Dominant white - AbsoluteAstronomy.com

Dominant white is a group of genetically

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

related coat color conditions in the horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

, best known for producing an all-white coat

White (horse)

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 genes, are rare...

, but also for producing some forms of white spotting

Pinto horse

A pinto horse has a coat color that consists of large patches of white and any other color. The distinction between "pinto" and "solid" can be tenuous, as so-called "solid" horses frequently have areas of white hair. Various cultures throughout history appear to have selectively bred for pinto...

and white markings

Horse markings

Markings on horses usually are distinctive white areas on an otherwise dark base coat color. Most horses have some markings, and they help to identify the horse as a unique individual. Markings are present at birth and do not change over the course of the horse's life...

. Dominant white horses are born with unpigmented pink skin and white hair with dark eyes, although the amount of white hair can vary. Dominant white is a rare condition, and under normal conditions at least one parent must be dominant white to produce dominant white offspring

Offspring

In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...

. However, there are documented cases where dominant white has occurred as a spontaneous mutation

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

.

Dominant white can occur in any breed, and has been studied in many different breeds. Two color breed

Color breed

A color breed is a term that refers to horses that are registered based primarily on their coat color, regardless of the horse's actual breed or breed type....

s, the American White Horse

American creme and white horse registry

The American White & American Creme Horse Registry was originally named the American Albino Horse Club and first established by Caleb Thompson and his wife, Ruth....

and Camarillo White Horse

Camarillo White Horse

The Camarillo White Horse is a rare horse breed less than 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 State Fair in Sacramento...

are characterized by their dominant white coats.

There are many different forms of dominant white; in genetics, they are labeled W1 through W11. All known dominant white coat colors are associated with the KIT
CD117
Mast/stem cell growth factor receptor also known as proto-oncogene c-Kit or tyrosine-protein kinase Kit or CD117 is a protein that in humans is encoded by the KIT gene...

gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

. As the name suggests, these known white coats are inherited dominantly, meaning that a horse only needs one copy of a W allele

Allele

An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

to have the white or white spotted coat.

Dominant white is genetically distinct from Sabino

Sabino horse

Sabino is a group of white spotting patterns in horses that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 gene, for which there is a DNA test...

and both genetically and visually distinct from gray

Gray (horse)

Gray or grey is a coat color of horses characterized by progressive silvering of the colored hairs of the coat. Most gray horses have black skin and dark eyes; unlike many depigmentation genes, gray does not affect skin or eye color Their adult hair coat is white, dappled, or white intermingled...

and cremello

Cream gene

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, if they are homozygous. Similarly, horses with a bay...

. Dominant white is not the same as lethal white syndrome

Lethal white syndrome

Lethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...

, nor are dominant white horses "albinos

Albinism

Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...

". Albinism has never been documented in horses. Some forms of dominant white are thought to result in nonviable embryos when a zygote

Zygote

A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

is has two W alleles (homozygous). However, this has not been verified for all dominant white genetic variations.

Identification

Dominant white horses are born with pink skin and a white coat, which they retain throughout their lives. 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 horse

Sabino horse

s. Non-white areas of skin and hair are most commonly seen along the dorsal midline

Dorsum (biology)

In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

of the horse, known as the topline, and are especially common in the mane

Mane (horse)

The mane is the hair that grows from the top of the neck of a horse or other equine, reaching from the poll to the withers, and includes the forelock or foretop. It is thicker and coarser than the rest of the horse's coat, and naturally grows to roughly cover the neck...

and on the ears. They may also have interspersed specks or spots of non-white skin and hair. In addition, the hooves

Horse hoof

A horse hoof is a structure surrounding the distal phalanx of the 3rd digit of each of the four limbs of Equus species, which is covered by complex soft tissue and keratinised structures...

are most often white, but may have striping if there is pigmented skin on the coronary band

Equine forelimb anatomy

The equine forelimb of the horse is attached to the trunk of the animal by purely muscular connections...

just above the hoof. In some cases, foal

Foal

A foal is an equine, particularly a horse, that is one year old or younger. More specific terms are colt for a male foal and filly for a female foal, but these terms are used until the horse is age three or four. When the foal is nursing from its dam , it may also be called a suckling...

s born with residual non-white hair may lose some or all of this pigment with age, without the help of the gray

Gray (horse)

factor. Dominant white spotting does not affect eye color, and most dominant white horses have brown eyes. The pink skin is devoid of pigment cells (melanocyte

Melanocyte

-External links: - "Eye: fovea, RPE" - "Integument: pigmented skin"...

s), and appears pink from the underlying network of capillaries

Capillary

Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...

