Dominant white

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This Thoroughbred stallion (W2/+) has one form of dominant white. His skin, hooves, and coat lack pigment cells, giving him a pink-skinned white coat. DominantWhiteHorsesD.jpg
This Thoroughbred stallion (W2/+) has one form of dominant white. His skin, hooves, and coat lack pigment cells, giving him a pink-skinned white coat.

Dominant white (W) [1] [2] is a group of genetically related coat color alleles on the KIT gene of the horse, best known for producing an all-white coat, but also able to produce various forms of white spotting, as well as bold white markings. Prior to the discovery of the W allelic series, many of these patterns were described by the term sabino, which is still used by some breed registries.

Contents

White-colored horses are born with unpigmented pink skin and white hair, usually with dark eyes. Under normal conditions, at least one parent must be dominant white to produce dominant white offspring. However, most of the currently-known alleles can be linked to a documented spontaneous mutation that began with a single ancestor born of non-dominant white parents. Horses that exhibit white spotting will have pink skin under the white markings, but usually have dark skin beneath any dark hair.

There are many different alleles that produce dominant white or white spotting; as of 2022 they are labeled W1 through W28 and W30 through W35, plus the first W allele discovered was named Sabino 1 (SB-1) instead of W1. [3] [4] [5] They are associated with the KIT gene. [B] The white spotting produced can range from white markings like those made by W20, to the irregularly-shaped or roaning patterns previously described as Sabino, to a fully white or almost fully white horse.

For many of the W alleles, the white coats are, as the name suggests, inherited dominantly, [D] meaning that a horse only needs one copy of the allele to have a white or white spotted coat. In fact, some such alleles may be embryonic lethal when homozygous. Others, such as SB-1 and W20, are incomplete dominants, capable of producing viable offspring with two copies of the gene, and who generally have more white than horses with only one copy. In addition, different alleles which on their own give a white-spotted but not completely white horse, such as W5 and W10, can combine to make a horse completely white.

White can occur in any breed, and has been studied in many different breeds. Because of the wide range of patterns produced, some suggest the family be called “white spotting” rather than “white.” Other researchers suggest the term "dominant white" be used only for the W alleles thought to be embryonic lethal when homozygous. [6]

White is both genetically and visually distinct from gray and cremello. Dominant white is not the same as lethal white syndrome, nor are white horses "albinos"—Tyrosinase negative albinism has never been documented in horses.

Description

This dominant white Franches Montagnes colt (W1/+) lost almost all his residual pigment by the time he was 3 years old (below) DominantWhiteHorsesB.jpg
This dominant white Franches Montagnes colt (W1/+) lost almost all his residual pigment by the time he was 3 years old (below)
The same foal as an adult horse. Some white spotted horses lose pigment with age, even though they do not possess the gray gene. The underlying skin remains dark. DominantWhiteHorsesC.jpg
The same foal as an adult horse. Some white spotted horses lose pigment with age, even though they do not possess the gray gene. The underlying skin remains dark.

Although the term "dominant white" is typically associated with a pure white coat, such horses may be all-white, near-white, partially white, or exhibit an irregular spotting pattern similar to that of sabino horses. [7] To add to the confusion, at least some horses in each of those groups might be referred to as "dominant white", "white spotted", or "sabino". The amount of white hair depends on which KIT alleles are involved. [8] At birth, most of the white hair is rooted in unpigmented pink skin. The pink skin lacks melanocytes, and appears pink from the underlying network of capillaries. White spotting is not known to affect eye color, and most white horses have brown eyes. [9]

White or near-white

White horses are born with pink skin and a white coat, which they retain throughout their lives. [10] The genetic factors that produce an all-white horse are often also capable of producing a near-white horse, which is mostly white but has some areas that are pigmented normally. Near-white horses most commonly have color in the hair and skin along the topline (dorsal midline) of the horse, in the mane, and on the ears. [7] The color is often interspersed as specks or spots on a white background. In addition, the hooves are usually white, but may have striping if there is pigmented skin on the coronary band just above the hoof. [11] [12] In some cases, foals born with residual non-white hair may lose some or all of this pigment with age, without the help of the gray factor. [13]

White spotting

White spotting from a W allele is difficult to identify visually, as it can range from small white markings in the case of a heterozygous W20 horse all the way to an obvious pinto pattern. In addition, even completely white horses can have genes which by themselves would only give white spotting, such as W20 combined with W22 [2] or W5 combined with W10. As such, the only reliable way to find out whether a horse has one of the known white spotting patterns from an allele on KIT is to have it genetically tested.

