Some background history

White markings in horses serve no biologic purpose, nor are they an adaptation to avoid predation or increase survivorship. On the contrary, white markings on a horse can make it easier for predators to detect them due to the way light interacts with different colors and how animals perceive contrast in their environment. Dark colors, such as black and brown, tend to absorb more light and blend more seamlessly into natural surroundings, whereas white reflects more light, making it more conspicuous, especially in low-light conditions or against darker backgrounds (Caro, 2005). This increased visibility can make a horse with white markings stand out from the rest of the herd, drawing the attention of predators.

Additionally, many predators rely on motion detection and contrast sensitivity rather than color perception when hunting. Studies on animal vision suggest that predators like wolves and big cats have dichromatic vision, meaning they perceive colors differently than humans, primarily distinguishing between blue and yellow wavelengths while having difficulty differentiating reds and greens (Jacobs, 1993). Because white markings create a stark contrast against darker coat colors or natural surroundings, they can enhance the visibility of movement, making a horse’s motion more detectable to a predator’s keen eyesight.

In some environments, particularly in wooded or shadowed areas, a horse with large white patches may stand out more distinctly compared to a uniformly dark-colored horse, which benefits from more effective camouflage (Caro, 2016). This is particularly relevant in species where natural selection has favored solid or muted coat colors in wild equines, such as zebras, which use disruptive coloration to confuse predators (Ruxton, Sherratt, & Speed, 2004).

While white markings may increase visibility in certain conditions, their impact varies based on habitat and lighting. In bright, open environments, such as snowy regions or sandy plains, white markings may offer some blending advantages. However, in forests, grasslands, or dusk and dawn lighting conditions—when many predators are most active—white markings can provide a disadvantage by increasing a horse’s contrast against the environment, making it easier for predators to spot and track them.

Why White Markings are Preferred

Humans selectively bred horses for white facial and leg markings due to a combination of aesthetic, practical, and genetic factors. These markings were often associated with visibility, tradition, and cultural significance, as well as linked to specific genetic patterns.

1. Increased Visibility

One of the primary reasons for selecting horses with white markings, especially on the face and legs, was to enhance their visibility in low-light conditions. White facial blazes and leg stockings made it easier to identify individual horses, particularly in herds or during nighttime activities (Gower, 1999). This was especially beneficial for cavalry, working horses, and herding livestock in dimly lit environments.

2. Aesthetic and Cultural Preferences

Throughout history, different cultures favored distinct coat patterns. In many European and Middle Eastern traditions, white markings were seen as noble and desirable. Medieval knights often preferred horses with white blazes and stockings because they were considered more striking and prestigious (Bennett, 1998). Similarly, in North America, flashy white markings became popular in breeds such as the American Paint Horse and Pinto horses.

3. Association with Temperament

Some studies suggest a correlation between coat color, white markings, and temperament. While anecdotal, many horse breeders believed that certain markings were linked to docility or alertness (Haase et al., 2021). This perception may have led to selective breeding for specific markings as a means of predicting and influencing behavior.

4. Genetic Linkages and Breed Standards

White facial and leg markings are largely controlled by genes affecting pigmentation, particularly the KIT and MITFgenes (Brooks et al., 2007). As breeding programs developed, horses with desirable traits—including markings—were preferentially selected. Some breed registries, such as the American Quarter Horse Association (AQHA), permit or even favor white markings within certain limits, reinforcing their propagation through generations (AQHA, 2023).

5. Cultural and Superstitious Beliefs

Certain markings have been historically linked to superstitions. For example, the “badger face” marking (inverted white blaze) was considered lucky in some cultures, while others believed extensive white on the face (e.g., a bald face) could indicate poor vision or sensitivity to sunlight (Sponenberg, 2009). These beliefs influenced breeding choices in different regions.

Humans bred horses to have white facial and leg markings primarily for their visibility, aesthetic appeal, perceived temperament traits, and genetic linkages. Over time, these preferences became ingrained in breed standards and cultural traditions, leading to the continued propagation of these markings in many modern horse breeds.

White Markings in Equine Ancestry

The absence of white markings in wild horses, such as the Przewalski’s horse, suggests that mustangs in America are not descended from native horses can be supported by several points drawn from the provided references.