. White hair is rooted in unpigmented pink skin. There are many other genetic

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

factors that produce white, near-white, and off-white coat colors in horses, some of which are visually very similar to dominant white.

Prevalence

Dominant white is one of several potential genetic causes for horses with "excessive white," 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, dominant white has been identified in multiple families of Thoroughbred

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...

s, American Quarter Horse

American Quarter Horse

The American Quarter Horse is an American breed of horse that excels at sprinting short distances. Its name came from its ability to outdistance other breeds of horses in races of a quarter mile or less; some individuals have been clocked at speeds up to 55 mph...

s, Frederiksborg horse

Frederiksborg horse

The Frederiksborger is Denmark's oldest horse breed. They were tremendously popular throughout the Renaissance and Baroque periods and were considered luxury items. Today the breed is rare, but has a loyal following. Stallions and mares undergo studbook inspections before being allowed to breed...

s, Icelandic horse

Icelandic horse

The Icelandic horse is a breed of horse developed in Iceland. Although the horses are small, at times pony-sized, most registries for the Icelandic refer to it as a horse. Icelandic horses are long-lived and hardy. In their native country they have few diseases; Icelandic law prevents horses from...

s, Shetland ponies

Shetland pony

The Shetland pony is a breed of pony originating in the Shetland Isles. Shetlands range in size from a minimum height of approximately 28 inches to an official maximum height of 42 inches at the withers. Shetland ponies have heavy coats, short legs and are considered quite intelligent...

, Franches Montagnes horses

Freiberger

The Freiberger, also known as Franches-Montagnes, is a draft horse from Switzerland. It was widely used as draft and pack horse in the Swiss army. Breeding used to be heavily subsidized in Switzerland. It is the last representative of the light cold blood horse in Europe...

, South German Draft horses, and in one family of the Arabian horse

Arabian horse

The Arabian or Arab horse is a breed of horse that originated on the Arabian Peninsula. With a distinctive head shape and high tail carriage, the Arabian is one of the most easily recognizable horse breeds in the world. It is also one of the oldest breeds, with archaeological evidence of horses...

. The American White Horse

American creme and white horse registry

The American White & American Creme Horse Registry was originally named the American Albino Horse Club and first established by Caleb Thompson and his wife, Ruth....

, which is descended primarily from one dominant white stallion

Stallion

A Stallion is a male horse.Stallion may also refer to:* Stallion , an American pop rock group* Stallion , a figure in the Gobot toyline* Stallion , a character in the console role-playing game series...

crossed on non-white mare

Mare

Female horses are called mares.Mare is the Latin word for "sea".The word may also refer to:-People:* Ahmed Marzooq, also known as Mare, a footballer and Secretary General of Maldives Olympic Committee* Mare Winningham, American actress and singer...

s, is known for its dominant white coat, as is the Camarillo White Horse

Camarillo White Horse

Inheritance

The W locus was mapped to the KIT gene

CD117

Mast/stem cell growth factor receptor also known as proto-oncogene c-Kit or tyrosine-protein kinase Kit or CD117 is a protein that in humans is encoded by the KIT gene...

in 2007. The terms "Kit oncogene" and "dominant spotting" gene, symbolized by KIT and W respectively, can be used interchangeably. Current research has now shown that there are multiple forms, or allele

Allele

s, of the W gene. All horses possess the KIT gene

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 forms of KIT. Each unique form is called an allele

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

Wild type

Wild type refers to the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard, "normal" allele at a locus, in contrast to that produced by a non-standard, "mutant" allele...

" allele. Thus, a dominant white horse has one KIT allele with a mutation associated with dominant white spotting, and one wild type KIT allele.

