As the previous posts on sabino show, my ideas about this group of patterns have evolved over the last three decades. The previous posts have been long, so I want to do a brief overview.
A large group of related patterns
My current understanding of sabino aligns with Sponenberg and Bellone, who have stated that this is a group of patterns with “overlapping phenotypes that defeat any attempt to neatly separate them out from one another.” I find it helpful to note the visual categories and how they fit within the group, but I would also stress that I see these as extremely loose guidelines. There is some degree of predictability, but it is more accurate to speak of tendencies rather than rules when it comes to these patterns. (See The Generic Sabino for more information on the Sponenberg/Bellone quote, along with others.)
The large-effect alleles
These are the mutations that result in significant amounts of white patterning. Because these phenotypes are striking – it is hard to miss when a foal like this appears unexpectedly! – most of the testable patterns in the sabino group fall into this category. The range of phenotypes for this group is shown below.
The pictures above represent the four visual categories for large-effect sabino alleles. They are (clockwise, starting at the top left) all-white, near-white, ticked sabino, and patchy sabino. The boundaries between them are not absolute, but the average expression of a single copy of most of the large-effect alleles are going to tend towards one of the four, or two adjacent categories (say, white and near-white).
Twenty-eight sabino alleles of this type have been formally identified. That includes Sabin0-1, W1-W19, and W21-28. There are likely more patterns in this group not yet identified.
The smaller-effect alleles
In their 2005 paper identifying the mutation for Sabino-1, Brooks and Bailey defined sabino patterning as meeting three of four traits: blaze, two or more white feet, spots or roaning on the midsection and ragged white edges. The alleles in this group result in less extensive white patterning than the previous group but still fulfill the Brooks and Bailey definition.
This is where the bragada phenotype from the previous post fits in the picture. The testable pattern that would fit in this group is W20, but there are likely more alleles in this category that cannot yet be detected by testing. The dividing line between the more modest patterns like bragada and the less extensive patchy sabinos is unclear, and some alleles may have an average that sits between the two.
The patterns in this group can be thought of as having a dosage effect with two possible outcomes: early embryonic loss or a viable foal that is white or near-white. It is not known for certain which alleles (or combinations of alleles) give the first outcome. Based on the combinations that have produced viable foals, and the historical records, it seems patterns, where the heterozygotes are white or nearly white, are more likely to be lethal.
The second outcome was initially documented in horses who were homozygous for the Sabino-1 allele. Another case of a white homozygote was noted with one of the patchy sabino alleles, W15. Compound heterozygotes – horses carrying two different alleles from this group – have also been white or nearly white. This was first noted in horses that carried both W5 and W20, but other combinations appear to have a similar effect. In this respect, some of the sabino alleles behave much like those at the Cream locus, where different allele combinations (cream/cream, cream/pearl, cream/snowdrop) have visually similar outcomes.
This second outcome is important to highlight because, in breeds where frame overo (LWOS) is a possibility, an all-white foal could be mistaken for a lethal white and needlessly euthanized. This is relevant not only for Sabino-1, where the homozygotes are known to be white but also for as-yet-untested combinations.
Some added complications
The situation with this group of patterns is already complex, but there are two additional points worth noting. The first is that the gene for the sabino patterns, KIT, is situated close to Tobiano. It is also believed to be the site for the roan and white ticking alleles. The interaction between sabino and tobiano, and the similarities between the sabino and the roan and white ticked patterns, are subjects I hope to cover in future posts.
The other point I want to stress is that what is true for the sabino group applies to white patterning in horses more broadly. The situation with white patterns is complicated; they overlap and they interact with one another. It is possible to speak in general terms about broad trends, but they rarely fit into neat boxes. This post was about those broad trends, but I will talk about outliers and phenotypic overlap in a future post.