1. Lack of White Markings in Wild Horses: Przewalski’s horse, the last truly wild horse species, lacks white markings, indicating that such traits are not inherent to wild equine populations. As stated, “the markings were not present, or were very rare, in the primeval wild horse, as evidenced by their absence in Przewalski’s horse” (Woolf, 1991, p. 3). This suggests that white markings are a trait associated with domestication rather than a natural occurrence in wild equines.
2. Selective Breeding and Aesthetic Preferences: Domestic horses have been selectively bred for various traits, including coat color and markings, primarily for aesthetic purposes. The document mentions that “white markings are assumed to be flashy, highlighting the limb action which is particularly important in shows” (Stachurska & Ussing, 2012, p. 75). This selective breeding has led to the proliferation of white markings in domesticated horse breeds, which are not found in their wild counterparts.
3. Genetic Evidence of Selective Breeding: The presence of white markings in domestic horses is associated with several genetic loci that are not prevalent in wild populations. The genetic basis for white markings is multifactorial and involves complex inheritance patterns, as discussed in multiple studies that highlight the heritability and genetic underpinnings of these traits (Woolf, 1990, p. 250). In contrast, wild horse populations like the Przewalski’s horse show a different genetic makeup, lacking the alleles responsible for white markings.
4. Implications for Mustang Ancestry: If mustangs were directly descended from native horses in the Americas, one would expect to find similar genetic traits, including white markings. However, the absence of such markings in their wild ancestors suggests that mustangs likely descended from domesticated horses, which were selectively bred for these traits in Europe and brought to the Americas by European settlers. The genetic diversity and presence of white markings in mustangs can thus be attributed to the introduction of domestic horses with these traits rather than a lineage from native equines.

In conclusion, the lack of white markings in wild horses like the Przewalski’s horse supports the notion that mustangs, as domesticated descendants, inherited these traits through selective breeding practices focused on aesthetics, rather than through lineage from native American horses. This argument is reinforced by genetic evidence indicating that the selective pressures for coat color and markings are a product of domestication rather than a natural evolution in wild horse populations. There are other reasons we know mustangs are descended from domestic horses, but that a topic we will touch upon later in this post.

Inheritance of White Markings

The white facial and leg markings on horses are described as multifactorial, meaning that the genes are not the only factor that determines their inheritance. It has been found that these markings are controlled by several genetic loci and at least some of the genes involved include MC1R, KIT and MITF. 

1. Multifactorial Inheritance: Some white facial and leg markings are not inherited in a simple Mendelian fashion but rather by a polygenic or multifactorial inheritance system where many genes interact to produce the phenotype. This pattern of inheritance suggests that the frequency of white markings can vary greatly between individuals because of the additive effects of several alleles (Woolf, 1990; Rieder et al., 2008). 

2. Key Genes Involved: Melanocortin 1 Receptor gene (MC1R): Mutation of this gene product causes changes in coat color and plays an important role in the extent of white markings especially in chestnut horses where they have more white markings than other colors (Woolf, 1990; Haase et al., 2013). KIT: The KIT gene is also responsible for the development of the white marking and the size of the area with white marking. Many alleles at this locus have been found to be associated with various forms of white spotting including dominant white (Haase et al., 2013; Rieder et al., 2008). MITF (Microphthalmia-associated Transcription Factor): This gene has been found to be responsible for certain white spotting phenotypes including the splashed white phenotype (Hauswirth et al., 2012). 

3. Genetic Correlation: Studies have shown that heritability for white markings is high, suggesting genetic basis. For instance, heritability estimates for facial markings can be 0.69, and total leg markings can be 0.68, which indicates that genetics is a significant determinant of these traits (Woolf, 1990; Rieder et al., 2008). 

4. Environmental Factors: In addition to genetic influences, environmental factors and stochastic (random) events can also affect the development and manifestation of white markings. This includes the possibility of developmental noise, which are stochastic variations that occur during embryonic development that may result in the variation in the migration and survival of melanoblasts (Mintz, 1974; Woolf, 1995). 

5. Complex Interactions: The interaction between the basic coat color (which is determined by MC1R and Agouti loci) and white markings is also rather complicated. For example, chestnut horses (e/e) are typically more heavily marked than bay (E/ –) and black (a/a; E/ –) horses, which shows the role of other genetic factors (Woolf, 1991). 

