- The Cream gene produces two types of diluted color. Horses with fully diluted colors, called Cremellos, perlinos, and smoky creams have rosy-pink skin, pale blue eyes, and cream-colored coats that can appear almost white. These coat colors, collectively called "double dilutes" or "blue-eyed creams", result when a horse is homozygous for the cream gene. The creme gene is an incomplete dominant, as when heterozygous, the dilution is less intense. In these cases, cream is responsible for palomino and buckskin. A few Palominos have a very light hair coat is occasionally mistaken for either cremello or white. White markings and patterns are visible against the slightly-pigmented coat and skin. The cream gene is not known to be associated with any health problems.
- Pearl-Cream pseudo-double dilute occurs when a horse has one cream gene and one pearl gene. These two distinct dilution factors interact to produce a cremello-like coat. Pearl-creams have pale but pigmented skin and blue-green eyes, and are distinctly pale cream-colored.[12] To date, the Pearl gene has been found in Quarter Horses, Paint horses, and some Iberian horses.[13] Pearl is not known to be associated with any health problems.
- Champagne-Cream pseudo-double dilute occurs when a horse has one cream gene and one champagne gene. Champagne and cream are another pair of unrelated dilution factors that interact to produce a cremello-like coat. Champagne-creams have freckled, pinkish skin, pale eyes, and pale coats. These colors were formerly referred to as "ivory champagnes".[14] Champagne is found in North American breeds such as the American Cream Draft, Tennessee Walking Horse, American Saddlebred, American Quarter Horse, and Miniature horse. It is not known to be associated with any health problems.
Albinism
Although white horses are sometimes called "albino" there are no reported cases of a true "albino" horse.[15] There are also references in literature calling white horses "albino".[16] Dominant white in horses is caused by the absence of pigment cells (melanocytes), whereas albino animals have a normal distribution of melanocytes.[17] In other animals, patches of unpigmented skin, hair, or eyes due to the lack of pigment cells (melanocytes) are called piebaldism, not albinism nor partial albinism.
All so-called "albino" horses have pigmented eyes, generally brown or blue. In contrast, many albino mammals, such as mice or rabbits, typically have a white hair coat, unpigmented skin and reddish eyes. The definition of "albinism" varies depending on whether humans, other mammals, or other vertebrates are being discussed.[18]
Despite this, some registries still refer to "albino" horses. For example, the Paso Fino Horse Association registers cremellos and other cream colors as "albino."[19] Until 1999, the American Quarter Horse Association (AQHA) described perlino or cremello horses as "albino" in rule 227(j).[20] The AQHA later replaced the word "albino" with "cremello or perlino," and in 2002 the rule was removed entirely. Among Connemara pony breeders, homozygous creams are called "blue-eyed creams" or sometimes "pseudo-albino".[21]
Types of albinism in humans and other animals
The best-known type of albinism is OCA1A, which impairs tyrosinase production. In other mammals, the diagnosis of albinism is based on the impairment of tyrosinase production through defects in the Color (C) gene.[22] Mice and other mammals without tyrosinase have unpigmented pink skin, unpigmented white hair, unpigmented reddish eyes, and some form of vision impairment. No mutations of the tyrosinase or C gene are known in horses.[15]
Humans exhibit a wide range of pigmentation levels as a species. However, the diagnosis of albinism in humans is based on visual impairment, which has not been described in white horses.[23] Vision problems are not associated with gray, dilute, or white coat colors in horses, and blue eyes in horses do not indicate poor vision. Eyes are pigmented at the front of the iris called the stroma, and in a thin layer at the back of the iris in tissue called the iris pigment epithelium. The iris pigment epithelium prevents damaging light scattering within the eye. Blue-eyed humans and mammals have little or no pigment in the stroma, but retain pigment in the iris pigment epithelium. If pigment is missing from both the stroma and the iris pigment epithelium, the only pigment in the eye is the hemoglobin in blood vessels. This accounts for the reddish appearance of eyes in some types of albinism.[24]
In research mammals, such as mice, albinism is more strictly defined. Albino mice occur due to a recessive mutation of the C gene. No such mutation exists in horses.[15] Albino mice lack pigment, but "...the inability of albino animals to produce pigment stems not from an absence of melanocytes...but from a deficiency and/or alteration of the structure of tyrosinase in melanocytes which are otherwise normal."[17] This definition of albinism in mice – the inability to make tyrosinase – is extended to other mammals.[22]
While mammals derive their pigments only from melanins, fish, reptiles and birds rely on a number of pigments apart from melanins: carotenoids, porphyrins, psittacofulvins, pterins, etc.[18][25] Most commonly, reptiles with a condition homologous to human OCA1A retain their reddish and orangish hues. As a result, birds and reptiles without the ability to manufacture tyrosinase are more accurately described as "amelanistic." Horses do not have non-melanin pigments and so if they were albino, would have no pigmentation. The retained pigment of dilute horses, like cremellos, is not comparable to the retained pigment of amelanistic "albino" birds and reptiles.
