This post gives an overview of this complex group of patterns and gives a brief explanation of how they have been named.
Perhaps no other pattern has captured my interest over the years as much as sabino. As an artist, I always gravitated towards patterns with a lot of detail, so it is not surprising that phenotypes like the one in the detail shot above (and shown in full below) would be very appealing. Sabinos were not just complex from an artistic standpoint; the pattern of their inheritance seemed equally complex. My proximity to the registry for Tennessee Walking Horses – and their willingness to let me spend hours poking through their records – gave me access to a lot of data. It was a puzzle that filled the pages of my research notebooks.
Although I have tended to focus on other areas in recent years, the patterns in this group continue to hold my interest. I hope to post more, and update some of my older posts, on the subject in the future. But for now, I do want to explain the terminology that will be used here.
Naming this group of patterns has always been problematic. Much of what is now known about them was not clear until recently, and new discoveries have complicated the situation. But it is also true that this has become a hot-button topic almost anywhere horse color is discussed online. Readers would not need to go far to find someone insisting that this or that term is “no longer correct” or that using it is wrong or “dishonest.” Preferred terminology is almost always presented as “scientifically correct,” even when it does not necessarily reflect the way the term is used in scientific publications.
For simplicity’s sake, I am going to explain how some of these terms will be used here on Equine Tapestry. Because I know this may generate some controversy in communities where different usages have been promoted, each of the following headers links to a page with passages from recently peer-reviewed publications. I will ask that those with questions read the linked pages and the quotes to get a sense of how these terms are commonly used by leading scientists in the field. That said, scientific terminology is less absolute than it is often portrayed and it is not my intent to replace one absolutist approach with another. Think of this as a translation guide for those more familiar with other approaches to talking about this complex topic. (My focus on communication over correctness is explained here.)
Readers who have spent any time in online horse color communities are probably aware that all manner of terms are considered “inappropriate” or are “no longer in use” because they are generic. Having written about horse color for decades now, it is my belief that contrary to this position, generic terms are an essential tool for talking about genetics. Generic terms allow us to speak about related groups of colors or patterns. They also allow us to talk about those patterns that, due to phenotype or pattern of inheritance, can be assumed to be related to other well-documented patterns. A full complement of generic and specific terms are needed – especially if someone is writing longer articles or books.
If terms for white patterns were ranked in terms of most generic to most specific, this is the top of the generic list. In genetics, white spotting is “the absence of melanocytes (pigment cells) from part or all of the locations in the body where they are normally found”. In horses, white spotting is a term often used to talk about sabino phenotypes. It is also useful as a clarification for the “dominant white” mutations that produce pinto phenotypes (as opposed to all-white) since “W” can mean either “white” or “white spotted”. However, the term is not actually unique to the W mutations and is regularly used in its broadest sense in peer-reviewed papers.
This term is used in the earliest published papers about patterns that are now known to be part of this group. The letter used to designate many of these patterns (W) comes from those papers. Although claims that this term is no longer in use are common online, that is not accurate. It is not a term I tend to use much, but it is not wrong.
My preference when speaking of this group of patterns is “sabino.” I believe that sabino, the traditional term for them, is useful because it allows these diverse phenotypes to be spoken of as a group. It also does not demand a categorical decision about issues that currently may be unclear, like the possibility of homozygotes. In my own writings (which reflect the way I keep research notes) I also use a handful of phenotypic subcategories. Four of those – all-white, near-white, sabino roan, patchy sabino – are pictured above. I’ll post more about those, and the one not pictured (bragada), in the near future.
There is an important distinction, however, between this use of the term and the names given to specific mutations. At this time, only one mutation that gives a sabino phenotype carries the word in its formal name. That is Sabino-1. The majority of the remaining sabino phenotypes (but not all) use White followed by a number. To keep confusion to a minimum, on this blog descriptive words for phenotypes appear without capitalization and in regular font. Specific mutations are capitalized and (when appropriate) numbered. Where possible using this blog’s style sheet, specific mutation names will also be italicized. This convention applies to adjectives for common traits – “roan hairs” versus the Roan gene – as well.
The important stuff
The situation with this group of patterns is complicated, and the terminology for them reflects that. What I have posted here is intended as a guide for readers of this blog. I have tried to align myself with the way the subject is discussed in peer-reviewed papers, or at the least speak in a way that does not create confusing contradictions. But as I stated in the guide for commenting, I am not a language purist. I am comfortable with the idea that I, and readers, may need to clarify and translate a bit as we muddle through communicating about this topic.
As with all aspects of horse color, what is important is the bigger picture of what is happening with these patterns. In the fourth edition of Equine Color Genetics, Phillip Sponenberg sums up the situation with these patterns best.
Sabino and white are therefore most accurately seen as a group of patterns rather than a single one, but the separate types have overlapping phenotypes that defeat any attempt to neatly separate them out from one another. Sabino and white are genetically related, so any discussion of them goes best when they are considered together.
That is how this blog will approach the patterns that produce a sabino phenotype, rather than focus on terminology. What has been learned about these patterns, and what is still left to discover, is far more interesting than what we call them.