| White horse | |
|---|---|
 Dominant purebred white stallion | |
| Genotype | |
| Base color | Dominant white |
| Skin | White |
Awhite horse is born predominantly white and stays white throughout its life. A white horse has mostly pink skin under its hair coat, and may have brown, blue, or hazel eyes. "True white" horses, especially those that carry one of thedominant white (W) genes, are rare. Most horses that are commonly referred to as "white" are actually"gray" horses whose hair coats are completely white. Gray horses may be born of any color and their hairs gradually turn white as time goes by and take on a white appearance. Nearly all gray horses have dark skin, except under anywhite markings present at birth. Skin color is the most common method for an observer to distinguish between mature white and gray horses.[1]
White horses have unpigmented skin and a white hair coat. Many white horses have dark eyes, though some have blue eyes. In contrast to gray horses which are born with pigmented skin they keep for life and pigmented hair that lightens to white with age, truly white horses are born with white hair and mostly pink, unpigmented skin. Some white horses are born with partial pigmentation in their skin and hair, which may or may not be retained as they mature, but when a white horse lightens, both skin and hair lose pigmentation. In contrast, grays retain skin pigment and only the hair becomes white.
White colorings, whetherwhite markings,white patterns ordominant white are collectively known as depigmentationphenotypes, and are all caused by areas of skin that lack pigment cells (melanocytes).[2] Depigmentation phenotypes have various genetic causes, and those that have been studied usually map to theEDNRB andKIT genes. However, much about the genetics behind various all-white depigmentation phenotypes are still unknown.
Dominant white (W) is a large group ofalleles best known for producing pink-skinned all-white horses with brown eyes, though some dominant white horses have residual pigment along the topline. Some W alleles produce white spotting on horses with a predominately dark coat. Dominant white has been studied inThoroughbreds,Arabian horses, theAmerican White horse, theCamarillo White Horse, and several other breeds. There are 32 identified variants of dominant white as of 2021, plussabino 1, each corresponding to a spontaneously-white foundation animal and amutation on theKIT gene. Researchers have suggested that at least some forms of dominant white result in nonviable embryos in the homozygous state, though others are known to be viable as homozygotes. While homologous mutations in mice are often linked toanemia andsterility, no such effects have been observed in dominant white horses. Dominant white horses typically have white noses that can be subject tosunburn.
Sabino-white horses are pink-skinned with all-white or nearly-white coats and dark eyes. They arehomozygous for the dominantSB1allele at theSabino 1locus, which has been mapped toKIT.[5] Sabino-white was one of the earliest dominant white alleles discovered, but was not originally recognized as such, hence the different name. TheSabino1 allele, and the associated spotting pattern, is found inMiniature horses,American Quarter Horses,American Paint Horses,Tennessee Walkers,Missouri Fox Trotters,Mustangs,Shetland Ponies, andAztecas.[6] Sabino 1 has not been found in theArabian horse,Clydesdale,[5]Thoroughbred,Standardbred horse, orShire horse. TheSabino 1 allele is not linked to any health defects, though sabino-whites may need some protection fromsunburn. Horses with only one copy of the Sabino1 gene usually have dramatic spotting, including two or more white legs, often with white running up the front of the leg, extensive white on the face, spotting on the midsection, and jagged or roaned margins to the pattern.[5]
The leopard complex, related to theLeopard (LP) gene, characterizes theAppaloosa andKnabstrupper breeds with their spotted coats. Leopard is genetically quite distinct from all other white and white-spotting patterns. Thefewspot leopard pattern, however, can resemble white. Two factors influence the eventual appearance of a leopard complex coat: whether one copy (heterozygousLP/lp) or two copies (homozygousLP/LP) Leopardalleles are present, and the degree of dense white patterning present at birth.[7] If a foal is homozygous for theLP allele and has extensive dense white patterning, they will appear nearly white at birth, and may continue to lighten with age. In other parts of the world, these horses are called "white born."[8] "White born" foals are less common among Appaloosa horses than Knabstruppers or Norikers, as the extensive dense white patterning is favored for producing dramatic full leopards. Homozygous leopards have theLP/LP genotype, and may bevarnish roan, fewspot leopard, or snowcap patterned. Homozygous leopards are substantially more prone to congenital stationarynight blindness.[9] Congenital stationary night blindness is present at birth and is characterized by impaired vision in dark conditions.
Lethal white syndrome is a genetic disorder linked to theFrame overo (O) gene and most closely studied in theAmerican Paint Horse. Affectedfoals are carried to term and at birth appear normal, though they have pink-skinned all-white or nearly-white coats and blue eyes. However, thecolon of these foals cannot function due to theabsence of nerve cells, and the condition cannot be treated. Foals with Lethal White Syndrome invariably die ofcolic within 72 hours, and are usually humanelyeuthanized. Carriers of the gene, who are healthy and normal, can be identified by aDNA test. While carriers often exhibit the "frame overo" pattern, this is not a dispositive trait and testing is necessary, as the pattern can appear in a minimal form as normalwhite markings or be masked by other white spotting genes.
Genetically white horses have unpigmented pink skin (except where a horse with a W allele may have some darker pigmented areas) and unpigmented white hair, though eye color varies. The lack of pigment in the skin and hair is caused by the absence of pigment-producing cells calledmelanocytes. Some coat colors are characterized by light or white-like coats and even pinkish skin, however these white-like coats are not lacking melanocytes. Instead, white-like coat colors result from various changes in the ways melanocytes producepigment.