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

Columbia University in the City of New York is a private, Ivy League university in Manhattan, New York City. Columbia is the oldest institution of higher learning in the state of New York, the fifth oldest in the United States, and one of the country's nine Colonial Colleges founded before the...

"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."

Sturtevant and his contemporaries agreed that this colt's blue eyes were inherited separately from his white coat. 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

Morgan horse

The Morgan is one of the earliest horse breeds developed in the United States. Tracing back to the stallion Figure, later named Justin Morgan after his best-known owner, the breed excels in many disciplines, and is known for its versatility....

mares to produce a breed of horse known today as the American White Horse

American creme and white horse registry

The American White & American Creme Horse Registry was originally named the American Albino Horse Club and first established by Caleb Thompson and his wife, Ruth....

. 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

Alfred University

Alfred University is a small, comprehensive university in the Village of Alfred in Western New York, USA, an hour and a half south of Rochester and two hours southeast of Buffalo. Alfred has an undergraduate population of around 2,000, and approximately 300 graduate students...

and Dr. Frederick B. Hutt of Cornell

Cornell University

Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...

. They concluded, based on test matings and progeny phenotype ratios, that the white coat was dominantly inherited and embryonic lethal

Gene lethality

Gene lethality occurs when two individuals of the same phenotype mate and a 2:1 phenotypic ratio exists in the offspring . Usually the homozygous dominant individuals will have been aborted before birth....

in the homozygous state. Other factors, such as variations in expressivity

Expressivity

Expressivity is a term used in genetics to refer to variations in a phenotype among individuals carrying a particular genotype. The term can be used to characterize qualitatively or quantitatively the extent of phenotypic variation given a particular genotype. The term is analogous to the...

and the influence of multiple genes

Polygene

A polygene, multiple factor, multiple gene inheritance, or quantitative gene is a group of non-allelic genes that together influence a phenotypic trait. The loci or identities of the non-allelic genes are often unknown to biologist...

, 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

Frederiksborg horse

. Wriedt described a range of what he considered to be homozygote phenotypes: all-white, white with pigmented flecks, or weißgraue, which transliterates to "white-gray." The German term for gray

Gray (horse)

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.

Other researchers prior to modern DNA analysis developed remarkably prescient theories. In another 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

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

, thus allowing for the varied expression of dominant white. He also proposed that other, distant genes might also influence the amount of white present.

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

Sabino horse

" became regarded by some as a more likely cause of white phenotypes. Sabino

Sabino horse

is a type of white spotting, and the one allele now mapped,the dominantly-inherited Sabino-1 (SB-1), is genetically related, though distinct. When homozygous, SB-1 can produce nearly all-white horses that resemble dominant white. Other genes responsible for all possible patterns labeled "Sabino" have not yet been identified.

In 2007, researchers from Switzerland

Switzerland

Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

and the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

published a paper identifying the genetic cause of dominant white spotting in horses from the Franches Montagnes horse

Freiberger

, Camarillo White Horse

Camarillo White Horse

, Arabian horse

Arabian horse

and Thoroughbred

Thoroughbred

breeds. Each of these dominant white conditions had occurred separately and spontaneously in the past 75 years, and each represents a different allele

Allele

(variation or form) of the same gene

Gene

. These same researchers identified a further seven unique causes of dominant white in 2009: three in distinct families of Thoroughbreds, one Icelandic horse

Icelandic horse

, one Holsteiner, a large family of American Quarter Horse

American Quarter Horse

s and a family of South German Draft horses.

Allelic series

The KIT gene contains over 2000 base pair

Base pair

In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...

s, and a change in any of those base pairs results in a mutant allele. Over forty such alleles have been identified by sequencing the KIT genes of various horses. The resultant phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

of most of these alleles is not yet known, however 11 have been linked to dominant white.

W1 is found in Franches Montagnes
Freiberger
The Freiberger, also known as Franches-Montagnes, is a draft horse from Switzerland. It was widely used as draft and pack horse in the Swiss army. Breeding used to be heavily subsidized in Switzerland. It is the last representative of the light cold blood horse in Europe...

horses descended from a white mare
Mare
Female horses are called mares.Mare is the Latin word for "sea".The word may also refer to:-People:* Ahmed Marzooq, also known as Mare, a footballer and Secretary General of Maldives Olympic Committee* Mare Winningham, American actress and singer...

named Cigale born in 1957. Cigale's parents' coats were not extensively marked. A single nucleotide polymorphism
Single nucleotide polymorphism
A single-nucleotide polymorphism is a DNA sequence variation occurring when a single nucleotide — A, T, C or G — in the genome differs between members of a biological species or paired chromosomes in an individual...

