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It’s all “White Spotting”

In genetics, mice are used as a model species for mammals. This is how white spotting is defined in The Colors of Mice, A Model Genetic Network (L. Lamoreux, 2010).

White spotting is defined as the congenital absence of pigment cells from portions of the body or from the entire body

Although terminology use can vary from the model species, in the case of horses, “white spotting” is used in the same way. The following quote is from “Impact of white spotting alleles, including W20, on phenotype in the American Paint Horse” (Brooks, S. A., et al., June 2019). (Emphasis is mine)

Genetic studies previously defined sequence variants corresponding to 35 alleles for white spotting in the horse. Here, we calculate the allele frequency for nine common white spotting alleles in the American Paint horse using a 32 sample of 1,054 registered animals.

The nine white spotting alleles in the study were frame overo, tobiano, Sabino-1, three W alleles (5, 10, 20), and three splashed white alleles (1, 2, 3). A figure that includes tobianos and frame overos talks about the “diverse white spotting phenotypes” in the breed and goes on to apply the term when talking about the frame overos.

It is not just the pinto patterns that are referred to as white spotting patterns. The term is used for appaloosa patterns as well, and some papers have referenced roan in this way. When pigmentation is viewed as a biological process, this does make sense, since these colors all fall under the “congenital absence of pigment cells.” This is from the most current edition of Horse Genetics (E. Bailey and S.A. Brooks, 2013). (Again, the emphasis is mine)

White spotting patterns such as tobiano, overo and appaloosa may not be readily visible as a coat pattern in a mature grey horse, but can be seen as pink skin patterns, particularly when the hair is wet.

That is one passage, but the usage is consistent throughout the text and figures. The usage is not unique to those particular authors. These quotes are from the paper identifying the mutation for Leopard Complex (Lp), “Evidence for a Retroviral Insertion in TRPM1 as the Cause of Congenital Stationary Night Blindness and Leopard Complex Spotting in the Horse” (R. Bellone, et al., 2013)

Leopard complex spotting (LP) is found in several breeds of horse and is characterized by the absence of pigment (white spotting) in the coat. These white spotting patterns tend to be symmetrical and centered over the hips.

Leopard complex spotting is a group of white spotting patterns in horses caused by an incompletely dominant gene (LP) where homozygotes (LP/LP) are also affected with congenital stationary night blindness.

Specifically, horses homozygous for a white spotting phenotype, known as leopard complex spotting, are affected by CSNB.

My goal with sharing these quotes was to give a sense of how the term “white spotting” is used by those directly involved in horse color research. I pulled quotes based on clarity and length, but I would encourage anyone interested (or skeptical) to read the full text of these publications to get a better sense of the context.

I also want to be clear that while I have highlighted how the term is used to refer to other white patterns, this is not meant to imply that the term is somehow incorrect when speaking of sabino phenotypes. White spotting is a particularly useful term when talking about white patterns that involve a lot of unknowns because it is correct no matter how the pattern is ultimately categorized. It is also helpful when talking about patterns in the W series which have a sabino (rather than white) phenotype because the letter can stand for either white or white spotted. What is problematic is the claims by some online speakers that “white spotting” is somehow the only proper term for sabino patterns, and that it replaces others which are “no longer correct.” That is not consistent with the published literature.