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Sunday, January 15, 2017



 THE TOBIANO PAINTS


Disclaimer: The mustang photographs on this blog post are presented without genetic testing; we do not know the actual chromosomal make-up of the mustangs. We rely solely upon the horse’s phenotype, or how they appear physically: coat colour, white markings, eye color, mane, and tail colour.

Mammalian Pigmentation

The colors found in mammalian hair, skin, irises, and some internal organs is produced by the pigment melanin. Melanin appears as colored granules in these pigmented cells and occurs in two forms, eumelanin, and phaeomelanin. Eumelanin is responsible for brown and black colour, and phaeomelanin is responsible for reds and yellows (Bailey & Brooks, 2013).

Black tobiano (muddy) (Black Hills Wild Horse SanctuarySouth Dakota) © Equus ferus- Wild Horse Photography ™ © Karen McLain

The absence of melanin will appear as white in mammals. The lack of color typically associated with Paint Horses is caused by the inability of these cells to produce the base colors from birth. The color loss results in large patches of white against a base coat of any colour (bay, chestnut, roan, grey, dun, champagne, silver dapple, palomino, brown, black, etc). Additionally, the white colour has pink skin beneath. 

Assorted tobiano horses (Black Hills Wild Horse SanctuarySouth Dakota) © Equus ferus- Wild Horse Photography ™ © Karen McLain

There are other forms of white colouration in equines; for example gray or roan. Grey horse color is caused by the failure of melanocytes over time, so the hair starts normally pigmented but loses the ability to maintain the pigment so the horse eventually turns white(Sponenberg, 2009). Roan horses are roan from birth although they are often not recognizable until after the foal coat has shed. Roan horses typically retain the base color on their head, legs, mane, and tail. The skin beneath grey and roan horses is dark and these colors are not actual colours, but rather modifiers that act upon a base coat (Gower, 2016). 
Black tobiano demonstrating the "shield" (Black Hills Wild Horse Sanctuary, South Dakota) © Equus ferus Wild Horse Photography © Karen McLain 

A tobiano horse may also be roan, appaloosa, sabinos, overo, or any other pattern as the tobiano pattern is not a mutually exclusive coat patterns.

Bay Roan tobiano   (Great Desert Basin, Utah) © Equus ferus- Wild Horse Photography ™ © Karen McLain

Roan tobiano foal, palomino tobiano mare  (Great Desert Basin, Utah) © Equus ferus- Wild Horse Photography ™ © Karen McLain

Equine chromosomes
The genes for four white coat patterns: roan (RN), sabinos (SB1), dominant white (W), and tobianos (TO) are located on the KIT gene.The KIT gene is responsible for sending instructions through cells that allow the cell to make specific proteins. The KIT proteins are found on the cell membrane where another protein called a “stem cell factor” binds to the KIT protein. When bound together, they activate the KIT protein, which in turn, activates other proteins within the cell. These proteins serve a variety of functions in mammalian cells such as growth, development, migration, and production of certain cell types such as interstitial gastrointestinal cells and melanocytes (Haase, Jude, Brooks, & Leeb, 2008).


Equine Chromosome #3
The KIT gene is located on the fourth chromosome at the 12 position and located very close is the ECA3 gene. The ECA3, or the third equine chromosome is the location of the chromosomal mutation responsible for tobianos. Although the KIT gene remains normal in these horses, the third chromosome has an area in the gene that has flipped. Approximately one-third the length of the chromosome is an area that is an exact mirror image in tobianos horses. Because the chromosomal inversion is adjacent to the KIT gene, it affects the KIT protein synthesis, and the cells cannot produce melanocytes- so the horse has areas of white. The test for tobianos examines the chromosome and looks for a ‘break’ (telomeric or centromeric) at the positions 13 and 21 on the third chromosome– this serves as an indication they separated and inverted during replication- the horse is genetically a tobianos (Bailey & Brooks, 2009).

The gene for tobiano horses is autosomal dominant. This means to be tobiano, a foal must have at least one tobiano parent, but they also may have two tobiano parents. If one parent is a tobiano, it does not matter what colour or pattern the other parent appears; the foal will be tobiano. If a horse matches the criteria for a tobiano, it is likely the horse has the genetic background of a tobiano, although there may be some mixing of other patterns (Gower, 2016). The patterns are not inherited exactly, however, the proportion of white to colour is inherited. In other words a horse with a lot of white will have offspring with a lot of white but this depends upon the other parent. Interestingly, the study by Woolf (1990) discovered male horses and those with chestnut coats have more white than female or bay horses. The researcher also noted that the inheritance of white leg markings and facial markings is multifactorial; there are many genes involved in the appearance of white markings (Woolf, 1990).