(SNP), a type of mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

in which a single nucleotide is accidentally exchanged for another, is thought to have occurred with Cigale. This mutation (c.2151C>G) is thought to severely affect the function of KIT. Some horses with the W1 mutation are born pure white, but many have residual pigment along the topline, which they may then lose over time. Based on studies of KIT mutations in mice, the severity of this mutation suggests that it may be nonviable in the homozygous state. However, horses with the W1 mutation have been found to have normal blood parameters and do not suffer from anemia
Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...

.

W2 is found in Thoroughbred
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...

horses descended from KY Colonel, a stallion
Stallion
A Stallion is a male horse.Stallion may also refer to:* Stallion , an American pop rock group* Stallion , a figure in the Gobot toyline* Stallion , a character in the console role-playing game series...

born in 1946. While KY Colonel was described as a chestnut with extensive white markings, he is known for siring a family of pure white horses through his white daughter, White Beauty, born in 1963. The W2 allele is also linked to a single nucleotide polymorphism (c.1960G>A).

W3 is found in Arabian horse
Arabian horse
The Arabian or Arab horse is a breed of horse that originated on the Arabian Peninsula. With a distinctive head shape and high tail carriage, the Arabian is one of the most easily recognizable horse breeds in the world. It is also one of the oldest breeds, with archaeological evidence of horses...

s descended from R Khasper, a near-white stallion
Stallion
A Stallion is a male horse.Stallion may also refer to:* Stallion , an American pop rock group* Stallion , a figure in the Gobot toyline* Stallion , a character in the console role-playing game series...

born in 1996. Neither of his parents were white, and the causative mutation (c.706A>T) is thought to have originated with this horse. Horses with the W3 allele often retain interspersed flecks or regions of pigmented skin and hair, which may fade with time. Some members of this family possess blue eyes, but these are thought to be inherited separately from the white coat. Based on similar studies in mice, researchers have named W3 as potentially homozygous nonviable.

W4 is found in Camarillo White Horse
Camarillo White Horse
The Camarillo White Horse is a rare horse breed less than 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 State Fair in Sacramento...

s, a breed characterized by a white coat, beginning with a spontaneous white stallion born in 1912 named Sultan. Like W1 and W3, horses with this type of dominant white may be pure white or near-white, with pigmented areas along the topline that fade with time. This mutation is also an SNP
Single nucleotide polymorphism
A single-nucleotide polymorphism is a DNA sequence variation occurring when a single nucleotide — A, T, C or G — in the genome differs between members of a biological species or paired chromosomes in an individual...

(c.1805C>T).

W5 is found in Thoroughbred
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...

s descending from Puchilingui, a 1984 stallion with sabino-like
Sabino horse
Sabino is a group of white spotting patterns in horses that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 gene, for which there is a DNA test...

white spotting and roaning. Horses with the W5 allele exhibit a huge range in white phenotype: a few have been pure white or near-white, while others have sabino-like spotting limited to high, irregular stockings
Horse markings
Markings on horses usually are distinctive white areas on an otherwise dark base coat color. Most horses have some markings, and they help to identify the horse as a unique individual. Markings are present at birth and do not change over the course of the horse's life...

and blazes that covered the face. Twenty-two members of this family were studied, and the 12 with some degree of dominant white spotting were found to have a deletion in exon
Exon
An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule either after portions of a precursor RNA have been removed by cis-splicing or when two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA...

15.

W6 is found in one near-white Thoroughbred
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...

born to non-white parents. The potential range of expressivity, therefore, is not yet known. The mutation (c.856G>A) is thought to have occurred spontaneously in this horse.

W7 is found in another near-white Thoroughbred
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...

born to unremarkable parents. The dam
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

did not possess the identified allele, which results from a splice site mutation
Splice site mutation
A splice site mutation is a genetic mutation that inserts or deletes a number of nucleotides in the specific site at which splicing of an intron takes place during the processing of precursor messenger RNA into mature messenger RNA. The abolishment of the splicing site results in one or more...