Prevalence

Dominant white is one of several potential genetic causes for horses with near-white or completely white coats; it may occur through spontaneous mutation, and thus may be found unexpectedly in any breed, even those that discourage excessive white markings. To date, forms of dominant white have been identified in Thoroughbreds, [13] Standardbreds, [14] American Quarter Horses, [7] Frederiksborg horses, [7] Icelandic horses, [7] Shetland ponies, [13] Franches Montagnes horses, [13] South German Draft horses, [7] and the Arabian horse. [13] The American White Horse, which is descended primarily from one white stallion crossed on non-white mares, is known for its white coat, as is the Camarillo White Horse. [15] [16]

Inheritance

The W locus was mapped to the KIT gene in 2007. [13] KIT is short for "KIT proto-oncogene receptor tyrosine kinase". [17] White spotting is caused by multiple forms, or alleles, of the KIT gene. [13] All horses possess the KIT gene, as it is necessary for survival even at the earliest stages of development. The presence or absence of dominant white is based on the presence of certain altered variants of KIT. Each unique form is called an allele, and for every trait, all animals inherit one allele from each parent. The original or "normal" form of KIT, which is expected in horses without dominant white spotting, is called the "wild type" allele. [A] Thus, a dominant white horse has at least one KIT allele with a mutation associated with dominant white spotting.

Allelic series

The KIT gene contains over 2000 base pairs, and a change in any of those base pairs results in a mutant allele. [7] Over forty seven such alleles have been identified by sequencing the KIT genes of various horses. [7] The resultant phenotype of many of these alleles is not yet known, but over 30 have been linked to white spotting. [18] [8] [19] DNA tests can identify if a horse carries the identified W alleles.

The Camarillo White Horse breed has a dominant white coat attributed to the W4 mutation. Whitecamarillo.JPG
The Camarillo White Horse breed has a dominant white coat attributed to the W4 mutation.
This palomino horse carries the W5 mutation, which usually causes irregular sabino-like markings. Sato - Palomino sabino Purebred Thoroughbred Stallion (5966320009).jpg
This palomino horse carries the W5 mutation, which usually causes irregular sabino-like markings.
This near-white mare is a daughter of Shirayukihime, the suspected founder of the W14 mutation. Horses with W14 are often fully white. Buchiko IMG 1120 R 20150307.JPG
This near-white mare is a daughter of Shirayukihime, the suspected founder of the W14 mutation. Horses with W14 are often fully white.
This horse tested homozygous for W20, and also exhibits a rabicano roaning pattern. Koning Albert KWPN Stallion.jpg
This horse tested homozygous for W20, and also exhibits a rabicano roaning pattern.
W20 has been found in many breeds including the German Riding Pony, German Warmblood, Thoroughbred, Oldenburger, Welsh pony, Quarter horse, Paint horse, Appaloosa, Noriker, Old-Tori, Gypsy horse, Morgan horse, Clydesdale horse, Franches-Montagnes, Marwari horse, South German Draft, Paso Peruano, Camarillo White Horse, and Hanoverian horse. [39]
W20 is a missense mutation on exon 14 (c.2045G>A; p.Arg682His). [26]

These alleles do not account for all dominantly inherited white spotting in horses. More KIT alleles are expected to be found with roles in white spotting. [7] Most W alleles occur within a specific breed or family and arise as spontaneous mutations. KIT appears to be prone to mutation, in part due to its many exons, so new alleles of W can occur in any breed. [8] There are likely many KIT variants in the global horse population that have not yet been investigated.

