Normal Facial and Leg Markings

The Genetics Behind Pure Thoroughbred Colors

It's been hypothesized that non-solid horses (such as the examples above with minimal white) contain a recessive gene (NnNn) which allows for the depigmented areas of white. It's believed that this gene in its dominant form (N+_) shows dominant epistasis over the alleles at the sabino gene (I'm not sure about other white genes, though I would assume it does the same to them - I do not know). Dominant epistasis is when "the dominant allele of one gene hides the expression of all alleles of another gene. For example, the albinism gene is dominant and hides the expression of the pigment in the eyes, skin, and hair." - Study.com Meaning in theory, N+ would cover up the Sabino gene of the specimen/not allow it to show despite the horse carrying Sb. 

Horse breeds that come to mind are Friesians, Norwegian Fjords, Sorraias, Przewalski, where the majority of horses of the breed are solid, with a few crop-outs displaying a small amount of white. The solid horses of these breeds are likely N+N+ or N+Nn; and when two N+Nn horses breed together, there is a chance for a crop-out NnNn that displays a small amount of white.

TS is NOT requiring the documentation of this gene in your horses genetics for registration. This info is here for fun and learning!

Disclaimer: Purebred Tbs do not carry Sabino, though this article is about Sabino horses:
    "The pattern of sabino horses is polygenic - that is, controlled by more than one gene. Two major genes have been hypothesized to work together to account for the occurrence of both normal white markings and the markings of sabino horses (Gower, 1999). One should remember that although the scheme proposed by Jeanette Gower is perfectly plausible it may turn out to be a simplified version of reality.
    In addition it is well recognised that several minor genes act together to affect the amount and placement of white in any particular horse, not just sabino horses. This character is also proven to be partly determined by environmental factors during a foals development before birth.
    The presence of any white (not just in sabino horses) is hypothesized to be recessive (NnNn), with the wild-type allele for no white being dominant (N+). The gene is also thought to show dominant epistasis over the alleles at the sabino gene. Thus any horse with the N+ allele is thought to be solid colored, without any white markings at all, regardless of its genotype at the sabino locus." - horse-genetics.com

Concerning Norwegian Fjord horses:
    "White or flesh-colored markings are very seldom seen on the Fjord Horse; but a white star on the forehead has existed as far back as we have written records. The white markings are inherited as a recessive gene and that both parents must have these genes for the offspring to exhibit visible markings." - Norwegian Fjord Horse Registry

Concerning Sorraia horses:
    "White markings usually occur as a by-product of domestication and inbreeding, the latter often being linked to domestication. The extreme inbreeding in today's Sorraias alone would explain the occurence of an occasional white marking. White markings occur in the Mongolian wild horse (Przewalski‘s horse) just as well." - Sorraia.org

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Lines (c) Sealle