The Next Generation

The inheritance of white facial and leg markings in horses is a complex trait controlled by several genetic loci and environmental factors with significant contributions from MC1R, KIT, and MITF genes and their products, which result in various phenotypic outcomes. The heritability of facial and leg markings is fairly high, with the heritability estimates for facial markings being estimated at 0.69 and for limb markings at 0.68, and the overall heritability for both facial and limb scores can be up to 0.77 (Woolf, 1990; Stachurska and Ussing, 2012). These markings are caused by several loci, and the two most important genes have been identified as MC1R and KIT that are involved in pigmentation. The MC1R gene codes for the receptor that regulates type of melanin produced while the KIT gene is associated with white spotting patterns (Rieder et al., 2008; Haase et al.,2013). Furthermore, it has been postulated that the expression of white markings is not only dependent on genetic factors but also on stochastic events that take place during embryonic development that affect melanoblast survival and migration (Woolf, 1990). This complexity suggests that other factors, including environmental factors, may also contribute to the variation in phenotypic expression of white markings in different individuals (Woolf, 1995; Rieder et al., 2008).

White markings can be inherited from parents to offspring. White markings in horses are known to be influenced by genetic factors, especially the mutation of certain alleles at certain loci. For example, a study showed that if both parents have white facial markings, the offspring are likely to have these markings and 61.8% of such offspring will have white markings (Document: Encina et al. – 2024). On the other hand, if both parents are without white markings, then a smaller percentage of the offspring (22.6%) have these markings.

Size matters?

The size of white markings on legs in horses is dependent on genetic and non-genetic factors. Key points include:

1. Genetic Factors: The heritability of white markings is quite high, and the studies have shown that the heritability values are between 0.68 and 0.77 for different parts of the body (Woolf, 1990; Rieder et al., 2008). The MC1R, KIT, and MITF genes have been associated with the frequency of white markings. For instance, the mutation of the recessive allele of the MC1R gene, which is responsible for the chestnut coat color, has been associated with more extensive white markings (Negro et al., 2017; Haase et al., 2013).

2. Interaction of Genes: There are interactions between different genetic loci such as MC1R and KIT, where the genetic control of the quantity of white marking depends on the base coat color of the horse (Patterson Rosa et al., 2022; Cuffe, 2024).

3. Environmental and Stochastic Factors: Other factors, which can be regarded as environmental variables, affecting the intrauterine development of the horse, can also influence the expression of the phenotype of white markings. Stochastic events that affect the survival, movement and mitosis of melanoblasts (the precursor of melanocytes) contribute to the variation in white markings (Woolf, 1995; Stachurska and Ussing, 2012).

4. Sex and Coat Color: It has been found that males have slightly more white markings than females and the base coat color greatly affects the frequency of white markings with chestnut horses having more extensive markings than bay or black horses (Woolf, 1990; Rieder et al., 2008). In summary, the quantity of white in leg markings is controlled by genetic inheritance, particular gene interactions, environmental factors and stochastic events that occur during development.

The Base Coat

Also, different coat colors have different effects on the chances of white markings in offspring. For example, chestnut horses are more likely to have white markings than black horses (Document: Encina et al. – 2024). In general, although genetics is an important factor in the expression of white markings, other factors like environmental conditions and developmental processes also play a role in the expression of these markings. This indicates that the above mentioned genes are involved in the determination of the shape of white markings, such as a star or a blaze.

For example, chestnut horses, which have a specific genotype (e/e), have more extensive white markings than bay or black horses and the amount of white marking is generally higher on these horses than on other horses (Woolf, 1990; Rieder et al., 2008; Haase et al., 2013). Furthermore, the heritability of markings suggests that the extent and perhaps the shape of these markings can be passed down from generation to generation and that certain genetic combinations are more likely to result in more pronounced markings (Woolf, 1990; Rieder et al., 2008). Therefore, even though there are stochastic events that can lead to the final appearance, the genetic predisposition to certain shapes of stars and blazes is indeed inherited. Specific coat patterns are associated with specific facial markings?