History:

Bay tobiano mare  (McCullough Peaks, Wyoming) © Equus ferus- Wild Horse Photography ™ © Karen McLain


The Tobiano paint pattern was named for General Tobías from Brazil. The General brought the paint horses to Argentina in the mid-1800’s. Before their arrival, tobiano horses were rare and had been grouped with other spotted-type horses. After General Tobías’ arrival, they were renamed after the general and placed into a unique paint coat classification(Kerson, 2015; Sponenberg, 2009)

Tobiano Characteristics:
Bay tobiano mare (McCullough Peaks, Wyoming) © Equus ferus- Wild Horse Photography ™ © Karen McLain
The tobiano is defined by several coat characteristics. As with all horses, unless genetically tested, we evaluate the coat pattern by the phenotype or the horses’ physical appearance. As a general rule, tobiano horses have the following characteristics (there are always exceptions to these rules):

  1. White cross the spine somewhere between the ears and the tail (Gower, 2016; Sponenberg, 2009)
  2. The body white appears to travel down in a vertical fashion (Gower, 2016)
  3. The edges of the white areas tend to be crisp and well-defined (Sponenberg, 2009)
  4. Legs are white and the edge of the socks/stockings is irregular (Gower, 2016; Sponenberg, 2009)
  5. Most tobiano have dark eyes although some tobiano horses have blue eyes  (Sponenberg, 2009)
  6.  Most tobianos have white areas within the mane and tail, this gives the appearance of a bicoloured tail, a trait usually seen only in tobiano horses. (Sponenberg, 2009)
  7. The predominantly solid coloured heads of tobianos are generally conservatively marked: thin blazes, simple stars (Sponenberg, 2009)
 Some tobianos have very little white
Black tobiano (McCullough Peaks, Wyoming) © Equus ferus- Wild Horse Photography ™ © Karen McLain



Minimally marked black tobiano stallion -note the bicoloured tail (McCullough Peaks, Wyoming) © Equus ferus- Wild Horse Photography ™ © Karen McLain


Bay tobiano stallion (McCullough Peaks, Wyoming) © Equus ferus- Wild Horse Photography ™ © Karen McLain

Black tobiano stallion (McCullough Peaks, Wyoming) © Equus ferus- Wild Horse Photography ™ © Karen McLain

Bay tobiano stallion (McCullough Peaks, Wyoming) © Equus ferus- Wild Horse Photography ™ © Karen McLain

Some tobiano horses have very little base colour but they tend retain normally coloured heads even when extensively white.


Minimally marked light bay chestnut tobiano (Black Hills Wild Horse SanctuarySouth Dakota) © Equus ferus- Wild Horse Photography ™ © Karen McLain


Light bay tobiano (note the minimal blaze) (Black Hills Wild Horse SanctuarySouth Dakota) © Equus ferus- Wild Horse Photography ™ © Karen McLain

Extensively white black marked tobiano (Black Hills Wild Horse SanctuarySouth Dakota) © Equus ferus- Wild Horse Photography ™ © Karen McLain
Some tobiano horses have marks within the white areas and there may be some bleeding of colour between the base colour and the white areas. The smaller spots in the white areas are called ink spots, bear tracks, cat’s paws. The areas of darker colour encroaching on the white areas are referred to as halos. There may also be some roaning at the edge of colour and white.

This is a historical link between these markings and homozygosity. No genetic link has been found, however anecdotally, homozygous horses often present with these marking whilst heterozygous generally do not show these markings. 

Cat's paws & halo effect (San Wash Basin, Colorado) © Equus ferus- Wild Horse Photography ™ © Karen McLain


References

Bailey, E., & Brooks, S. (2009). Method for screening for a tobiano coat color genotype  #USPatent 8101354 B2.

Bailey, E., & Brooks, S. (2013). Horse Genetics (2nd ed.). Boston, Massachusetts: CABI.

Gower, J. (2016). Horse Color Explained: A Breeder's Perspective. Brattleboro, Vermont: Echo Point Books & Media, Inc.

Haase, B., Jude, R., Brooks, S. A., & Leeb, T. (2008). An equine chromosome 3 inversion is associated with the tobiano spotting pattern in German horse breeds. 
Animal Genetics, 39(3), 306-309. doi:10.1111/j.1365-2052.2008.01715.x

Kerson, N. (2015). What Color is that?  A quick guide to horse color identification: Nancy Kerson- Self Published

Sponenberg, D. (2009). Equine Color Genetics (3rd ed.). Ames, Iowa: Wiley-Blackwell.

Woolf, C. M. (1990). Multifactorial inheritance of common white markings in the Arabian horse. J Hered, 81(4), 250-256.
A special thanks to Nancy Kerson for her brilliant book "What Color is that? A quick guide to horse color identification" and to the Black Hills Wild Horse Sanctuary. A worth sanctuary for wild horses.

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