Why cream is not albino
The cream gene, which is responsible for palomino, buckskin, and cremello coat colors, was mapped to the MATP gene in 2003 (now known as SLC45A2).[26] This gene is sometimes called the OCA4 gene, because one mutation on SLC45A2 is associated with Oculocutaneous albinism type 4. However, other mutations in SLC45A2 are responsible for normal variations in skin, hair, and eye color in humans and .[27] Although SLC45A2 is not the "albino gene"; one of many mutations of the human SLC45A2 is responsible for a form of albinism[citation needed].
Famous white horses
Many famous horses, past and present, were alleged to be "white" by observers, but were actually grays with hair coats turned fully white. Likewise, most white horses used in movies are actually grays, in part because they are easier to find. However, there are a few truly white horses who were used in film. One of the best-known examples was "Silver," ridden by the Lone Ranger, a role actually played by two different white horses. At least one horse who played "Topper," ridden by Hopalong Cassidy, was also white.
Another famous white horse is Sodashi, a Japanese Thoroughbred racehorse who won Grade 1 races including Hanshin Juvenile Fillies, Oka Sho (Japanese 1,000 Guineas), and Victoria Mile.[28][29]
Mythology
Throughout history, white horses have been mythologized in many cultures. For example, Herodotus reported that white horses were held as sacred animals in the Achaemenid court of Xerxes the Great (ruled 486–465 BC),[30] In more than one tradition, a white horse carries patron saints or the world saviour in the end times, including Hinduism, Christianity, and Islam.[citation needed]
See also
https://en.wikipedia.org/wiki/White_horse#Sabino-white
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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:
General
Diluted coat colors have melanocytes, but vary from darker colors due to the concentration or type of these pigment-producing cells, not their absence. Pigment dilution, sometimes referred to as hypomelanism, has been called leucism, albinism (perfect, impartial, or dilute), ghosting, paling, and isabellinism.[1]
- Albinism describes a condition where pigment cells synthesize little or no pigment
- Leucism describes a condition that creates loss of pigment cells
Heredity diagram
Left: black eumelanin, right: brown eumelanin
https://en.wikipedia.org/wiki/Dilution_gene
Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and pink or blue eyes.[1][2] Individuals with the condition are referred to as albino.
https://en.wikipedia.org/wiki/Albinism
Pearl gene, also called the "Barlink factor", is a recessive gene. One copy of the allele has no effect on the coat color of black, bay or chestnut horses. Two copies on a chestnut horse produce a pale, uniform apricot color of body hair, mane and tail as well as pale skin. It also interacts with Cream dilution to produce "pseudo-double" Cream dilutes with pale skin and blue or green eyes.
https://en.wikipedia.org/wiki/Dilution_gene
https://en.wikipedia.org/wiki/Rabicano
https://en.wikipedia.org/wiki/PAX3
https://en.wikipedia.org/wiki/Microphthalmia-associated_transcription_factor
https://en.wikipedia.org/wiki/Syntaxin
https://en.wikipedia.org/wiki/Tiger_eye
https://en.wikipedia.org/wiki/Melanocortin_1_receptor
https://en.wikipedia.org/wiki/Leucism
| Black | |
|---|---|
 Black Irish Draught horse | |
| Variants | Fading, non-fading, possibly genetic |
| Genotype | |
| Base color | Extension "E" |
| Modifying genes | none |
| Description | Solid black base color uniform over entire body other than markings |
| Skin | Black |
| Eyes | Brown |
Horses described as "homozygous black" are simply homozygous for the dominant extension gene (EE); they are homozygous "not-red". Such horses are only "guaranteed" to never produce a red foal. The actual horse may carry additional genetic modifiers that could make it bay, buckskin, gray, bay roan, perlino, silver bay, and so on. A visually black horse that is tested "homozygous black" is EE and has no other color modifiers.
https://en.wikipedia.org/wiki/Black_horse
https://en.wikipedia.org/wiki/Lethal_white_syndrome
Origin
The effort to produce a Siamese-style pointed cat in colors other than the traditional four began in England and in America in the 1940s, carried out by breeders who used foundation crossings between the Siamese, Abyssinian, and the red domestic shorthair. The American Shorthair also became part of the matrix. Initially, the Colorpoint breeders experienced setbacks and failures; in the effort to achieve the proper colors in the proper places, the Siamese body type was often sacrificed. The breeding was further complicated by the difficulty of working with the red coloration because it is a sex-linked
https://en.wikipedia.org/wiki/Colorpoint_Shorthair
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