Relation to sabino

Sabino can refer either specifically to Sabino 1 (SB1) or to a variety of visually similar spotting patterns. SB1 creates a nearly pure white horse when homozygous, and bold spotting when heterozygous. To add to the confusion, white spotting created by several W alleles, such as W5, W15, and W19 creates patterns that historically were called sabino. For that reason, the use of the word "sabino" is evolving. Genetically, Sabino 1 is simply another allele on KIT, [20] and thus can be classified in the same “family” of KIT mutations as the alleles labeled W or dominant white. [56]

The arrangement of irregular white markings on this Paso Fino is typical of heterozygous Sabino-1, other sabino-like patterns, and some alleles of dominant white. Such ambiguous white markings are usually called "sabino" in the absence of a DNA test to determine the genetic mechanism actually involved. Sabino-Pinto-Puerto-Rican-Paso-Fino.jpg
The arrangement of irregular white markings on this Paso Fino is typical of heterozygous Sabino-1, other sabino-like patterns, and some alleles of dominant white. Such ambiguous white markings are usually called "sabino" in the absence of a DNA test to determine the genetic mechanism actually involved.

In its homozygous form, Sabino 1 can be confused with dominant white alleles such as W1, W2, W3, or W4 that create a white or near-white horse with only one copy. Both dominant white and "Sabino-White" horses are identified by all-white or near-white coats with underlying pink skin and dark eyes, often with residual pigment along the dorsal midline. However, it takes two copies of Sabino 1 to produce a Sabino-white horse, and Sabino 1 is not homozygous lethal. [57]

Initially, dominant white was separated from sabino on the grounds that the former had to be entirely white, while the latter could possess some pigment. [58] However, the 2007 and 2009 studies of dominant white showed that many dominant white alleles produce a range of white phenotypes that include horses with pigmented spots in their hair and skin. [7] Each of the larger families of dominant white studied included pure-white horses, horses described as having "sabino-like" white markings, as well as white horses described as "maximal sabino". [7] [13]

More recently, dominant white and sabino were distinguished from one another on the grounds that dominant white alleles produce nonviable embryos in the homozygous state, while Sabino 1 was viable when homozygous. [59] However, not all KIT alleles currently identified as "dominant white" have been proven lethal, [21] and in fact W20 is known to be viable in the homozygous form. [60]

The similarities between Dominant White and Sabino 1 reflect their common molecular origin: The W series and SB1 have both been mapped to KIT. The researchers who mapped Sabino 1 in 2005 suggested that other sabino-like patterns might also map to KIT, [20] which has been the case for many other alleles discovered since that time, including major alleles for white leg and facial markings that have also been mapped to or near to the KIT gene. [61]

Molecular genetics

Skin biopsies of non-white (left) and white horses. Blue staining identifies Kit protein activity, while melanin is visible in the non-white sample as brown granules. The sample from the white horse shows reduced Kit activity, no melanocytes, and no melanin. DominantWhiteHorsesGH.jpg
Skin biopsies of non-white (left) and white horses. Blue staining identifies Kit protein activity, while melanin is visible in the non-white sample as brown granules. The sample from the white horse shows reduced Kit activity, no melanocytes, and no melanin.

The KIT gene encodes a protein called steel factor receptor, which is critical to the differentiation of stem cells into blood cells, sperm cells, and pigment cells. A process called alternative splicing, which uses the information encoded in the KIT gene to make slightly different proteins (isoforms) for use in different circumstances, may impact whether a mutation on KIT affects blood cells, sperm cells, or pigment cells. Steel factor receptor interacts chemically with steel factor or stem cell factor to relay chemical messages. These messages are used during embryonic development to signal the migration of early melanocytes (pigment cells) from the neural crest tissue to their eventual destinations in the dermal layer. The neural crest is a transient tissue in the embryo that lies along the dorsal line. Melanocytes migrate along the dorsal line to a number of specific sites: near the eye, near the ear, and the top of the head; six sites along each side of the body, and a few along the tail. At these sites, the cells undergo a few rounds of replication and differentiation, and then migrate down and around the body from the dorsal aspect towards the ventral aspect and the limb buds. [62]

The timing of this migration is critical; all white markings, from a small star to a pure white coat, are caused by the failed migration of melanocytes. [61]

