The perils of breeding horses for color

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Registered as white, thoroughbred mare Buchiko has almost dappled colouring.
Registered as white, thoroughbred mare Buchiko has almost dappled coloring. Read more about her here.

The topic of coat color and coat patterns is of great interest to horse breeders. Color can substantially increase a horse’s marketability.

All else being equal, a buyer is often willing to pay more for a horse with an attractive color pattern. This has led to a wide variety of colors and patterns in many breeds, and an increase in the number of breeders looking to include coat color in their breeding program.

However, color should not be the sole objective of a breeding program. The desire to add a particular color gene to a herd may lead to selection of breeding stock for their color first, rather than for good conformation or a proven performance record.

Over time, breeding program that focus too intently on color may compromise quality, if care is not taken to select for other important traits at the same time.

Additionally, certain colors or patterns are associated with health or medical problems, which can then be disseminated within the population.

A colt affected by Naked Foal Syndrome with its unaffected mother. The affected colt has almost no hair. What little coat he has is cremello, which leads to a very strong dilution of the pigmentation but is unrelated to the syndrome. Bauer et al. https://doi.org/10.1534/g3.117.039511
A colt affected by Naked Foal Syndrome with its unaffected mother. The affected colt has almost no hair. What little coat he has is cremello, which leads to a very strong dilution of the pigmentation but is unrelated to the syndrome. Bauer et al. Read more on Naked Foal Syndrome.

The incredible advancements in the field of genetics, and equine genetics in particular, over the last 15 years have allowed researchers to identify many genes and mutations responsible for coat colors and patterns in the horse. These discoveries led to the development of DNA tests for color genes which breeders can use to make informed decisions about their breeding stock and to increase the chances of producing foals with desirable colors and patterns. The diagnostic genetic tests have the added benefit of allowing breeders to identify color traits in their breeding stock that could have detrimental effects.

These tests are an important tool that breeders can use to breed horses responsibly while still incorporating desirable colors and patterns into their herds. Genetic tests for horse coat color genes are available through laboratories that offer such diagnostic services such as the Veterinary Genetics Laboratory at the University of California, Davis.

Breeding for color

Most coat color genes associated with particular color phenotypes (color which is visible to the eye) have no harmful effects. Examples of these genes include those that determine the base colors (chestnut, bay, black, brown) and the modifications of these such as cream, dun, champagne and tobiano. As long as diversity within a herd is maintained and breeding choices are made responsibly, there is no reason to expect detrimental effects from propagating these colors throughout a breeding program. Breeders can easily incorporate specific colors into their herds by simply selecting quality breeding stock that also have the colors of interest. Genetic tests, now available for nearly all of the major coat color genes, can be used by breeders wishing to identify potential carriers of colors of interest. However, there remain several coat color patterns for which the specific genes or mutations have not been identified or have only recently been identified, including: dun, roan, white spotting (Appaloosa, splash white and some forms of sabino), and flaxen chestnut.

Flaxen chestnut represents one example of a coat color with no known detrimental effects. It is characterized by a white, blonde, or silver mane or tail (or both) and is expressed only on chestnut horses. The flaxen mane and tail are often very striking and are considered desirable by some breeders.

The flaxen trait appears to result from a different gene, or set of genes, from those already known to produce blond to nearly-white mane and tail phenotypes such as the Cream or Silver dilution genes. Flaxen chestnut has been studied at the Veterinary Genetics Laboratory (VGL), University of California, Davis.

The research was focused on the Morgan breed and involved pedigree analyses to determine the mode of inheritance of the trait as well as molecular research using DNA samples collected from flaxen and non-flaxen chestnuts. Results from the pedigree analyses are summarized here:

 Flaxen inheritance patterns in chestnut morgan horses.
Flaxen inheritance patterns in chestnut morgan horses: The preliminary results found that a flaxen chestnut sire mated with a flaxen chestnut dam consistently produces flaxen chestnut offspring, while a non-flaxen chestnut sire mated with a non-flaxen chestnut dam consistently produces non-flaxen chestnut offspring. The expression of the trait among parents and offspring is highly variable (see photos above), and the inheritance mode appears to be more complicated than a simple recessive when a flaxen mates with a non-flaxen or when non-chestnut horses are involved. The genetics of the flaxen trait in Morgans are still not well understood.
 Flaxen inheritance patterns in chestnut morgan horses: The preliminary results found that a flaxen chestnut sire mated with a flaxen chestnut dam consistently produces flaxen chestnut offspring, while a non-flaxen chestnut sire mated with a non-flaxen chestnut dam consistently produces non-flaxen chestnut offspring. The expression of the trait among parents and offspring is highly variable (see photos above), and the inheritance mode appears to be more complicated than a simple recessive when a flaxen mates with a non-flaxen or when non-chestnut horses are involved. The genetics of the flaxen trait in Morgans are still not well understood. Non-flaxen parents may produce flaxen chestnut offspring.
Non-flaxen parents may produce flaxen chestnut offspring: The two flaxen chestnut Morgan offspring above are among the progeny of the bay sire and black dam. Neither parent show the flaxen trait, yet they produced three very flaxen chestnut offspring, which are this mare’s only offspring. With other mares, the stallion produced 9 other chestnuts, but none were flaxen. It is known that the flaxen trait is expressed only in horses with red (chestnut) body hair pigment, not black or bay body colors, but the trait may be hidden within the sire’s and dam’s genetic code and can be passed on to offspring.

A few coat color genes have been found to be associated with health problems, and it is therefore important for breeders to educate themselves about the risks involved with these traits. One of the best-known examples is that of Overo Lethal White Foal Syndrome (OLWFS), which is most often associated with the frame overo pattern.