Horse coat patterns are accompanied by specific facial markings due to genetic control of pigmentation. Here are some notable associations:

1. Tobiano Pattern: Tobiano horses are homozygous for white and have large patches of white that cross the spine and have face marks that are usually star or stripe like. This is associated with the KIT gene which is involved in white spotting phenotypes (Haase et al., 2013).

2. Sabino Pattern: This pattern has a patchy white marking on the face and other parts of the body and is similar to the leopard complex. The KIT gene is also involved here, that is through mutations that result in different levels of white marking including facial patches (McFadden et al., 2024).

3. Splashed White: Horses with this type of splashed white pattern have a lot of white on the face including up to a full blaze or completely white face. This phenotype is caused by mutations in MITF and PAX3 genes (Hauswirth et al., 2012).

4. Leopard Complex: Horses with this pattern, such as Appaloosa, may have distinctive facial markings with their coat pattern. The gene that causes the leopard spotting is TRPM1 and this can include facial spots (Neves et al., 2017).

5. Frame Overo: This pattern has patchy white patches which do not run across the back and may have facial markings. This is associated with the EDNRB gene which can result in significant white facial markings (Patterson Rosa et al., 2022).

Blue Eyes

The analysis of the coat patterns and facial markings shows that genetics is a crucial determinant of equine coat colour and patterns, and certain genes can lead to specific phenotypic characteristics such as blue eyes. Blue eyes are usually associated with certain white markings in horses. In particular, horses with the splashed white phenotype, which is characterized by significant white markings, have blue eyes. This is explained in the literature on the genetics of white spotting patterns where mutations in genes such as MITF and PAX3 are responsible for both the splashed white markings and blue eyes (Hauswirth et al., 2012; McFadden et al., 2024). Also, in the context of the Lethal White Overo (LWO) syndrome, which is also characterized by white markings, affected horses may have blue eyes as well (Neves et al., 2017).

Cut-outs, reverse colour, negative shapes, superimposed colour…

White marking with ‘cut-outs’, or the base color seen within the blaze, similar to freckles, but on a larger scale are inherited. The body color “cut-outs” in a white blaze on horses, which are the areas of depigmentation, are caused by mutations in genes regulating the migration and differentiation of melanocytes, the pigment producing cells of neural crest origin. Indeed, mutations in genes such as MITF (Microphthalmia-associated transcription factor) and PAX3 (Paired box gene 3) have been linked to splashed white phenotype, which is characterized by extensive white markings including “cut-outs” or unpigmented areas in the coat.

These traits are inherited in a composite fashion. Some white spotting patterns, including splashed white, can be inherited in an autosomal dominant manner; however, the expression can be highly variable among horses with similar pedigree because of the involvement of other genetic factors and possibly environmental factors. The inheritance is not strictly monogenic since many mutations can contribute to the final phenotype and thus result in varying levels of depigmentation from minimal to near complete (Hauswirth et al., 2012, Neves et al., 2017). In summary, the ‘cut-outs’ in a white blaze are the result of particular genetic mutations that affect the development of melanocyte and these traits can be inherited, but the inheritance is not very clear.

Variations in a horse’s health including skin and coat conditions may also affect how much white markings are seen in its life. In summary, the variability of white markings that come and go during the lifetime of a horse can be explained by the genetic, stochastic developmental events, environmental factors, and the health of the horse. Blazes or partial blazes on horses’ faces are forms of white facial marking and their inheritance is quite complex. According to Woolf (1990), the existence and intensity of white markings including blazes can be influenced by several genetic loci and the frequency of these markings may vary among the offspring depending on the genetic contribution of the parents (Woolf, 1990, p. 250-256). However, the patterns of white markings can be complicated and horses with highly marked parents may have more genes that cause white markings and tend to pass on similar markings to their offspring (Woolf, 1990, p. 250-256). Therefore, if a horse has a broken or incomplete blaze, it is possible that this trait can be inherited by their offspring, but the expression of the trait may vary depending on the genes of other factors. In conclusion, although there is a genetic basis for a broken or incomplete blaze, the specific expression in offspring is not fixed and may be influenced by other genes as well as environmental factors.

Clean edge or jagged?

