I mentioned in the last post that the known White-spotting (W) mutations tended to fall into three basic categories, at least in terms of appearance: white, sabino, and white markings. At the moment, there is only one mutation in that last category, known as W20. The paper that documented the pattern, published just last year, had this to say about it:
We previously discovered a missense variant… but did not immediately recognize its functional role… With sequence and phenotype data of more than 200 horses, we now realized that this variant actually appears to have a subtle effect on pigmentation and consequently termed it W20. This variant is common and segregates in many horse breeds.
The paper goes on to explain that of the 145 horses tested, 52 had the W20 allele. According to the supplementary material, the breeds where it was found included:
German Riding Pony
South German Draft
That is an impressively broad range of breeds. Along with the Thoroughbred and warmblood breeds, there is a mountain and moorland pony (Welsh), a number of drafts including one of the older ones (Noriker), a colonial Spanish breed (Paso Peruano), and even the Marwari. Taking that into account, and then looking at the known examples of W20, it is tempting to wonder if this might in fact be the mutation most often responsible for the common “flashy white” type of sabino. Those are the sabinos with blazes, stockings and perhaps some white on the belly. It is also the type of pattern thought to amplify the white in other patterns, producing what is sometimes called “sabino boost”.
When the Sabino1 test became available, there was some disappointment that most horses thought of as sabino tested negative. Unlike the Splashed White patterns, which have one common and widespread mutation (SW1), there has not yet been a widespread testable form of sabino patterning. Looking at the list of breeds above, it does seem that if W20 is not the ”flashy white” sabino gene, it is at least one of them. If that is the case, garden variety sabinos like the Shire at the top of this post, or the Paint Horse above and Arabian below, might carry it.
There are still a lot of questions about W20. The original paper describes horses with the mutation as having “extended white markings”. From photographs of the known individuals with the mutation, it appears that homozygous W20 horses are more likely to have belly spots, but perhaps not much more than that. That does fit with what is known about breeding programs that center around horses with flashy white markings, and the relative rarity of loudly marked individuals in those lines. It is also clear that W20 interacts with at least one other White-spotting pattern (W5) to produce an all-white horse. What is not known is what it does with other patterns, both other White-spotting variants and unrelated white patterns. If the mutation is present in the Clydesdale, for instance, then what makes white-born foals so rare there? Does it play a role in horses with more extensive patterning or roaning? What is the maximum (and minimum) expression for W20, both when heterozygous and when homozygous?
These kinds of questions are important. For breeders, a better understanding of the way these patterns interact would make it easier to produce the “blaze and white socks” image that many find appealing, without getting excessive white on the body. Likewise a breeder of pintos might need to know how to ensure that enough white fell on the body to obtain regular papers.
That is why I am encouraging anyone interested in pinto patterning to participate in the W20 Project. The project is to raise funds as well as collect samples for a more comprehensive study of the W20 mutation. To participate, owners pay $38 – a fee lower than most commercial tests – and submit 2 samples (30 hairs each) for each horse they want tested. Participants will also need to send three pictures (both sides and a head shot showing any markings) and a copy of the horse’s pedigree. Horses with white patterning as well as those without are wanted. Results will be available in June of this year. More information, including information on where to send samples, can be found by clicking the link below, which takes you to the W20 Project page. There is, however, only a brief window for getting involved as all funds and samples must be submitted by April 15, 2014.
[Thank you, Jackie, for letting me know that the lovely heavy horse at the top of the post was actually a Shire, and not a Clydesdale!]