(c.338-1G>C).

W8 is found in one Icelandic horse
Icelandic horse
The Icelandic horse is a breed of horse developed in Iceland. Although the horses are small, at times pony-sized, most registries for the Icelandic refer to it as a horse. Icelandic horses are long-lived and hardy. In their native country they have few diseases; Icelandic law prevents horses from...

with sabino-like white spotting
Pinto horse
A pinto horse has a coat color that consists of large patches of white and any other color. The distinction between "pinto" and "solid" can be tenuous, as so-called "solid" horses frequently have areas of white hair. Various cultures throughout history appear to have selectively bred for pinto...

, mottling, and roaning
Roan (horse)
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 more solid-colored. The roan pattern is dominantly-inherited, and is found in many horse breeds...

. Both parents and four maternal half-siblings, all non-white, were found without the W8 allele. The W8 allele is also a splice site mutation (c.2222-1G>A).

W9 is found in one all-white Holsteiner horse with a single nucleotide polymorphism (c.1789G>A). No relatives were studied, but both parents are non-white.

W10 is found in a family of 27 American Quarter Horse
American Quarter Horse
The American Quarter Horse is an American breed of horse that excels at sprinting short distances. Its name came from its ability to outdistance other breeds of horses in races of a quarter mile or less; some individuals have been clocked at speeds up to 55 mph...

s, 10 of which are white or spotted. Those 10 family members had a deletion in exon
Exon
An exon is a nucleic acid sequence that is represented in the mature form of an RNA molecule either after portions of a precursor RNA have been removed by cis-splicing or when two or more precursor RNA molecules have been ligated by trans-splicing. The mature RNA molecule can be a messenger RNA...

7 (c.1126_1129delGAAC). Like W5, a wide range of phenotypes were observed. The most modestly marked had large amounts of white on the face and legs and some medium-sized belly spots, while another was nearly all-white.

W11 is found in a family of South German Draft horses descending from a single white stallion, in which the causative mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

is thought to have originated. The mutation responsible for the W11 phenotype is a splice site mutation of intron 20 (c.2684+1G>A).

These eleven 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. Horses outside of the families described here are not likely to have any of these eleven alleles. The genetic causes behind the American White Horse

American creme and white horse registry

The American White & American Creme Horse Registry was originally named the American Albino Horse Club and first established by Caleb Thompson and his wife, Ruth....

, at least three Thoroughbred families and an Arabian family with dominant white spotting remain to be discovered.

Molecular genetics

The KIT gene

Gene

encodes a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

called steel factor receptor, which is critical to the differentiation of stem cells into blood cells

Haematopoiesis

Haematopoiesis is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells...

, sperm cells

Spermatogenesis

Spermatogenesis is the process by which male primary germ cells undergo division, and produce a number of cells termed spermatogonia, from which the primary spermatocytes are derived. Each primary spermatocyte divides into two secondary spermatocytes, and each secondary spermatocyte into two...

, and pigment cells

Melanocyte

-External links: - "Eye: fovea, RPE" - "Integument: pigmented skin"...

. A process called alternative splicing

Alternative splicing

Alternative splicing is a process by which the exons of the RNA produced by transcription of a gene are reconnected in multiple ways during RNA splicing...

, which uses the information encoded in the KIT gene to make slightly different proteins (isoforms

Protein isoform

A protein isoform is any of several different forms of the same protein. Different forms of a protein may be produced from related genes, or may arise from the same gene by alternative splicing. A large number of isoforms are caused by single-nucleotide polymorphisms or SNPs, small genetic...