A certain degree of the eventual amount of white, and its "design", is completely random. The development of an organism from single-celled to fully formed is a process with many, many steps. Even beginning with identical genomes, as in clones and identical twins, the process is unlikely to occur the same way twice. A process with this element of randomness is called a stochastic process, and cell differentiation is, in part, a stochastic process. [63] The stochastic element of development is partly responsible for the eventual appearance of white on a horse, potentially accounting for nearly a quarter of the phenotype. [64] The research team that studied dominant white cited "subtle variations in the amount of residual KIT protein" as a potential cause for the variability in phenotype of horses with the same allele. They also speculated that variability in the phenotype of horses with W1 might be caused by "different efficacies of [nonsense-mediated decay] in different individuals and in different body regions." That is, some horses destroy more of the mutant KIT protein than others. [7]

Lethality

Early embryonal lethality, also known as early embryonic death or a non-viable embryo, may occur when the embryo possesses two copies of certain dominant white alleles. [65] The reason for this is that several mutations of W are caused by nonsense mutations, frameshift mutations or DNA deletions, which, if homozygous, would make it impossible to produce a functional KIT protein. However, it appears that not all W alleles are embryonic lethals. Homozygous embryos from alleles of certain missense and splice site mutations are sometimes viable, apparently because they have less effect on gene function. [8] For instance, W1 is a nonsense mutation and it is thought that horses with the genotype W1/W1 would die in utero, while W20 is a missense mutation and living horses with the W20/W20 genotype have been found. A 2013 study also located horses that were compound W5/W20 heterozygotes, almost completely white, essentially with greater depigmentation than could be accounted for by either allele alone. [26]

"White" horses that are not dominant white

The dark skin under a white hair coat, easily seen at the muzzle and genitals, shows that this white-looking horse is actually a gray. Most horses that look "white" are actually grays. Andalusian.jpg
The dark skin under a white hair coat, easily seen at the muzzle and genitals, shows that this white-looking horse is actually a gray. Most horses that look "white" are actually grays.
Pale blue eyes, rosy-pink skin and cream-colored hair identify the presence of some sort of dilution gene, most often the cream gene. This cremello is neither white nor gray. Akhalteke craem.jpg
Pale blue eyes, rosy-pink skin and cream-colored hair identify the presence of some sort of dilution gene, most often the cream gene. This cremello is neither white nor gray.

White horses are potent symbols in many cultures. [66] An array of horse coat colors may be identified as "white", often inaccurately, and many are genetically distinct from "dominant white".

"Albino" horses have never been documented, despite references to so-called "albino" horses. [67] [68] Dominant white is caused by the absence of pigment cells (melanocytes), whereas albino animals have a normal distribution of melanocytes. [69] Also, a diagnosis of albinism in humans is based on visual impairment, which has not been described in horses with dominant white nor similar coat colors. [70] In other mammals, the diagnosis of albinism is based on the impairment of tyrosinase production. [71] No mutations of the tyrosinase gene are known in horses, however, cream and pearl colors result from mutations to a protein involved in tyrosinase transport. [72]

Non-white colors

This "white-born" or "fewspot" Appaloosa foal is hard to distinguish from white without familiarity with the leopard complex and the animal's pedigree. WeissgeborenerAppaloosa.jpg
This "white-born" or "fewspot" Appaloosa foal is hard to distinguish from white without familiarity with the leopard complex and the animal's pedigree.

Lethal white overo

Foals with lethal white syndrome (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 within a few days of birth. [83] 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" (non-tobiano) phenotype, or horses with multiple patterns, the LWS allele can be detected by DNA test. [84]

Mosaicism

Mosaicism in horses is thought to account for some spontaneous occurrences of white, near-white, spotted, and roan horses. [85] Mosaicism refers to mutations that occur after the single-cell stage, and therefore affect only a portion of the adult cells. [86] Mosaicism may be one possible cause for the rare occurrence of brindle coloring in horses. [87] 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, 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. [88] Though this is not always the case, genetic mutations can occur spontaneously in one sex cell of a parent during gametogenesis. [89] In these cases, called germline mutations, the mutation will be present in the single-celled zygote conceived from the affected sperm or egg cell, and the condition can be inherited by the next generation. [85]

