (PDF). Institutionen för husdjursgenetik. Retrieved 2008-02-17.^{[ permanent dead link ]}

[4] Locke, M. M.; Ruth, L. S.; Millon, L. V.; Penedo, M. C. T.; Murray, J. D.; Bowling, A. T. (2001). "The cream dilution gene, responsible for the palomino and buckskin coat colours, maps to horse chromosome 21". Animal Genetics. 32 (6): 340–3. doi:10.1046/j.1365-2052.2001.00806.x. PMID 11736803.

[castle-5] Castle, W. E. (1954). "Coat Color Inheritance in Horses and in Other Mammals". Genetics. 39 (1): 35–44. doi:10.1093/genetics/39.1.35. PMC 1209634 . PMID 17247465.

[Horse33863-6] 1 2 "Examining Eye Issues in Silver Dapple Horses". thehorse.com. May 13, 2014.

[7] Ségard, E. M.; Depecker, M. C.; Lang, J; Gemperli, A; Cadoré, J. L. (2013). "Ultrasonographic features of PMEL17 (Silver) mutant gene-associated multiple congenital ocular anomalies (MCOA) in Comtois and Rocky Mountain horses". Veterinary Ophthalmology. 16 (6): 429–35. doi:10.1111/vop.12021. PMID 23278951.

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