An important correction

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Louis XIV as the Roman Emperor in the Carrousel du Roi, by Israel Silvestre the Younger, c.1662. The depiction of his mount, and a similar one for the Duke d’Anguein, suggest that the silver dilution might once have been present in the Spanish horse population. 

A few years ago, I reported that a Pura Raza Espanola (PRE) mare has tested positive for silver. This was significant since a previous case of what appeared to be a bay silver stallion had been called into question, regarding both a reported test and registration status. Unlike the stallion, the mare in question was not visibly silver – she was chestnut – but was said to be a carrier. Unfortunately, she has since proven to be negative for the dilution. It appears that while the owners used standard testing notation, what they were doing was what the dog geneticist, Sheila Schmutz, refers to as “guessotyping“. More often when people guess the color of a horse, they use the name of the color. This is especially true when a new and interesting color with an appealing name (champagne, pearl) comes to light. Most of the time there are other clues that the owner is guessing, though that was not true with this particular case, where the particulars were a little unusual.  With the proliferation of websites and forums dedicated to the discussion of horse color, and the more widespread use of testing notations in the descriptions of horses,  this is likely to be an increasingly common problem going forward.

From the standpoint of this blog, I have tried to keep a balance between verifying information and being unnecessarily antagonistic with owners and breeders. In the past, owners of horses with unexpected colors or test results have encountered very aggressive – and at times outright rude – questioning from color enthusiasts. What is often lost in these exchanges is an awareness that breeders do not owe enthusiasts copies of test results. Certainly a potential buyer, or a mare owner interested in breeding to a stallion, have every right to ask to see results if the true color of a horse is in question. But what we are often asking is that someone satisfy our curiosity. Owners and breeders are a valuable resource for anyone interested in horse color. Unlike the situation with the model animal for coat color, the mouse, it is not feasible to breed large, expensive animals as part of a research program. Breeders are doing the work, and paying the bills, for that. What’s more, they are in a unique position to observe nuances of color that people less directly involved with the individual animals might miss. Maintaining a positive atmosphere encourages an open exchange of information, and reduces the chance that someone with a unique horse feels burned by the research community.

That said, it is clear that testing notations cannot be assumed to represent an actual test. As much as it would be a good idea if people did not use them without test results in hand, because they are more widely understood than in the past, the temptation to use them as shorthand for a guessotype is there.

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Reminder – deadline for W20 samples

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I wanted to post a quick reminder that the deadline for having your horse included in the upcoming W20 study is this Tuesday, April 15, 2014. As mentioned in the previous post, the W20 mutation is one of the newly identified White-spotting (W) patterns. The majority of these patterns have been new mutations that originated in specific families of horses within the last few decades. The W20 mutation is different in that it has been found in a number of unrelated breeds (a current list can be found here), which suggests that the pattern is old. That means it may be widespread in the equine population.

Unlike the other patterns in the W series, this one does not put a lot of white on the horse – at least not unless it is paired with another pattern. The exact range of expression is not known, however, nor is it clear how it interacts with some of the other forms of white patterning. To better understand that, one of the research groups in Germany is offering the W20 test for a brief time. The idea is to both collect a range of samples and raise the funds necessary to do the work. (Horses, unfortunately, do not tend to attract third party research grants, so the biggest impediment to progress is lack of funding.)

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Dr. Rony Jude, one of the scientists involved in this project, has indicated that the horses submitted need not be pintos, or even have obviously flashy markings. Horses like the two included in this post would be helpful. I have submitted my own Appaloosa mare, who has only a moderate blaze and a white hind pastern. Groups of related horses are also helpful. Other good candidates are horses that have known, tested patterns but that have more white than might be expected.

The (English-language) form needed to have your horse included is located here. Owners in the United States will need to send their samples in now in order to meet the deadline. For those that have not done genetic testing before, collecting samples involves pulling 30 tail hairs (the root bulb must be visible). For this study, each horse needs two samples: two ziplock bags of 30 hairs. I would also recommend sending the materials in a padded envelope – not a package – to avoid delays in customs. The form indicates that you need to submit three photos (both sides and a face shot showing markings) and a pedigree. Pedigrees are useful for finding possible sources for the mutation, but they are not required for inclusion in the study; you can submit unregistered horses. Dr. Jude has also set up a PayPal account (rjude-consulting@hotmail.com) for participants in the United States, where foreign bank transfers tend to be costly. The necessary photos and the form can also be sent to that address, which is a good idea since we are so close to the deadline.

I hope that Dr. Jude and her team get a good turnout for the study. With luck, W20 could prove to be one of the bigger pieces of the puzzle when it comes to understanding how white markings and white patterns work in horses.

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A heritable form of brindle discovered in horses

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Most brindle horses are chimeras, formed when fraternal twins fuse early in development. As the fused fetus develops, the pigment of the two individuals migrate along the Lines of Blashko. If the two individuals were destined to have different colors, then those lines take on the two-toned pattern associated with brindle. The results can be very striking, but because the color comes from an accident of development rather than the DNA itself, chimeric brindles cannot reproduce their color.

Breeders interested in brindle have long sought a true-breeding brindle. This past winter, the first heritable form of brindling was identified in a single family of horses. Incontinentia Pigmenti (IP) causes areas of darker pigmentation that follow the Lines of Blaschko, producing a pattern much like that of the chimeras. The mutation is dominant and sex-linked, so all the living examples are female descendants of the founder. Females that inherit the mutation are born brindle, while the males die in utero.

Unfortunately while Incontinentia Pigmenti (IP) is heritable, it is not something any breeder would actually want. A close look at the mare above shows that the affected horses have areas of hairlessness as well as hypopigmentation. The disease – and that is what this is – causes lesions in the skin, and sometimes the hair does not return after the area heals. A comparable disease in human beings causes problems with teeth, nails and eyes. According to the researchers, the affected mares also had abnormalities of the teeth, hooves and eyes. Obviously this is not the hoped-for true breeding brindle.

So like the recently identified Lavender Foal Syndrome, this is not really something that would be classified as a color. It is a genetic disease where the altered color is one of the symptoms. Breeders looking for a genetic form of brindling will have to continue looking.

The above photo is of a third-generation mare from this line, taken from the original paper. That paper is available by open access at PLOS One. The breed of the test family was not given.

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