Frame overos are common in Paint Horses, but the pattern can also be found in a few other breeds. Frame overo usually manifests as irregular white patches along the neck and barrel of the horse but the pattern is highly variable, with some horses being so minimally marked that they appear to be solid.

Horses homozygous for frame overo (those that receive the mutation from both parents) are born completely white and suffer from intestinal abnormalities that prevent the passage of food (due to a lack of important structures called “enteric ganglia”).

Paint Us A Picture at Callaghan Park.
Overo thoroughbred Paint Us A Picture. © Darryn Nufer/Central Queensland Thoroughbred Breeders Assn

Affected foals usually die within days of birth. A genetic test for OLWFS is available so breeders can test their horses before breeding.

When two overo horses that carry the OLWFS mutation are bred together, the risk of producing an affected foal is 25%. Therefore it is recommended that horses with any overo in their pedigrees be tested for this mutation before breeding in order to avoid producing a lethal white foal, especially because the frame overo pattern does not always express itself with loud markings and OLWFS carrier status cannot be determined based on spotting pattern alone.

Paint Us A Picture's sire Profile in Style.
Paint Us A Picture’s sire Profile in Style, a frame overo.

Breeding a frame overo to a solid horse offers the same chance of producing frame overo foals as breeding two frame overos to each other but without the risk of a lethal white foal, so breeders can make responsible choices about breeding for frame without decreasing their probability of producing frame overo foals.

Overo breeding and the changes of producing Lethal White offspring.
LEFT: Breeding of two overos producing a 25% chance of Lethal White offspring and 50% overo. RIGHT: Breeding of overo with non-overo solid colour also produces 50% overo but with no risk of Lethal White.
A Black Silver horse exhibiting strongly diluted long hair with darker roots and flat gray, dappled body colour. Photo: Tim Kvick CC BY 2.0 via Wikimedia Commons
A Black Silver horse exhibiting strongly diluted long hair with darker roots and flat gray, dappled body colour. Photo: Tim Kvick CC BY 2.0 via Wikimedia Commons

Another example of a coat color gene associated with health problems is the silver dilution, which can be associated with an eye defect known as Anterior Segment Dysgenesis (ASD). ASD is a genetic malformation of the anterior segment of the eye that causes development of cysts, misshapen pupils, and defects of the iris, cornea, and lens.

The association between the silver gene and ASD has been documented and studied in the Rocky Mountain Horse breed in which silver-diluted colors are prevalent. Silver dilution is also present in other horse and pony breeds such as Miniature, Icelandic, Morgan, and Shetland, and there are indications that ASD may not be restricted to Rocky Mountain Horses.

While there is a clear association between the silver gene and ASD, the specific mechanism leading to the eye defect is not known. It is hypothesized that ASD is caused by a gene located near the gene controlling the silver dilution, and that selection for silver-colored breeding pairs increased the incidence of ASD over time through a “hitch-hiking” effect.

Two Black Silver Rocky Mountain horses, one of the breeds affected by the Silver dilution gene. Photo: Bob Langrish, Tim Kvick, Laura Behning. CC BY 2.0 via Wikimedia Commons
Two Black Silver Rocky Mountain horses, one of the breeds affected by the Silver dilution gene. Photo: Bob Langrish, Tim Kvick, Laura Behning. CC BY 2.0 via Wikimedia Commons

Because the silver color is highly desired in the Rocky Mountain breed, ASD presents a particular challenge to these breeders. While the silver gene may not directly cause ASD, the silver coat color is associated with ASD in Rocky Mountain Horses, perhaps because the gene causing ASD is located near the silver gene and has been propagated with the color in this breed. A genetic test for silver is available, but because the genetic basis of ASD has not been identified, horses cannot yet be DNA tested for the disease itself.

There is a great deal of future research needed regarding the diseases associated with coat color in horses. Horses with a coat pattern that indicate they’re homozygous for LP (the ‘appaloosa’ gene), for example, are prone to night blindness, and grey horses have a high risk of developing skin melanomas.

The reasons for these associations and the mutations that led to them are still being studied and may someday be elucidated. Hopefully this will lead to the development of additional genetic tests that will be at the disposal of horse owners looking to breed for particular colors without compromising health.

Conclusion

Production of horses with attractive coat color patterns is part of horse breeding culture and is prevalent within the industry.

Quarter Horse with the rare MITFC280Sfs*20 allele.
Quarter Horse with the rare MITFC280Sfs*20 allele. This horse has a very pronounced splashed white phenotype with blue eyes and a largely unpigmented head and belly.

While breeding for color alone can sometimes be problematic, many problems can be avoided by a combination of breeder education and continued research. Additional knowledge about the genetic basis of equine coat colour will enable better diagnostic and predictive tests for colour and associated traits. The more that is learned about the horse genome, the more breeders can be assured that a particular mating will not produce an undesirable result.

With an eye for quality stock and the implementation of rational and responsible genetic testing, there is no reason that a breeding operation cannot introduce new colours and patterns into a herd while maintaining diversity and herd health.

Article by Sonya Sokolow, Leah Brault and Cecilia Penedo.

Further reading on equine genetics and colour:
Horse Genetics, by Ann T. Bowling
The Genetics of the Horse, edited by Ann T. Bowling and Anatoly Ruvinsky
Equine Color Genetics, by D. Phillip Sponenberg
Horse Colour Explained – A Breeder’s Perspective, by Jeanette Gower
Hair Colour in the Horse, by R. Geurts

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