[
]
"White" horses that are not dominant white
White horses
''The White Horses'' is a 1965 television series co-produced by RTV Ljubljana (now RTV Slovenija) of Yugoslavia and German TV (''Südwestfunk'').
Plotline
The story follows the adventures of a teenage girl Julia (Helga Anders) who leaves Belgrad ...
are potent symbols in many cultures. An array of horse coat colors may be identified as "white", often inaccurately, and many are genetically distinct from "dominant white".
"Albino
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. Individuals with the condition are referred to as albino.
Varied use and interpretation of the term ...
" horses have never been documented, despite references to so-called "albino" horses. Dominant white is caused by the absence of pigment cells ( melanocytes), whereas albino animals have a normal distribution of melanocytes. Also, a diagnosis of albinism in humans is based on visual impairment
Visual impairment, also known as vision impairment, is a medical definition primarily measured based on an individual's better eye visual acuity; in the absence of treatment such as correctable eyewear, assistive devices, and medical treatment ...
, which has not been described in horses with dominant white nor similar coat colors. In other mammals, the diagnosis of albinism is based on the impairment of tyrosinase
Tyrosinase is an oxidase that is the rate-limiting enzyme for controlling the production of melanin. The enzyme is mainly involved in two distinct reactions of melanin synthesis otherwise known as the Raper Mason pathway. Firstly, the hydroxy ...
production. No mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
s of the tyrosinase gene are known in horses, however, cream
Cream is a dairy product composed of the higher-fat layer skimmed from the top of milk before homogenization. In un-homogenized milk, the fat, which is less dense, eventually rises to the top. In the industrial production of cream, this process ...
and pearl
A pearl is a hard, glistening object produced within the soft tissue (specifically the mantle) of a living shelled mollusk or another animal, such as fossil conulariids. Just like the shell of a mollusk, a pearl is composed of calcium carb ...
colors result from mutations to a protein involved in tyrosinase transport.
Non-white colors
* Cremello
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, ...
or Blue-eyed cream horses have rosy pink skin, pale blue eyes and cream-colored coats, indicating that pigment cells and pigment are present in the skin, eyes, and coat, but at lower levels. White horses do not have pigment cells, and thus no pigment, in the skin or coat. In addition, dominant white horses seldom have blue eyes. Other genetic factors, or combinations of genetic factors, such as the pearl gene
The Pearl gene, also known as the "Barlink factor", is a dilution gene at the same locus as the cream gene, which somewhat resembles the cream gene and the champagne gene but is unrelated to champagne. It is a somewhat rare dilution gene found in ...
or champagne gene
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 ...
, can also produce cremello-like coats. These coat colors may be distinguishable from dominant white by their unusually colored eyes.
* Gray
Grey (more common in British English) or gray (more common in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning literally that it is "without color", because it can be composed o ...
horses are born any color and progressively replace their colored coat with gray and white hairs. Most gray horses have dark skin, unless they happen to also carry genes for pink or unpigmented skin. Unlike white horses, grays are not born white, nor is their skin color affected by their coat color change.
* Leopard complex
The leopard complex is a group of genetically related Equine coat color, coat patterns in horses. These patterns range from progressive increases in interspersed white hair similar to Gray (horse), graying or Roan (horse), roan to distinctive, Da ...
horses, such as the Appaloosa
The Appaloosa is an American horse breed best known for its colorful spotted coat pattern. There is a wide range of body types within the breed, stemming from the influence of multiple breeds of horses throughout its history. Each horse's colo ...
and Knabstrupper
The Knabstrupper or Knabstrup is a Danish breed of warmblood horse. It is principally a riding horse, but is also used as a harness horse and as a circus animal.
It is broadly similar to the Frederiksborger, but often has a spotted coat. In ...
breeds, are genetically quite distinct from all other 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 or two copies of the Leopard allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
s are present, and the degree of dense leopard-associated white patterning that is 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, which tend to have blankets and varnish roans, than Knabstruppers or Norikers, which tend to be full leopards.
* Tovero
The Tovero (also known as Tobero)PHAA
Retrieved on 13 January 2009 c ...
