Tiger eye orgoat eye is a gene causing diluted eye color in horses. There are two variants, Tiger-eye 1 (TE1) and Tiger-eye 2 (TE2), which are bothrecessive.[1] Horses displaying tiger eye typically have a yellow, orange, or amberiris. Tiger eye has only been found inPuerto Rican Paso Fino horses. Horses of related breeds were tested (90Colombian Pasos, 20Mangalargas, 44Lusitanos, and 42Andalusian horses), and none were found to have either tiger eye allele. No obvious link between eye shade and coat color was seen, making this the first studied gene in horses to affect eye color but not coat color. Tiger eye does not appear to affect vision, and there were no signs of reduced pigment on theretina orretinal pigment epithelium.[2]
Thegene involved codes forSLC24A5, a solute carrier known to be involved in pigmentation in other species.SLC24A5 is found on equine chromosome 1 base pairs 141,657,837–141,678,329 and theprotein is a potassium-dependent sodium–calcium ion exchanger involved inmelanocyte maturation. The protein is believed to be located in thetrans-golgi network of melanocytes. Tiger-eye 1 is a missense mutation (c.272A>T and p.Phe91Tyr) in which a singleadenine is replaced with athymine inexon 2, changing aphenylalanine to atyrosine in the resulting protein. Tiger-eye 2 is a deletion (c.875-340_1081+82del) in which the entirety of exon 7, and a bit of theintrons on either side, are removed, resulting in a protein that is 69 amino acids shorter. Both mutations are predicted to be deleterious to protein function.[2][3]
Another mutation at this same gene is associated with the shade of black horses. One variant is mainly found in jet black horses, while the other variant is mainly found in fading black horses. However, the association is not perfect and there are probably other mutations that can also cause the fading black appearance.[4]
SLC24A5 is involved in pigmentation in humans, mice, and zebrafish. In mice a targeted mutation was found which diluted the eye color without visibly affecting coat color, though a closer examination found that themelanosomes were smaller and paler than thewild type.[5] Unlike tiger eye in horses, the mice showed reduced pigment in the retinal pigment epithelium. In humans a widespread mutation to the homologous gene plays a large role in the light skin color of European humans,[6] and another mutation can cause oculocutaneous albinism (OCA) type 6 (OCA6), which impairs vision. No vision impairment is seen in horses, and the coat color is not visibly affected.[2]
| Base coat colors |
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|---|---|---|---|---|---|---|---|---|---|---|---|
| Gray | |||||||||||
| Dilution genes |
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| White | |||||||||||
Horse markings and patterns |
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| Other | |||||||||||
| Genetics and breeding | |||||||||||