Tiger eye or goat 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 both

recessive In genetics, dominance is the phenomenon of one variant ( allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant an ...

. Horses displaying tiger eye typically have a yellow, orange, or amber

iris Iris most often refers to: *Iris (anatomy), part of the eye *Iris (mythology), a Greek goddess * ''Iris'' (plant), a genus of flowering plants * Iris (color), an ambiguous color term Iris or IRIS may also refer to: Arts and media Fictional ent ...

. Tiger eye has only been found in Puerto Rican Paso Fino horses. Horses of related breeds were tested (90 Colombian Pasos, 20 Mangalargas, 44

Lusitano The Lusitano, also known as the Pure Blood Lusitano or PSL (''Puro Sangue Lusitano''), is a Portuguese horse breed. Horses were known to be present on the Iberian Peninsula as far back as 20,000 BC, and by 800 BC the region was renowned fo ...

s, and 42

Andalusian horse The Andalusian, also known as the Pure Spanish Horse or PRE (Spanish language literally translates to “Spanish pure breed”. This name is sometimes capitalized when used in English-language publications, but is all lower-case in Spanish, wh ...

s), 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 the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...

retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual ce ...

. *

Genotypes

Paso-Fino-Equitation-Manati2009 865- Platanera Dorada

* ''TE1/TE1'' or ''TE1/TE2'': Yellow, amber, or bright orange eyes. * ''TE2/TE2'': Blue eyes when in combination with

cream Cream is a dairy product composed of the higher-fat layer skimmed from the top of milk before homogenization. In un-homogenized milk, the fat, which is less dense, eventually rises to the top. In the industrial production of cream, this process ...

. It is not known what this looks like without cream. Only one horse has been tested to carry this genotype. * ''TE1/n'', ''TE2/n'', or ''n/n'': Dark eyes.

Molecular genetics

The

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

involved codes for

SLC24A5 SLC may refer to: Places * Salt Lake City, Utah * Salt Lake City International Airport, IATA Airport Code Education * Sarah Lawrence College, NY * School Leaving Certificate (Nepal) * St. Lawrence College, Ontario, Canada * Small Learning Com ...

, 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 the

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

is a potassium-dependent sodium–calcium ion exchanger involved in

melanocyte Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and hear ...

maturation. The protein is believed to be located in the ''trans''-golgi network of melanocytes. Tiger-eye 1 is a missense mutation (c.272A>T and p.Phe91Tyr) in which a single

adenine Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its deriv ...

is replaced with a

thymine Thymine () ( symbol T or Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidin ...

exon An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequen ...

2, changing a

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the formula . It can be viewed as a benzyl group substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of alanine. This essential amin ...

to a

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the G ...

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 the

intron An intron is any Nucleic acid sequence, nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of ...

s 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.

In other species

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 the

melanosome A melanosome is an organelle found in animal cells and is the site for synthesis, storage and transport of melanin, the most common light-absorbing pigment found in the animal kingdom. Melanosomes are responsible for color and photoprotection i ...

s were smaller and paler than the

wild type The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, "m ...

. 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, * 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.

References

External links

{{DEFAULTSORT:Tiger eye Eye color Horse coat colors Iris (anatomy) Single-nucleotide polymorphisms