, Medicine hat
Medicine Hat is a city in southeast Alberta, Canada. It is located along the South Saskatchewan River. It is approximately east of Lethbridge and southeast of Calgary. This city and the adjacent Town of Redcliff to the northwest are with ...
or War bonnet are terms sometimes applied to Pinto horses with residual non-white areas only around the head, especially the ears and poll, while most of the remaining coat is white. While dominant white horses may have areas of residual pigment only around the ears and poll, the term "medicine hat" usually refers to horses with more commonly known white spotting genes, most often tobiano
Tobiano is a spotted color pattern commonly seen in pinto horses, produced by a dominant gene. The tobiano gene produces white-haired, pink-skinned patches on a base coat color. The coloration is almost always present from birth and does not c ...
, combined with frame overo, sabino or splashed white.
Lethal white overo
Foals with lethal white syndrome
Lethal white syndrome (LWS), also called overo lethal white syndrome (OLWS), lethal white overo (LWO), and overo lethal white foal syndrome (OLWFS), is an autosomal genetic disorder most prevalent in the American Paint Horse. Affected foals are bo ...
(LWS) have two copies of the frame overo gene and are born with white or nearly white coats and pink skin. However, unlike dominant white horses, foals with LWS are born with an underdeveloped colon that is untreatable, and if not euthanized, invariably die of colic
Colic or cholic () is a form of pain that starts and stops abruptly. It occurs due to muscular contractions of a hollow tube (small and large intestine, gall bladder, ureter, etc.) in an attempt to relieve an obstruction by forcing content out. ...
within a few days of birth. Horses that carry only one allele of the LWS gene are healthy and typically exhibit the " frame overo" spotting pattern. In cases of "solid" horses with frame overo ancestry, uncertain "overo
Overo refers to several genetically unrelated pinto coloration patterns of white-over-dark body markings in horses, and is a term used by the American Paint Horse Association to classify a set of pinto patterns that are not tobiano. ''Overo'' is ...
" (non-tobiano) phenotype, or horses with multiple patterns, the LWS allele can be detected by DNA test
Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, o ...
.
Mosaicism
Mosaicism
Mosaicism or genetic mosaicism is a condition in multicellular organisms in which a single organism possesses more than one genetic line as the result of genetic mutation. This means that various genetic lines resulted from a single fertilized ...
in horses is thought to account for some spontaneous occurrences of white, near-white, spotted, and roan horses.[Haase, B. ''et al'' (2009). "Whenever a white foal is born out of solid-coloured parents, the most likely explanation is a KIT mutation in the germline of one of its parents or alternatively a mutation in the early developing embryo itself, which might lead to mosaic foals."] Mosaicism refers to mutations that occur after the single-cell stage, and therefore affect only a portion of the adult cells. Mosaicism may be one possible cause for the rare occurrence of brindle
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.
Brindle ty ...
coloring in horses. Mosaic-white horses would be visually indistinguishable from dominant whites. Mosaicism could produce white or partially white foals if a stem cell in the developing foal underwent a mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
, or change to the DNA, that resulted in unpigmented skin and hair. The cells that descend from the affected stem cell will exhibit the mutation, while the rest of the cells are unaffected.
A mosaic mutation may or may not be inheritable, depending on the cell populations affected.[Haase, B. ''et al'' (2009) "our study included several founder animals where mosaicism cannot be excluded. One example for such a scenario is the W8 allele observed in a single "mottled" Icelandic horse, which represents the founder animal for this mutation (Fig. 1g). This horse might be a mosaic, and it remains to be determined whether it will consistently produce offspring with the mottled phenotype."] Though this is not always the case, genetic mutations can occur spontaneously in one sex cell of a parent during gametogenesis
Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic di ...
.[Strachan, Tom & Andrew Read (1999) "A common assumption is that an entirely normal person produces a single mutant gamete. However, this is not necessarily what happens. Unless there is something special about the mutational process, such that it can happen only during gametogenesis, mutations may arise at any time during post-zygotic life."] In these cases, called germline mutation
A germline mutation, or germinal mutation, is any detectable variation within germ cells (cells that, when fully developed, become sperm and ova). Mutations in these cells are the only mutations that can be passed on to offspring, when either a m ...
s, the mutation will be present in the single-celled zygote
A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism.
In multicell ...
conceived from the affected sperm or egg cell, and the condition can be inherited by the next generation.
History of dominant white research
Dominant white horses were first described in scientific literature in 1912. Horse breeder William P. Newell described his family of white and near-white horses to researcher A. P. Sturtevant of Columbia University
Columbia University (also known as Columbia, and officially as Columbia University in the City of New York) is a private research university in New York City. Established in 1754 as King's College on the grounds of Trinity Church in Manhatt ...