Gray horses have the most common "white-like" coat color. However, the most noticeable difference between a gray horse whose hair coat is completely white and a white horse is skin color: most gray horses have black skin and dark eyes, white horses have light, unpigmented skin. The gray gene does not affect skin or eye color, so grays typically have dark skin and eyes, as opposed to the unpigmented pink skin of true white horses.[10] The skin and eyes may be other colors if influenced by other factors such aswhite markings, certainwhite spotting patterns ordilution genes. Gray foals may be born any color, but the colored hairs of their coat become progressively silvered as they age, eventually giving mature gray horses a white or nearly-white hair coat. Gray is controlled by a single dominantallele of agene that regulates specific kinds ofstem cells.[11] Gray horses are at an increased risk for melanoma; 70-80% of gray horses over the age of 15 have a melanoma tumor.[11]
True white hair is rooted in unpigmented skin that lacks melanocytes. In contrast, diluted coat colors have melanocytes, but vary due to the concentration or chemical structure of the pigments made by these pigment-producing cells, not the absence of the cells themselves. There are at least five known types of pigment dilution in horses, three which, as described below, can act to produce off-white phenotypes. Horses with strongly diluted coat colors usually have pale eyes (usually blue), cream-colored coats, and rosy-pink skin that contains a minimal amount of pigment.White markings are usually visible upon closer inspection.
Although white horses are sometimes called "albino" there are few 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 calledpiebaldism, not albinism nor partial albinism.
All so-called "albino" horses have pigmented eyes, generally brown or blue. While true albino horses will have a pale blue or white eye. 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, someregistries still refer to "albino" horses. For example, thePaso Fino Horse Association registerscremellos and other cream colors as "albino."[19] Until 1999, theAmerican 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. AmongConnemara pony breeders, homozygous creams are called "blue-eyed creams" or sometimes "pseudo-albino".[21]
The best-known type of albinism isOCA1A, which impairstyrosinase production. In othermammals, the diagnosis of albinism is based on the impairment oftyrosinase production through defects in theColor (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. Nomutations of the tyrosinase orC 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 onvisual 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 thestroma, and in a thin layer at the back of the iris in tissue called theiris 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 theC 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 ofpigments apart from melanins: carotenoids, porphyrins, psittacofulvins, pterins, etc.[18][25] Most commonly, reptiles with a condition homologous to humanOCA1A 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.
The cream gene, which is responsible for palomino, buckskin, and cremello coat colors, was mapped to theMATP gene in 2003 (now known asSLC45A2).[26] This gene is sometimes called theOCA4 gene, because onemutation onSLC45A2 is associated withOculocutaneous albinism type 4. However, other mutations inSLC45A2 are responsible for normal variations in skin, hair, and eye color in humans and .[27] AlthoughSLC45A2 is not the "albino gene"; one of many mutations of the humanSLC45A2 is responsible for a form of albinism.[28]
Many famous horses, past and present, were alleged to be "white" by observers, but were actuallygrays with hair coats turned fully white. Likewise, most white horses used in movies are actuallygrays, 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 theLone Ranger, a role actually played by two different white horses. At least one horse who played "Topper," ridden byHopalong Cassidy, was also white.
Another famous white horse isSodashi, a JapaneseThoroughbred racehorse who wonGrade 1 races includingHanshin Juvenile Fillies,Oka Sho (Japanese 1,000 Guineas), andVictoria Mile.[29][30]
Throughout history, white horses have beenmythologized in many cultures. For example,Herodotus reported that white horses were held assacred animals in theAchaemenid court ofXerxes the Great (ruled 486–465 BC),[31] In more than one tradition, a white horse carriespatron saints or the world saviour in the end times, includingHinduism,Christianity, andIslam.[citation needed]
Phenotypes may vary from tiny depigmentated body spots to white head and leg markings, further on to large white spotting and finally nearly complete depigmentation in white-born horses...White markings result from the lack of melanocytes in the hair follicles and the skin...A completely pigmented head or leg depends on the complete migration and clonal proliferation of the melanoblasts in the mesoderm of the developing fetus, thus ensuring that limbs and the head acquire a full complement of melanocytes
Horses with 2 copies of the Sabino1 gene, are at least 90% white and are referred to as Sabino-white.
The progressive loss of colour in the hair of gray horses is controlled by a dominantly inherited allele at the Gray locus (GG). Foals are born any colour depending on the alleles present at other colour determining loci. After birth, horses carrying the GG allele begin to show white hairs that are intermixed with their original hair colour. Although the rate at which horses will turn gray is variable, the amount of white hair increases with age until the coat is completely white at maturity. Pigmentation of the skin and eyes is not affected by GG. Dark skin distinguishes the gray phenotype from that of pink-skinned cremello and white horses.
Pearl is known to interact with Cream dilution to produce pseudo-double Cream dilute phenotypes including pale skin and blue/green eyes.
No true albino mutation of the color gene is known among horses, though several varieties of white horse are popularly known as albinos.
...the inability of albino animals to produce pigment stems not from an absence of melanocytes
Albinism results from a structural gene mutation at the locus that codes for tyrosinase; that is, albino animals have a genetically determined failure of tyrosine synthesis.
In the most severe form, the latter may look pink since the only pigment present is hemoglobin within the iris blood vessels
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