) for use in different circumstances, may impact whether a mutation

Mutation

on KIT affects blood cells, sperm cells, or pigment cells. Steel factor receptor interacts chemically with steel factor

Stem cell factor

Stem Cell Factor is a cytokine that binds to the c-Kit receptor . SCF can exist both as a transmembrane protein and a soluble protein...

or stem cell factor to relay chemical messages. These messages are used during embryonic development to signal the migration of early melanocyte

Melanocyte

-External links: - "Eye: fovea, RPE" - "Integument: pigmented skin"...

s (pigment cells) from the neural crest

Neural crest

Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia....

tissue to their eventual destinations in the dermal layer. The neural crest

Neural crest

is a transient tissue in the embryo that lies along the dorsal

Dorsum (biology)

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

Self-replication

Self-replication is any behavior of a dynamical system that yields construction of an identical copy of that dynamical system. Biological cells, given suitable environments, reproduce by cell division. During cell division, DNA is replicated and can be transmitted to offspring during reproduction...

and differentiation

Cellular differentiation

In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...

, and then migrate down and around the body from the dorsal aspect towards the ventral aspect and the limb buds.

The timing of this migration is critical; all white markings

Horse markings

, from a small star to a pure white coat, are caused by the failed migration of melanocytes.

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 genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

s, as in clones

Cloning

Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...

and identical twins

TWINS

Two Wide-Angle Imaging Neutral-Atom Spectrometers are a pair of NASA instruments aboard two United States National Reconnaissance Office satellites in Molniya orbits. TWINS was designed to provide stereo images of the Earth's ring current. The first instrument, TWINS-1, was launched aboard USA-184...

, the process is unlikely to occur the same way twice. A process with this element of randomness is called a stochastic

Stochastic

Stochastic refers to systems whose behaviour is intrinsically non-deterministic. A stochastic process is one whose behavior is non-deterministic, in that a system's subsequent state is determined both by the process's predictable actions and by a random element. However, according to M. Kac and E...

process, and cell differentiation is, in part, a stochastic process. 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. 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 kind of dominant white.

Lethality

Early embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

nal lethality, also known as early embryonic death or a non-viable embryo, may occur when the embryo possesses two dominant white allele

Allele

s, or have the homozygous genotype. This hypothesis was supported by Pulos and Hutt's 1969 study of Mendelian

Mendelian inheritance

Mendelian inheritance is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics...

progeny ratios

Progeny testing

Progeny Testing is a test of the value for selective breeding of an individual's genotype by looking at the progeny produced by different matings.- Progeny Testing :...

. Conclusions about Mendelian traits that are controlled by a single gene

Gene

can be drawn from test breedings with large sample sizes. However, traits that are controlled by allelic series or multiple loci

Quantitative trait locus

Quantitative traits refer to phenotypes that vary in degree and can be attributed to polygenic effects, i.e., product of two or more genes, and their environment. Quantitative trait loci are stretches of DNA containing or linked to the genes that underlie a quantitative trait...

are not Mendelian characters, and are not subject to Mendelian ratios.

Pulos and Hutt knew that if the allele

Allele

that created a white coat was recessive

Recessive

In genetics, the term "recessive gene" refers to an allele that causes a phenotype that is only seen in a homozygous genotype and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father...

, 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.

Pulos and Hutt reported that neonatal death rates in white foal

Foal

s were similar to those in non-white foals, and concluded that homozygous white fetus

Fetus

A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

es died during gestation

Gestation

Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....

. No aborted

Miscarriage

Miscarriage or spontaneous abortion is the spontaneous end of a pregnancy at a stage where the embryo or fetus is incapable of surviving independently, generally defined in humans at prior to 20 weeks of gestation...

fetuses were found, suggesting that death occurred early on in embryonic or fetal development and that the fetus was "resorbed."

Prior to Pulos and Hutt's work, researchers were split on the mode of inheritance of white and whether it was deleterious (harmful). 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. Therefore, it remains to be proven whether all equine dominant white mutations cause embryonic lethality in the homozygous state.

The white (W) locus

Locus (genetics)

In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...

was first recognized in mice

House mouse

The house mouse is a small rodent, a mouse, one of the most numerous species of the genus Mus.As a wild animal the house mouse mainly lives associated with humans, causing damage to crops and stored food....

in 1908. The mutation

Mutation

of the same name produces a belly spot and interspersed white hairs on the dorsal

Dorsum (biology)

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. A mutation which affects multiple systems is "pleiotropic

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...