:
"The colour of skin is white or so-called pink, usually with a few small dark specks in skin. Some have a great many dark spots in skin. These latter usually have a few dark stripes in hoofs; otherwise the hoofs are almost invariably white. Those that do not have dark specks in skin usually have glass or watch eyes, otherwise dark eyes ... I have one colt coming one year old that is pure white, not a coloured speck on him, not a coloured hair on him, and with glass lue Lue or LUE may refer to:
People
* Andrew Lue (born 1992), Canadian retired football player
* Cachet Lue (born 1997), Canadian-born Jamaican footballer
* Lue Gim Gong (1860–1925), Chinese-American horticulturalist
* Lee Lue (1935–1969), Laotian ...
eyes."
Sturtevant and his contemporaries agreed that this colt's blue eyes were inherited separately from his white coat.[Sturtevant, AH (1912). "Since "glass" eyes occur not infrequently in pigmented horses it seems probable that this white-eyed albino '' ic' is really an extreme case of spotting, plus an entirely independent "glass" eye."] In 1912, Sturtevant assigned the "white" trait to the ''White'' or ''W'' locus. At the time there was no means of assigning ''W'' to a position on the chromosome, or to a gene.
This family of white horses produced Old King in 1908, a dark-eyed white stallion that was purchased by Caleb R. and Hudson B. Thompson. Old King was bred to Morgan mares to produce a breed of horse known today as the American White Horse. A grandson of Old King, Snow King, was at the center of the first major study of the dominant white coat color in horses, conducted in 1969 by Dr. William L. Pulos of Alfred University and Dr. Frederick B. Hutt of Cornell
Cornell University is a private statutory land-grant research university based in Ithaca, New York. It is a member of the Ivy League. Founded in 1865 by Ezra Cornell and Andrew Dickson White, Cornell was founded with the intention to teach a ...
. They concluded, based on test matings and progeny phenotype ratios, that the white coat was dominantly inherited and embryonic lethal in the homozygous state. Other factors, such as variations in expressivity and the influence of multiple genes, may have influenced the progeny ratios that Pulos and Hutt observed. The white coat of the American White Horse has not yet been mapped.
A 1924 study by C. Wriedt identified a heritable white coat color in the Frederiksborg horse. Wriedt described a range of what he considered to be homozygote phenotypes: all-white, white with pigmented flecks, or '' weiß graue'', which transliterates to "white-gray."[WL Pulos & FB Hutt (1969). "Although Wriedt referred to Sturtevant's report in his genetic analysis of records of the Frederiksborg white horses, he considered the latter to be recessive whites, with homozygotes white, white with gray spots, or gray white ("weissgraue"). Heterozygotes were believed to vary all the way from dilute gray to full color."] The German term for gray
Grey (more common in British English) or gray (more common in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning literally that it is "without color", because it can be composed o ...
horse is '' schimmel'', not ''weißgraue''. Heterozygotes, according to Wriedt, ranged from roaned or diluted to more or less solid white horses. Reviewers, such as Miguel Odriozola, reinterpreted Wriedt's data in successive years, while Pulos and Hutt felt that his work had been "erroneous" because Wriedt never concluded that white was lethal when homozygous.[WL Pulos & FB Hutt (1969). "In the light of more recent evidence, these conclusions now seem to have been erroneous ..."]
Other researchers prior to modern DNA analysis developed remarkably prescient theories. The gene itself was first proposed and named ''W'' in 1948.[ In a 1969 work on horse coat colors, ''A los colores del caballo'', Miguel Odriozola suggested that various forms of dominantly inherited white spotting might be arranged sequentially along one ]chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
, thus allowing for the varied expression of dominant white. He also proposed that other, distant genes might also influence the amount of white present.[WL Pulos & FB Hutt (1969). "Odriozola added no new data on dominant white, but ... suggested that different forms of W arranged linearly in the chromosome might be responsible for the differing degrees of white ... and that the expression of white is also influenced by modifying genes."]
The embryonic lethality hypothesis was originally supported by Pulos and Hutt's 1969 study of Mendelian
Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...
progeny ratios. Conclusions about Mendelian traits that are controlled by a single gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
can be drawn from test breedings with large sample sizes. However, traits that are controlled by allelic series or multiple loci are not Mendelian characters, and may not be subject to Mendelian ratios.
