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.

Population Analysis Identifies 15 Multi-Variant Dominant White Haplotypes in Horses

“…For melanogenesis, the KIT protein phosphorylates Melanocyte Inducing Transcription Factor (MITF), which upregulates pigment-related genes in its phosphorylated state [1,2]. Loss-of-function KIT mutations result in hypopigmentation in mice, humans and horses, likely due to interruptions in the migratory signaling [2][3][4][5][6]. Mutations can also affect other KIT protein pathways, leading to embryonic death, sterility, growth issues and immunodeficiencies [5].…”

“…Over 35 mutations associated with or causing depigmentation were previously identified within equine KIT [3][4][5][6][7][8][9][10][11][12][13][14]. These include Sabino 1 (SB1) and the Dominant White series of alleles (W1-W35) which are a combination of coding and non-coding mutations within KIT.…”

“…These include Sabino 1 (SB1) and the Dominant White series of alleles (W1-W35) which are a combination of coding and non-coding mutations within KIT. Many Dominant White variants arose recently and have known founders [4]. Most of these alleles are predicted to affect the expression of the wild-type protein or produce a less active KIT protein and are inherited in an autosomal dominant or incomplete dominant manner [3][4][5][6][7][8][9][10][11][12][13][14].…”

“…Many Dominant White variants arose recently and have known founders [4]. Most of these alleles are predicted to affect the expression of the wild-type protein or produce a less active KIT protein and are inherited in an autosomal dominant or incomplete dominant manner [3][4][5][6][7][8][9][10][11][12][13][14]. The Tobiano (TO) allele is a large paracentric inversion outside of KIT thought to interrupt its regulation, resulting in large white areas on the coat [4].…”

“…Most of these alleles are predicted to affect the expression of the wild-type protein or produce a less active KIT protein and are inherited in an autosomal dominant or incomplete dominant manner [3][4][5][6][7][8][9][10][11][12][13][14]. The Tobiano (TO) allele is a large paracentric inversion outside of KIT thought to interrupt its regulation, resulting in large white areas on the coat [4]. Individuals of diverse breeds including Paint Horses and Gypsy Cobs were observed carrying both the TO and W20 or SB1 alleles [15].…”