." Following the mapping

Genetic linkage

Genetic linkage is the tendency of certain loci or alleles to be inherited together. Genetic loci that are physically close to one another on the same chromosome tend to stay together during meiosis, and are thus genetically linked.-Background:...

of the KIT
CD117
Mast/stem cell growth factor receptor also known as proto-oncogene c-Kit or tyrosine-protein kinase Kit or CD117 is a protein that in humans is encoded by the KIT gene...

gene to the W locus in 1988, researchers began identifying other mutations as part of an allelic series

Allele

of W. There are over 90 known W 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

Infertility

Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...

. Some alleles, such as sash 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. A recent study showed that blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

parameters in horses with the W1 mutation were normal.

"White" horses that are not dominant white

White horses

White (horse)

are potent symbols

White horse (mythology)

White horses have a special significance in the mythologies of cultures around the world. They are often associated with the sun chariot, with warrior-heroes, with fertility , or with an end-of-time saviour, but other interpretations exist as well...

in many cultures. An array of horse coat colors may be identified as "white," often inaccurately, and many are genetically distinct from "dominant white."

Additionally, "albino

Albinism

Albinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...

" horses have never been documented, despite references to so-called "albino" horses. Dominant white is caused by the absence of pigment cells (melanocyte

Melanocyte

-External links: - "Eye: fovea, RPE" - "Integument: pigmented skin"...

s), whereas albino animals have a normal distribution of melanocytes. Also, a diagnosis of albinism in humans is based on visual impairment

Visual impairment

Visual impairment is vision loss to such a degree as to qualify as an additional support need through a significant limitation of visual capability resulting from either disease, trauma, or congenital or degenerative conditions that cannot be corrected by conventional means, such as refractive...

, which has not been described in horses with dominant white nor similar coat colors. In other mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, the diagnosis of albinism is based on the impairment of tyrosinase

Tyrosinase

Tyrosinase also known as monophenol monooxygenase is an enzyme that catalyses the oxidation of phenols and is widespread in plants and animals...

production through defects in the Color (C) gene

Gene

. No mutation

Mutation

s of the tyrosinase or C gene are known in horses.

Non-white colors

Cremello
Cream gene
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, if they are homozygous. Similarly, horses with a bay...

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. Dominant 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
Pearl gene
The Pearl gene, also known as the "Barlink factor," is a dilution gene that somewhat resembles the cream gene and the champagne gene, but is neither. It is a somewhat rare dilution gene found in the American Quarter Horse, American Paint Horse, and Peruvian Paso. The same mutation appears in...

or champagne gene
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 at birth, respectively...

, can also produce cremello-like coats. These coat colors may be distinguishable from dominant white by their unusually colored eyes.
Gray
Gray (horse)
Gray or grey is a coat color of horses characterized by progressive silvering of the colored hairs of the coat. Most gray horses have black skin and dark eyes; unlike many depigmentation genes, gray does not affect skin or eye color Their adult hair coat is white, dappled, or white intermingled...

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 dominant white horses, grays are not born white, nor is their skin color affected by their coat color change.
Leopard complex
Leopard complex
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...

horses, such as the Appaloosa
Appaloosa
The Appaloosa is a horse breed best known for its colorful leopard-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 color pattern is genetically the result of various spotting...

and Knabstrupper 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
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

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
Tovero
The Tovero coloration is a mix of tobiano and overo colorations in Pinto horses and American Paint Horses. The genetics of pinto coloration are not always fully understood, and some horses have a combination of patterns that does not fit cleanly in either category.Some characteristics of a Tovero...

, Medicine hat or War bonnet are terms sometimes applied to Pinto horse
Pinto horse
A pinto horse has a coat color that consists of large patches of white and any other color. The distinction between "pinto" and "solid" can be tenuous, as so-called "solid" horses frequently have areas of white hair. Various cultures throughout history appear to have selectively bred for pinto...

s with residual non-white areas only around the head, especially the ears and poll, while most the remaining of the 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
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 present from birth and does not change throughout the horse's lifetime, unless the horse also carries...

, combined with frame overo
Lethal white syndrome
Lethal white syndrome , also called overo lethal white syndrome , lethal white overo , and overo lethal white foal syndrome , is an autosomal genetic disorder most prevalent in the American Paint Horse...

