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Tiger Eye

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Tiger eye is a dilute iris color found in Puerto Rican Paso Fino Horses. In contrast to the brown-colored eyes of most horses, “tiger eye” is characterized by a yellow, amber, or bright orange color. Researchers at the Veterinary Genetics Laboratory investigated the genetic basis of this phenotype and identified two variants responsible for tiger eye, named Tiger eye 1 (TE1) and Tiger eye 2 (TE2). The tiger eye phenotype is inherited as a recessive trait. Horses with tiger eye are most frequently homozygous (two copies) for the TE1 variant. Some tiger eye horses are compound heterozygous (one copy each) for both variants (TE1/TE2). Horses with the genotype TE2/TE2 are rare; the one documented case had a very light yellow/blue iris color. Brown-eyed horses can carry one copy of either TE1 or TE2 but they will also have one copy of the normal gene. Unlike the iris color dilution associated with the cream and champagne mutations, there does not appear to be a connection between tiger eye and dilute coat pigmentation. The tiger eye phenotype has been seen on all three non-dilute base coat colors (black, bay, and chestnut) and in both males and females. While to date TE1 and TE2 have only been detected in Puerto Rican Paso Finos, it is possible that these variants may explain lighter eye color in closely related breeds. 

The VGL offers a DNA test for the tiger eye iris color dilution. Since brown-eyed horses can carry either TE1 or TE2, owners breeding for or against this unique eye color can use this diagnostic tool to inform mating decisions.

Tiger Eye results are reported as:

N/N No copies of the Tiger Eye variants
N/TE1 Carrier of the Tiger Eye 1 variant.
TE1/TE1 2 copies of the Tiger eye 1 variant, horse should display a lighter eye color.
N/TE2 Carrier of the Tiger Eye 2 variant.
TE2/TE2 2 copies of the Tiger eye 2 variant horse should display a lighter eye color.
TE1/TE2 1 copy of Tiger eye variant 1 and 1 copy of Tiger eye 2 variant, horse should display a lighter eye color

References:
Kowalski, E.J.A. & Bellone, R.R. (2011) Investigation of HERC2 and OCA2 SNP for iris color variation in Puerto Rican Paso Fino horses. Journal of Equine Veterinary Science 31(5):319.

Mack, M, Kowalski, E., Grahn, R.A., Bras, D., Penedo M.C.T., and Bellone, R.  Two Mutations in Solute Carrier Family 24 Member 5 Are Associated with "Tiger Eye" Iris Pigmentation in Puerto Rican Paso Fino Horses. Proceedings from the Plant and Animal Genome Conference XXV. San Digeo, CA, January 2017.

 
Veterinary Genetics Laboratory, Tel 530-752-2211, Email VGL