Pulos and Hutt knew that if the allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
that created a white coat was recessive
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
, then white horses would have to be homozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
for the condition and therefore breeding white horses together would always result in a white foal. However, this did not occur in their study and they concluded that white was not recessive. Conversely, if a white coat was a simple autosomal dominant, ''ww'' horses would be non-white, while both ''Ww'' and ''WW'' horses would be white, and the latter would always produce white offspring. But Pulos and Hutt did not observe any white horses that always produced white offspring, suggesting that homozygous dominant (''WW'') white horses did not exist. As a result, Pulos and Hutt concluded that white was semidominant
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
and lethal in the homozygous state: ''ww'' horses were non-white, ''Ww'' were white, and ''WW'' died.[Pulos & Hutt (1969). "Each of the five white stallions used in the stud sired one or more colored foals. Similarly, all of the eight white mares that were adequately tested produced at least one colored foal. The fact that these 13 white horses were all proven to be heterozygotes agrees with previous reports that white horses with colored eyes did not breed true to type, but always produced some colored progeny. This, in turn, suggests that the genoytpe WW is not viable."]
Pulos and Hutt reported that neonatal
An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used to ...
death rates in white foals were similar to those in non-white foals, and concluded that homozygous white fetus
A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...
es died during gestation.[Pulos & Hutt (1969). "Among six white foals (from parents both white) that died soon after birth, one had been unable to stand and nurse; death of another was attributed to exposure, one was strangled and another killed by the mare. The possibility that any of these might have been homozygotes is refuted by the fact that similar conditions caused death of several foals from the colored pony mares. Some of those foals were white, and some colored, but none could have been WW."] No aborted
Aborted is a Belgian death metal band formed in 1995 in Waregem. The group currently consists of vocalist, founder and only constant member Sven de Caluwé, guitarist Ian Jekelis, bassist Stefano Franceschini and drummer Ken Bedene. Although t ...
fetuses were found, suggesting that death occurred early on in embryonic or fetal development and that the fetus was "resorbed."[Pulos & Hutt (1969). "As aborted foetuses were not found although a constant watch was maintained for them, it is possible that the homozygotes die early in gestation and are resorbed."]
Prior to Pulos and Hutt's work, researchers were split on the mode of inheritance of white and whether it was deleterious (harmful).[Pulos & Hutt (1969). "... in his genetic analysis of records of the Frederiksborg white horses, riedtconsidered ]hem
A hem in sewing is a garment finishing method, where the edge of a piece of cloth is folded and sewn to prevent unravelling of the fabric and to adjust the length of the piece in garments, such as at the end of the sleeve or the bottom of the g ...
to be recessive whites, with homozygotes white, white with gray spots, or gray white ("weissgraue") ... He considered that the gene for white could not itself be lethal because four fertile white mares produced from 46 matings a total of 37 foals, none of which was dead or weak, and that good record (80 percent fertility) was better than could have been expected if the gene for white color were lethal. Subsequently von Lehmann-Mathildenhoh reported evidence of a dominant white in the Bellschwitz and Ruschof studs ... He did not consider the possibility that it might be associated with any lethal action ... alisburymade no reference to effects of the gene in homozygotes ... Berge lists dominant white horses as heterozygotes, and follows Castle in suggesting that homozygosity for W is lethal." Recent research has discovered several possible genetic pathways to a white coat, so disparities in these historical findings may reflect the action of different genes. It is also possible that the varied origins of Pulos and Hutt's white horses might be responsible for the lack of homozygotes. It now appears that not all equine dominant white mutations cause embryonic lethality in the homozygous state.[Haase, B. ''et al'' (2007). "However, this report on the embryonic lethality was derived from the analysis of offspring phenotype ratios in a single herd segregating one or more unknown mutations."]
