Spotting the Pattern: A Review on White Coat Color in the Domestic Horse

Abstract

Traits such as shape, size, and color often influence the economic and sentimental value of a horse. Around the world, horses are bred and prized for the colors and markings that make their unique coat patterns stand out from the crowd. The underlying genetic mechanisms determining the color of a horse's coat can vary greatly in their complexity. For example, only two genetic markers are used to determine a horse's base coat color, whereas over 50 genetic variations have been discovered to cause white patterning in horses. Some of these white-causing mutations are benign and beautiful, while others have a notable impact on horse health. Negative effects range from slightly more innocuous defects, like deafness, to more pernicious defects, such as the lethal developmental defect incurred when a horse inherits two copies of the Lethal White Overo allele. In this review, we explore, in detail, the etiology of white spotting and its overall effect on the domestic horse to Spot the Pattern of these beautiful (and sometimes dangerous) white mutations.

Keywords: coat color; depigmentation; horse; mutation; white spotting.

Figure 1

Various phenotypes caused by variants at the W locus, including full depigmentation, a sabino-like pattern, and minimal markings. Coat color genotypes and breeds are as follows: (A) a/a E/e (black) W13/n, Friesian-American White Horse cross. (B) A/a E/e Cr/n (Buckskin). (C) Foal—A/A E/E (bay) W15/15, Arabian, Dam—A/A E/e (bay) W15/n, Arabian.

Figure 2

Varying degrees of the tobiano phenotype in the Hucul horse. The horse in (A) displays cryptic tobiano markings that can be confused with a solid horse with white markings despite carrying a TO variant. The horse in (B) shows minimal tobiano markings. The horse in (C) displays classic tobiano. All horses possess at least one TO variant. All horses display their base coat color as well as white markings. Figure adapted from [24]. Photographs by M. Pasternak.

Figure 3

Incomplete dominance of the sabino trait. The American Saddlebred horse in (A) genotypes as SB1/sb1 and displays white socks, a blaze, and white spots stretching across the body while the Paint horse foal in (B) genotypes as SB1/SB1 and displays an all-white phenotype. Pink skin is exposed on the faces of both individuals.

Figure 4

(A) Chestnut Roan Quarter horse with a mixture of white and chestnut hairs over the body (B) Bay Roan Quarter horse with a mixture of white and dark hairs over the body. The head and lower legs are solid in color for both horses.

Figure 5

Common splashed white markings. (A) Paint horse with SW7 displaying typical splashed markings on the face, legs, and body and blue eyes. This individual was reported as deaf and possesses one Cream variant diluting the coat color. Image adapted from [48]. (B) Pura Raza Española horse with SW9 displaying blue eyes, four white limbs, and a white face. (C) Franches-Montagnes horse with the Macchiato variant, which causes a phenotype similar to splashed white and a coat color dilution similar to Cream. This individual was reported to not carry any coat dilutions other than Macchiato. Image adapted from [44].

Figure 6

Phenotypes of horses with EDXW1-3. (A) Shire horse homozygous for both EDXW1 and EDXW3 displaying white spotting with white spots on the abdomen and blue eyes. (B) A Gypsy Vanner horse heterozygous for EDXW2 showing a similar, but less extensive, phenotype as Horse A. There is slight depigmentation below the back quarter. (C) Shire horse heterozygous for EDXW3 displaying white spotting on the abdomen, legs, and face. The horses pictured possess no white-causing variants except for the alleles explicitly mentioned.

Figure 7

Common phenotypes of Paint horses with Lethal White Overo (LWO). (A) LWO/n individual displays a blue eye and white blaze (photograph by M. Simmons). (B) Jagged body marks as a result of one copy of LWO. (C) All white homozygous LWO foal and a heterozygous LWO mare. The photographs in (B,C) were adapted from [57]).

Figure 8

Phenotypes of Appaloosa horses attributed to LP and PATN1. (A) LP/lp patn1/patn1 individual with a depigmented hind quarter. (B) LP/LP patn1/patn1 individual with a depigmented flank and the few spot pattern. (C) LP/lp PATN1/patn1 individual with full leopard spotting. (D) LP/LP PATN1/PATN1 individual that is fully white with no other white spotting alleles.