The variation in the edges of facial markings in horses, whether they appear clean or jagged, can be attributed to several factors related to the developmental processes of melanoblasts, which are the precursor cells that develop into melanocytes (the cells responsible for pigment production).

1. Melanoblast Migration and Proliferation: The presence of clean or jagged edges in white markings is influenced by how well melanoblasts migrate and proliferate in the developing tissues. If melanoblasts migrate uniformly and proliferate evenly, the resulting markings tend to have clean edges. Conversely, if there are irregularities or disturbances in the migration and proliferation processes, this can lead to markings with jagged edges (Woolf, 1995).
2. Developmental Noise: The concept of “developmental noise” refers to random fluctuations that occur during the developmental processes. This can lead to variations in how melanoblasts settle in the skin and hair follicles. Such random events can result in differently shaped edges of the markings, with some being more defined (clean edges) and others being less so (jagged edges) (Woolf, 1995; Stachurska and Ussing, 2012).
3. Genetic Control: The genetic makeup of the horse also plays a significant role in how these markings develop. The interaction between multiple genes (a polygenic inheritance) affects the extent and pattern of white markings, which can contribute to differences in edge definitions (Woolf, 1990; Rieder et al., 2008).

Half a Head… Harlequin?

The phenomenon where a blaze covers half of a horse’s face in an almost perfect midline demarkation is probably due to the failure of melanoblasts (the cells that become melanocytes, which produce pigment) to migrate or proliferate properly during embryonic development. This migration and proliferation occurs independently on each side of the embryo to produce symmetrical or asymmetrical pigmentation patterns. If the melanoblasts on one side of the midline survive and proliferate while those on the other side do not, this can result in a clearly demarcated blaze covering one half of the face. This condition represents the concept of ‘developmental noise’, where random events during development can result in alterations in pigmentation. This explanation is in conformity with the fact that such markings are genetically and environmentally determined (Woolf, 1990; Mintz, 1967, 1974).

The Environment

The environment can greatly affect the phenotypic expression of white markings in horses through genetic and epigenetic changes. In particular, the migration and proliferation of melanoblasts, The cells that produce melanin in the skin are referred to as melanocytes and these cells migrate from the neural crest to their destination in the skin and hair follicles during early embryonic development. Specifically, the migration and proliferation of melanoblasts, The cells that produce melanin in the skin are referred to as melanocytes and these cells migrate from the neural crest to their destination in the skin and hair follicles during early embryonic development. However, environmental factors and epigenetic mechanisms can also affect the development and function of the melanocytes and, therefore, the formation of white markings. Stochastic factors that alter gene expression during early development can also influence the phenotype of the markings. Furthermore, epigenetic mechanisms such as DNA methylation, histone modifications, and microRNA activity can influence gene expression without affecting the underlying DNA sequence. The environment can also influence the development of white markings through stochastic (random) events that occur during melanoblast migration and survival. Epigenetic mechanisms such as DNA methylation and histone modifications can also affect gene expression in response to environmental factors. Together, these genetic and epigenetic mechanisms help to determine the pattern of white markings in horses. The environment can also cause white markings through stochastic (random) events that affect the development of melanoblasts and their survival.

Environmental factors can also influence gene expression through epigenetic mechanisms such as DNA methylation and histone modifications. These two types of mechanisms acting in concert determine the configuration of white markings in horses. The environment can also lead to white markings through stochastic (random) events that affect the development of melanoblasts and their survival. Epigenetic mechanisms can also be involved in the modulation of gene expression in response to environmental cues through DNA methylation and histone modifications. Both of these types of mechanisms are involved in the control of white markings in horses and they act in concert to determine the final outcome.

The most common markings

The most common marking in horses is the “half-stocking,” which was found to be present on the left back leg in a study of Arabian and Thoroughbred horses,with a prevalence of 16.9% (Kocakaya et al., 2023). The most common facial marking in horses is typically referred to as a “star.” In studies of various horse breeds, including Arabian horses, the presence of a star marking is frequently noted. For example, in the study by Woolf (1990), different types of facial markings were categorized, and the star marking is recognized as one of the common types observed.The prevalence of white facial markings was also mentioned in the Pura Raza Española horses, and it was mentioned that small white markings like stars are common (Encina et al., 2024).In general, depending on the breed, the star marking is one of the most common types of facial markings observed in horses.