The ''white'' (''W'') locus
Locus (plural loci) is Latin for "place". It may refer to:
Entertainment
* Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front
* ''Locus'' (magazine), science fiction and fantasy magazine
** ''Locus Award' ...
was first recognized in mice in 1908.[Durham, F.M. ''A preliminary account of the inheritance of coat colour in mice.'' Reports to the Evolution Committee IV: 41-53, 1908.] The mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
of the same name produces a belly spot and interspersed white hairs on the dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
* Dorsal c ...
aspect of the coat in the heterozygote (''W/+'') and black-eyed white in the homozygote (''W/W''). While heterozygotes are healthy, homozygous ''W'' mice have severe macrocytic anemia
The term ''macrocytic'' is from Greek words meaning "large cell". A macrocytic class of anemia is an ''anemia'' (defined as blood with an insufficient concentration of hemoglobin) in which the red blood cells (erythrocytes) are larger than their ...
and die within days. A mutation which affects multiple systems is "pleiotropic
Pleiotropy (from Greek , 'more', and , 'way') occurs when one gene influences two or more seemingly unrelated phenotypic traits. Such a gene that exhibits multiple phenotypic expression is called a pleiotropic gene. Mutation in a pleiotropic ge ...
." Following the mapping of the '' KIT'' gene to the ''W'' locus in 1988, researchers began identifying other mutations as part of an allelic series of ''W''. There are dozens of known alleles, each representing a unique mutation on the ''KIT'' gene, which primarily produce white spotting from tiny head spots to fully white coats, macrocytic anemia from mild to lethal, and sterility. Some alleles, such as ''splash'' produce white spotting alone, while others affect the health of the animal even in the heterozygous state. Alleles encoding small amounts of white are no more likely to be linked with anemia and sterility than those encoding conspicuous white. Presently, no anecdotal or research evidence has suggested that equine ''KIT'' mutations affect health or fertility.[Haase, B. ''et al'' (2009). "Currently, there is little known about possible pleiotropic effects of KIT mutations in horses."] A recent study showed that blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
parameters in horses with the ''W1'' mutation were normal.
Between the time of Pulos and Hutt's study in 1969 and the beginning of molecular-level research into dominant white in the 21st century, a pattern known as " Sabino" began to describe certain white phenotypes. The first allele of the W series identified by researchers was an incomplete dominant that was named ''Sabino-1'' (''SB-1''). It is found on the same locus as other ''W'' alleles. When homozygous, ''SB-1'' can produce nearly all-white horses.
In 2007, researchers from Switzerland and the United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
published a paper identifying the genetic cause of dominant white spotting in horses from the Franches Montagnes horse, Camarillo White Horse
The Camarillo White Horse is a rare horse breed just over 100 years old known for its pure white color. It dates back to 1921, when Adolfo Camarillo, one of the last Californios, purchased a 9-year-old stallion named Sultan at the California ...
, Arabian horse and Thoroughbred
The Thoroughbred is a horse breed best known for its use in horse racing. Although the word ''thoroughbred'' is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thoroughbred breed. Thoroughbreds are ...
breeds. Each of these dominant white conditions had occurred separately and spontaneously in the past 75 years, and each represents a different allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
(variation or form) of the same gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
. These same researchers identified a further seven unique causes of dominant white in 2009: three in distinct families of Thoroughbreds, one Icelandic horse, one Holsteiner
The Holsteiner is a breed of horse originating in the Schleswig-Holstein region of northern Germany. It is thought to be the oldest of warmblood breeds, tracing back to the 13th century. Though the population is not large, Holsteiners are a do ...
, a large family of American Quarter Horses and a family of South German Draft horses.
Homologous conditions
In humans, a skin condition called piebaldism
Piebaldism refers to the absence of mature melanin-forming cells (melanocytes) in certain areas of the skin and hair. It is a rare autosomal dominant disorder of melanocyte development.James, William; Berger, Timothy; Elston, Dirk (2005). ''Andrews ...
is caused by more than a dozen distinct mutations in the ''KIT'' gene. Piebaldism in humans is characterized by a white forelock, and pigmentless patches of skin on the forehead, brow, face, ventral trunk and extremities. Outside of pigmentation, piebaldism is an otherwise benign condition. In pig
The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous, domesticated, even-toed, hoofed mammal. It is variously considered a subspecies of ''Sus ...
s, the "patch," "belted," and commercial "white" colors are caused by mutations on the ''KIT'' gene. The best-known model for ''KIT'' gene function is the mouse, in which over 90 allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
s have been described. The various alleles produce everything from white toes and blazes to black-eyed white mice, ''panda-white'' to ''sashed'' and ''belted''. Many of these alleles are lethal in the homozygous state, lethal when combined, or sublethal due to anemia. Male mice with ''KIT'' mutations are often sterile.
Notes
References
External links
White Spotting
- This page includes pictures of many of the forms of dominant white.
{{DEFAULTSORT:Dominant White
Horse coat colors