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:

"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 [blue] eyes." [12]

Sturtevant and his contemporaries agreed that this colt's blue eyes were inherited separately from his white coat. [90] In 1912, Sturtevant assigned the "white" trait to the White or W locus. [12] 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. [15] 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. They concluded, based on test matings and progeny phenotype ratios, that the white coat was dominantly inherited and embryonic lethal in the homozygous state. [91] Other factors, such as variations in expressivity and the influence of multiple genes, may have influenced the progeny ratios that Pulos and Hutt observed. [92] 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. [93] 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." [94] The German term for gray horse is schimmel , not weißgraue. [95] 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. [96]

Other researchers prior to modern DNA analysis developed remarkably prescient theories. The gene itself was first proposed and named W in 1948. [8] 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, thus allowing for the varied expression of dominant white. He also proposed that other, distant genes might also influence the amount of white present. [97]

The embryonic lethality hypothesis was originally supported by Pulos and Hutt's 1969 study of Mendelian progeny ratios. [10] Conclusions about Mendelian traits that are controlled by a single gene 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. [98]

Pulos and Hutt knew that if the allele that created a white coat was recessive, then white horses would have to be homozygous 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 and lethal in the homozygous state: ww horses were non-white, Ww were white, and WW died. [99]

Pulos and Hutt reported that neonatal death rates in white foals were similar to those in non-white foals, and concluded that homozygous white fetuses died during gestation. [100] No aborted fetuses were found, suggesting that death occurred early on in embryonic or fetal development and that the fetus was "resorbed." [101]

Prior to Pulos and Hutt's work, researchers were split on the mode of inheritance of white and whether it was deleterious (harmful). [102] 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. [92]

This mare (W14/+) is the daughter of Shirayukihime, who is thought to be the founder of the W14 mutation. Yukichan 20080427P2.jpg
This mare (W14/+) is the daughter of Shirayukihime, who is thought to be the founder of the W14 mutation.

The white (W) locus was first recognized in mice in 1908. [104] The mutation of the same name produces a belly spot and interspersed white hairs on the dorsal 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 and die within days. [105] A mutation which affects multiple systems is "pleiotropic." 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. [106] 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. [105] 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. [107] A recent study showed that blood parameters in horses with the W1 mutation were normal. [22]

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. [108] 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 published a paper identifying the genetic cause of dominant white spotting in horses from the Franches Montagnes horse, Camarillo White Horse, Arabian horse and Thoroughbred breeds. [13] Each of these dominant white conditions had occurred separately and spontaneously in the past 75 years, and each represents a different allele (variation or form) of the same gene. 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, a large family of American Quarter Horses and a family of South German Draft horses. [7]

Homologous conditions

Some spotted patterns on pigs, such as this one, are caused by polymorphisms of the porcine KIT gene. Gloucester Old Spot Boar, England.jpg
Some spotted patterns on pigs, such as this one, are caused by polymorphisms of the porcine KIT gene.

In humans, a skin condition called piebaldism 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. [109] In pigs, the "patch," "belted," and commercial "white" colors are caused by mutations on the KIT gene. [110] The best-known model for KIT gene function is the mouse, in which over 90 alleles 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. [111]

Notes

  1. ^
    Use of the term "wild type" is subjective, as genes undergo changes, called mutation, at statistically regular intervals called mutation rates.
  2. ^
    A gene is a unit of heredity which encodes the instructions to make molecules. [112] An allele is a specific version of a gene. [113] Geneticists often discuss only two alleles at a time: the "wildtype" or normal allele which encodes the correct molecule, and the mutant allele. When more than two alleles are known, they form an allelic series. A locus is the physical location of a gene on a chromosome. [113]
  3. ^
    For any particular gene, when an individual inherits two identical alleles, one from each parent, it is homozygous, or a homozygote. When an individual inherits two different alleles, one from each parent, it is heterozygous or a heterozygote. [113]
  4. ^
    Mendelian traits are characteristics of an organism that are controlled by a single gene. Mendelian traits can be described as dominant if the characteristic is found in heterozygotes, or recessive if not. Dominance and recessiveness are properties of traits, not genes. Defining a trait as dominant (the word dominate is a verb) or recessive depends on how the trait is defined. [114]

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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.

