From Wikipedia, the free encyclopedia - View original article
White horses are born white and stay white throughout their life. White horses may have brown, blue, or hazel eyes. "True white" horses, especially those that carry one of the dominant white (W) genes, are rare. Most horses that are commonly referred to as "white" are actually "gray" horses whose hair coats are completely white.
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, whether white markings, white patterns or dominant white are collectively known as depigmentation phenotypes, and are all caused by areas of skin that lack pigment cells (melanocytes). Depigmentation phenotypes have various genetic causes, and those that have been studied usually map to the EDNRB and KIT genes. However, much about the genetics behind various all-white depigmentation phenotypes are still unknown.
Dominant white is best known for producing pink-skinned all-white horses with brown eyes, though some dominant white horses have residual pigment along the topline. Dominant white is, as the name implies, a genetically dominant color. At least one parent must be dominant white and it does not "skip" generations because it is not recessive. Nonetheless, new variations or mutations producing dominant white do occur spontaneously from time to time. Dominant white is rare, but has occurred in many breeds. It has been studied in Thoroughbreds, Arabian horses, the American White horse and the Camarillo White horse. There are 11 identified variants of dominant white, each corresponding to a spontaneously-white foundation animal and a mutation on the KIT gene. No horse has been identified as homozygous dominant white, and researchers have suggested that at least some forms of dominant white results in nonviable embryos in the homozygous state. While homologous mutations in mice are often linked to anemia and sterility, no such effects have been observed in dominant white horses. Dominant white horses typically have white noses that can be subject to sunburn.
Sabino-white horses are pink-skinned with all-white or nearly-white coats and dark eyes. They are homozygous for the dominant SB1 allele at the Sabino 1 locus, which has been mapped to KIT. Without a DNA test, Sabino-white horses are indistinguishable from dominant white horses. The Sabino1 allele, and the associated spotting pattern, is found in Miniature horses, American Quarter Horses, American Paint Horses, Tennessee Walkers, Missouri Fox Trotters, Mustangs, Shetland Ponies, and Aztecas. Sabino 1 has not been found in the Arabian horse, Clydesdale, Thoroughbred, Standardbred horse, or Shire horse. The Sabino 1 allele is not linked to any health defects, though sabino-whites may need some protection from sunburn. 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.
The leopard complex, related to the Leopard (LP) gene, characterizes the Appaloosa and Knabstrupper breeds with their spotted coats. Leopard is genetically quite distinct from all other white and white-spotting patterns. The fewspot leopard pattern, however, can resemble white. Two factors influence the eventual appearance of a leopard complex coat: whether one copy (heterozygous LP/lp) or two copies (homozygous LP/LP) Leopard alleles are present, and the degree of dense white patterning present at birth. If a foal is homozygous for the LP 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." "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 the LP/LP genotype, and may be varnish roan, fewspot leopard, or snowcap patterned. Homozygous leopards are substantially more prone to congenital stationary night blindness. 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 the Frame overo (O) gene and most closely studied in the American Paint Horse. Affected foals are carried to term and at birth appear normal, though they have pink-skinned all-white or nearly-white coats and blue eyes. However, the colon of these foals cannot function due to the absence of nerve cells, and the condition cannot be treated. Foals with Lethal White Syndrome invariably die of colic within 72 hours, and are usually humanely euthanized. Carriers of the gene, who are healthy and normal, can be identified by a DNA 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 normal white markings or be masked by other white spotting genes.
True white horses have unpigmented pink skin 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 called melanocytes. 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 produce pigment.
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. The skin and eyes may be other colors if influenced by other factors such as white markings, certain white spotting patterns or dilution 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 dominant allele of a gene that regulates specific kinds of stem cells. Gray horses are at an increased risk for melanoma; 70-80% of gray horses over the age of 15 have a melanoma tumor.
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 blue eyes, cream-colored coats, and rosy-pink skin. White markings are usually visible upon closer inspection.
Pearl is known to interact with Cream dilution to produce pseudo-double Cream dilute phenotypes including pale skin and blue/green eyes.To date, the Pearl gene has been found in Quarter Horses, Paint horses, and some Iberian horses. Pearl is not known to be associated with any health problems.
There is no reported case of a true "albino" horse, despite references to white horses that are called "albino". There are also references in literature calling white horses "albino". All so-called "albino" horses have pigmented eyes, usually brown or blue, and thus are not true albinos. In contrast, many albino mammals, such as mice or rabbits, typically have a white hair coat, unpigmented skin and reddish eyes.
Dominant white in horses is caused by the absence of pigment cells (melanocytes), whereas albino animals have a normal distribution of melanocytes. 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.
Despite this, some registries still refer to "albino" horses. For example, the Paso Fino Horse Association registers cremellos and other cream colors as "albino." Until 1999, the American Quarter Horse Association (AQHA) described cremellos as "albino" in rule 227(j). 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".
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. 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.
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. 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.
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. 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." This definition of albinism in mice - the inability to make tyrosinase - is extended to other mammals.
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. 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.
The cream gene, which is responsible for palomino, buckskin, and cremello coat colors, was mapped to the MATP gene in 2003. This gene is sometimes called the OCA4 gene, because one mutation on MATP is associated with oculocutaneous albinism type 4. However, other benign mutations on MATP are responsible for normal variations in skin, hair, and eye color in humans. MATP is not the "albino gene"; one of many mutations of the human MATP is responsible for a form of albinism.
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 Yukichan, a Japanese Thoroughbred racehorse who won the Kanto Oaks at Kawasaki Racecourse.
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), In more than one tradition, a white horse carries patron saints or the world saviour in the end times, including Hinduism, Christianity, and Islam.
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 grey horses is controlled by a dominantly inherited allele at the Grey 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 grey 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 grey phenotype from that of pink-skinned cremello and white horses.
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
|Wikimedia Commons has media related to White horses.|