, sabino
Sabino horse
Sabino is a group of white spotting patterns in horses that affect the skin and hair. A wide variety of irregular color patterns are accepted as sabino. In the strictest sense, "sabino" refers to the white patterns produced by the Sabino 1 gene, for which there is a DNA test...

or splashed white
Splashed white
Splashed white or splash is a horse coat color pattern 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"...

.

Lethal White

Foals with Lethal white syndrome

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

Colon (anatomy)

The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...

that is untreatable, and if not euthanized, invariably die of colic

Colic

Colic is a form of pain which starts and stops abruptly. Types include:*Baby colic, a condition, usually in infants, characterized by incessant crying*Renal colic, a pain in the flank, characteristic of kidney stones...

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

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...

" (non-tobiano) phenotype, or horses with multiple patterns, the LWS allele can be detected by DNA test.

Sabino

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

Dorsum (biology)

. However, there are genetic differences. The term "dominant white" is reserved for known W alleles. Dominant white horses are heterozygous for any one of 11 known allele

Allele

s of the KIT
CD117
Mast/stem cell growth factor receptor also known as proto-oncogene c-Kit or tyrosine-protein kinase Kit or CD117 is a protein that in humans is encoded by the KIT gene...

gene (e.g. W8/+). Homozygosity for some of the 11 known alleles may not create a viable embryo. In contrast, Sabino-White horses are homozygous for the Sabino 1 allele of the KIT gene (SB1/SB1).

Another type of sabino patterning, called simply "sabino," "minimal sabino" if slight, or if particularly dramatic, "maximum sabino," refers to horses that test negative for any of the Dominant White alleles, negative for Sabino 1, and also negative for Tobiano

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 present from birth and does not change throughout the horse's lifetime, unless the horse also carries...

and Frame overo. 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. 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 hair and skin. 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."

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. However, not all KIT alleles currently identified as "dominant white" have been proven lethal.

The similarities between Dominant White, Sabino 1, and other forms of sabino may reflect their common molecular origin: The W1-W11 series and SB1 have been mapped to KIT. The researchers who mapped Sabino 1 suggested that other sabino-like patterns might also map to KIT. Similarly, major alleles for white leg and facial markings

Horse markings

have also been mapped to or near to the KIT gene.

Mosaicism

Mosaicism
Mosaic (genetics)
In genetic medicine, a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one individual who has developed from a single fertilized egg...

in horses is thought to account for some spontaneous occurrences of white, near-white, spotted, and roan

Roan (horse)

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 more solid-colored. The roan pattern is dominantly-inherited, and is found in many horse breeds...

horses. Mosaicism refers to mutations that occur after the single-cell

Zygote

stage, and therefore affect only a portion of the adult cells. Mosaicism may be one possible cause for the rare occurrence of brindle 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

Stem cell

This article is about the cell type. For the medical therapy, see Stem Cell TreatmentsStem cells are biological cells found in all multicellular organisms, that can divide and differentiate into diverse specialized cell types and can self-renew to produce more stem cells...

in the developing foal underwent a mutation

Mutation

, or change to the DNA, that resulted in unpigmented skin and hair. The cells that descend

Cellular differentiation

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. Though this is not always the case, genetic mutations can occur spontaneously in one sex cell of a parent during gametogenesis

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 division of diploid gametocytes into various gametes,...

. In these cases, called germline mutation

Germline mutation

A germline mutation is any detectable and heritable variation in the lineage of germ cells. Mutations in these cells are transmitted to offspring, while, on the other hand, those in somatic cells are not. A germline mutation gives rise to a constitutional mutation in the offspring, that is, a...

s, the mutation will be present in the single-celled zygote

Zygote

conceived from the affected sperm or egg cell, and the condition can be inherited by the next generation.

Homologous conditions

In humans, a skin condition called piebaldism

Piebaldism

Piebaldism is a rare autosomal dominant disorder of melanocyte development. Common characteristics include a congenital white forelock, scattered normal pigmented and hypopigmented macules and a triangular shaped depigmented patch on the forehead....

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

Pig

A pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives...

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

Mouse

A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

, in which over 90 allele

Allele

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. To date, no such pleiotropic

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...

effects have been described in horses with KIT mutations.

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