<span class="mw-page-title-main">Equine coat color genetics</span> Genetics behind the equine coat color

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">Tobiano</span> Spotted color pattern in horses

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 change throughout the horse's lifetime, unless the horse also carries the gray gene. It is a dominant gene, so any tobiano horse must have at least one parent who carries the tobiano gene.

<span class="mw-page-title-main">Overo</span> Group of colouration patterns of horses

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 a Spanish word, originally meaning "like an egg". The most common usage refers to frame overo, but splashed white and sabino are also considered "overo". A horse with both tobiano and overo patterns is called tovero.

<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.

<span class="mw-page-title-main">Equine coat color</span> Horse coat colors and markings

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

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

A white 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 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. 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 any white markings present at birth. Skin color is the most common method for an observer to distinguish between mature white and gray horses.

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

Black is a hair coat color of horses in which the entire hair coat is black. It is not uncommon to mistake dark chestnuts or bays for black.

<span class="mw-page-title-main">Sabino horse</span> Color pattern in horses

Sabino describes a distinct pattern of white spotting in horses. In general, Sabino patterning is visually recognized by roaning or irregular edges of white markings, belly spots, white extending past the eyes or onto the chin, white above the knees or hocks, and "splash" or "lacy" marks anywhere on the body. Some sabinos have patches of roan patterning on part of the body, especially the barrel and flanks. Some sabinos may have a dark leg or two, but many have four white legs. Sabino patterns may range from slightly bold face or leg white markings—as little as white on the chin or lower lip—to horses that are fully white.

<span class="mw-page-title-main">Leopard complex</span> Coat pattern in horses

The leopard complex is a group of genetically related coat patterns in horses. These patterns range from progressive increases in interspersed white hair similar to graying or roan to distinctive, Dalmatian-like leopard spots on a white coat. Secondary characteristics associated with the leopard complex include a white sclera around the eye, striped hooves and mottled skin. The leopard complex gene is also linked to abnormalities in the eyes and vision. These patterns are most closely identified with the Appaloosa and Knabstrupper breeds, though its presence in breeds from Asia to western Europe has indicated that it is due to a very ancient mutation.

<span class="mw-page-title-main">Splashed white</span> Horse coat colour

Splashed white or splash is a horse coat color pattern in the "overo" group of spotting patterns that produces pink-skinned, white markings. Many splashed whites have very modest markings, while others have the distinctive "dipped in white paint" pattern. Blue eyes are a hallmark of the pattern, and splash may account for otherwise "solid" blue-eyed horses. Splashed white occurs in a variety of geographically divergent breeds, from Morgans in North America to Kathiawari horses in India. The splashed white pattern is also associated with congenital deafness, though most splashed whites have normal hearing. Splashed white can be caused by multiple variants across two different genes, for which genetic testing is available.

<span class="mw-page-title-main">Roan (horse)</span> Horse coat color pattern characterized by an even mixture of colored and white hairs on the body

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.

<span class="mw-page-title-main">Dog coat genetics</span> Genetics behind dog coat

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.

The agouti gene, the Agouti-signaling protein (ASIP) is responsible for variations in color in many species. Agouti works with extension to regulate the color of melanin which is produced in hairs. The agouti protein causes red to yellow pheomelanin to be produced, while the competing molecule α-MSH signals production of brown to black eumelanin. In wildtype mice, alternating cycles of agouti and α-MSH production cause agouti coloration. Each hair has bands of yellow which grew during agouti production, and black which grew during α-MSH production. Wildtype mice also have light-colored bellies. The hairs there are a creamy color the whole length because the agouti protein was produced the whole time the hairs were growing.

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  88. 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."
  89. 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."
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  99. 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."
  100. 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."
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  102. Pulos & Hutt (1969). "... in his genetic analysis of records of the Frederiksborg white horses, [Wriedt] considered [them] 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 ... [Salisbury] made 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."
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