Mustangs are not Native (no matter what you read online)

The primary genetic reason that mustangs in America are considered descendants of domestic horses rather than a native species is that their ancestry can be traced back to domesticated horses introduced to the Americas by Europeans, not to the prehistoric native horse species native to North America.

Key Genetic and Evolutionary Factors:

1.         Extinction of Native North American Horses     
Horses were first found in North America about 55 million years ago and evolved into many species. However, at the end of the Pleistocene epoch (~10,000 years ago) these horses went extinct, possibly due to climate change and hunting by early humans.This means that there was a genetic discontinuity – there were no wild horses in North America for a thousand of years.

2.         European Reintroduction of Domesticated Horses
 The modern Equus caballus was reintroduced to the Americas by the Spanish in the 16th century.  These horses were domesticated breeds from Europe and not the wild horses of North America.      Some escaped or were let loose, or were released, and formed feral populations, but their genetic origin is from the domestic stock, and not from the extinct native species.

3.         Genetic Studies Confirm Domestic Origins.  
DNA analysis of mustangs supports their origin from Iberian (Spanish) horse breeds, with later input from other European breeds (e.g. draft horses, Thoroughbreds). Their genetic continuity with the extinct North American horse species is absent.

4.         Legal and Conservation Classification
Mustangs are classified as feral and not wild because they are not genetically close to the prehistoric species and are descended from domesticated horses. The U.S. government has classified them as “wild” under the Wild Free-Roaming Horses and Burros Act of 1971, but from a biological point of view, they are domesticated feral animals and not a native species. In conclusion, mustangs in America are not considered to be a native species because they are descended from domesticated European horses and not the extinct native horses of prehistoric North America.

The following is a list of genetic traits that are present in true wild horses and are lacking in domestic horses: traits associated with domestication syndrome and the loss of genetic variability in regions linked to tameness and morphology. The analysis of the available data.

Key Genetic Differences Between Wild and Domestic Horses

1.         Greater Genetic Diversity in Przewalski’s Horse: The Przewalski’s horse (Equus ferus przewalskii) is considered the only living wild horse, that was never tamed. Genetic analyses show that Przewalski’s horses are genetically closer to their ancestral stock than domestic horses, especially with regard to the regions linked to the immune system and open environments. Librado et al. (2015) reported that Przewalski’s horses have unique haplotypes that are not seen in modern domestic horses, particularly at the mitochondrial DNA level.

2.         Distinct Y-Chromosome Haplotypes:  He noted that domesticated horses are suffering from a relative lack of variety at the Y-chromosome level as compared to true wild horses. Przewalski’s horses have a distinct Y-chromosome lineage that is not seen in domesticated horses (Lippold et al., 2011).

3.         Mutation in the G-Protein-Coupled Receptor (GPR143) Gene: In a study by Gaunitz et al. (2018), it was determined that domesticated horses have been selected at genes associated with behavior, including GPR143, which is linked to visual perception and may have influenced tameness. The true wild horses, including Przewalski’s, do not have these same mutations, suggesting a difference in their sensory perception and behavioral response.

4.         Genetic Selection in Neural and Behavioral Genes: Domesticated horses have been selected for tamer and less aggressive conformation through genes like SORCS1 and NRXN1 that are involved in the development of the nervous system and plasticity of synapses (Schubert et al., 2014).Przewalski’s horses do not have all of these domestication-related genetic changes and are therefore more skittish and less submissive than domestic breeds (Schubert et al., 2014).

5.         Difference in Coat Color Genetics: The ancient wild horses (including Przewalski’s) were mostly of the dun coat color, which is controlled by the TBX3 gene and is camouflage color. Many domestic horses lost the dun dilution allele in the process of selecting for other coat colors, whereas Przewalski’s horses retained this ancestral trait (Ludwig et al., 2009).

Horses, including the Przewalski’s horse, do not have the genetic markers of domestication that are seen in modern horses, especially in the behavioural, neurologic, and coat colour genes. They own higher Y-chromosome diversity, have conserved immune system genes, and have not been selectively bred for tameness and coat color, which makes them different from the domesticated breeds.

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Dr Meredith Hudes-Lowder & Karen McLain

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