Sunday, January 29, 2017

The Frame Overo

When I first sat down to go through my photographs, I was hoping to have five or six good examples of the Frame Overo trait. I was pleasantly surprised to find so many mustangs with this particular pattern. Interestingly, I found Frame Overo mustangs only at Sand Wash Basin and Black Hills Wild Horse Sanctuary. It is entirely possible there are some Frame Overos at other management areas, but neither Karen nor I had photographed any of them. The Frame Overo is also unique to North America. This paint pattern is common only in Spanish Colonial Horses or their descendants. However, the trait occasionally but rarely appears in European-derived horses such as Thoroughbreds, Ethiopian breeds, and miniature horses (Sponenberg, 2009).

The Frame Overo Paints are horses that have white patches superimposed on a background of any base colour such as roan, bay, palomino and so on. The white colour often begins as a patch on the neck or barrel and spreads horizontally. The back almost always remains solid coloured between the withers and tail. The white patches are irregularly edged and splashy (Bailey & Brooks, 2013) although some white markings are clean and crisp, similar to tobianos (Sponenberg, 2009). The markings usually do not have the lacy or frosted appearance of Sabino Paints (Kerson, 2015)

The tail is usually one colour and at least one leg is usually solid coloured although often all four legs are solid. If the legs are solid, they may have socks or stockings one might find on a non-paint horse. The markings on the head are often extensive, bonnet or apron-faces are common. Additionally, they may have a pigmented upper or lower lip, or 'moustache' (Sponenberg, 2009).

The genetics are simple, the Frame trait is autosomal dominant, which means a Frame foal must have one Frame parent. However, if both parents have the Frame gene, there is a 25% possibility the foal will be homozygous and have two Frame genes. This foal will not survive because a homozygous Frame is linked to another genetic defect which causes loss of peristalsis (the wave-like action that moves food through the intestines), or more rarely, an incomplete colon. (Bailey & Brooks,2013). More information and examples are located at the bottom of this page.
FF= Normal color
Ff-= Frame Overo
ff= Overo Lethal White

Perhaps one of the best-known Frame Overos is Picasso from the Sand Wash Basin. This handsome Bay horse is one of the most photographed mustangs and he has a Breyer Horse modelled after him. He is a Bay Frame Overo (and possibly carries other paint traits). Many of his numerous offspring are Frame Overos including two know Lethal White foals. There will be more information on this genetic anomaly presented below.


This is an excellent example of a Bay Frame Overo stallion.
Note the white markings on neck and barrel as well as the four solid coloured legs.
Sand Wash Basin, Colorado

The horse on the right is a Chestnut Frame Overo
Sand Wash Basin, Colorado

A small family band with a Sorrel Frame Overo Stallion and a Frame Overo foal. In the management ranges, we cannot be certain of a foal's parentage. Identifying a foal's dam is generally more reliable than the sire. However, mares have been known to steal foals from other mares. Additionally, fillies will leave their natal bands briefly, breed with another stallion, and return to their natal band to deliver and raise the foal within the band they were born. Without genetic testing, we cannot be certain.
Sand Wash Basin, Colorado

A Chestnut Frame Overo. The pigmented lip is common and may appear on the upper lip, lower lip, or both as this mustang mare demonstrates.
Sand Wash Basin, Colorado

The Chestnut Frame Overo is unusual because the right hind leg is extensively marked with white. This may be an anomalous finding, or an indication there are other paint genes present, possibly Sabino. The heavily white face and body markings are more characteristic of Frame Overos.
Sand Wash Basin, Colorado

This charmingly marked dark (liver) Chestnut Frame Overo mare is called 'Crazy Horse'. The freckles on her face are also known as Belton Spots and similar to the spotting found in English Setter dogs. Many horses with Belton Spots on their facial markings, also show "ermine spots" on their legs.  From Sand Wash Basin.

Although not perfectly focused, this Bay Frame Overo has Belton spots on his blaze as well as ermine spots (black spots on socks that are usually found along the coronet band). The ermine spots can cause the hoof to darken and appear striped. From Sand Wash Basin

These two beautiful sparring stallions are from Sand Wash. Kiowa on the left (Bay) and Haze on the right (Sorrel). Haze is a minimally marked Frame Overo- can you find the tiny white mark on Haze?

This is Miss Fleck- she is a Chestnut Frame Overo from Sand Wash Basin: the first image is her as a foal, the second image as a young mare. She was born into Voodoo's band a but later joined Picasso's band (2013). Picasso has since lost his band but he has been doing well as a bachelor. Fleck gave birth to a Lethal White Foal.
Fleck: foal, right side
Fleck: left side

This is Kiowa from the near side (left) from Sand Wash Basin. He had just been in a fight and you can see lacerations on his hip and shoulder. There is also a significant scar on Kiowa's left haunch just below the laceration. It is believed he tangled with either a mountain lion or a wound from fighting that became infected. It is also possible he ran into a fence along the border of Sand Wash Basin and neighbouring ranches.

This is Raindancer. A lovely Chestnut Frame Overo with blue eyes from Sand Wash Basin.

This is an excellent example of a Bay Frame Overo mare from Sand Wash Basin, Colorado with a pigmented lip, flank markings and a small shoulder marking.

Here is a lovely colt Named Van Gogh, he is one of Picasso's many Frame Overo offspring from Sand Wash Basin.

This is Yatzee on the left and a grey stallion on the right from Sand Wash Basin. The grey is actually a Frame Overo - if you look closely at his neck, you'll see the faint outline of the original white mark in the middle, just under his mane. He also has some white markings on his barrel. Some people refer to grey paints as "ghost paints". In the winter it is impossible to tell them apart from solid grey colour horses. A wet grey paint in a summer coat is the easiest to recognise because you can see the underlying skin colour: pink under white markings and darker under the base colour

Another "Ghost Paint- Frame Overo Grey from Sand Wash Basin. The horse on the left has several white Frame markings on his neck, barrel and just above his stifle. He also has the distinctive white apron face with the pigmented lip. The pink skin of the white marking is especially noticeable on this horse's muzzle. If you see a horse with a pigmented lip, there is a good chance they are a paint- often a Frame Overo. The middle horse is also a Frame Overo, this time black with four white stockings.  Four white stockings may indicate this mustang has other paint genes.  The last horse is a Bay Frame Overo with a usual number of white leg markings. Generally, the leg markings do not rise above the knees in Frame Overos.

This family band has two Frame Overos from Sand Wash Basin. The foal, a chestnut beginning to grey-out, and the sorrel stallion. Again, notice the solid coloured legs which help distinguish Frame Overos from Tobiano Paints.

A handsome Grullo Frame Overo gelding from Black Hills Wild Horse Sanctuary Black Hills is a wonderful rescue organisation, please click on the link to find out more

A Black Frame Overo from Black Hill  Wild Horse Sanctuary

Bay Frame Overo with a stunning pattern from Black Hill  Wild Horse Sanctuary. 

Two Black Frame Overos from Black Hill  Wild Horse Sanctuary. Note the tendency for Frame Overos to have solid colour on the spine from the withers to the dock.

Sugar, a Grulla Frame Overo from Sand Wash Basin, Colorado

This stallion is a Chestnut Frame Overo from Sand Wash Basin. It is evident he is thin but these horses receive no care. Many of the horses are underweight from untreated injuries, dental issue, and other injuries or infections that go untreated. It is truly survival of the fittest and only the strongest survive to reproduce. He was a very fiesty stallion, sparring with the bachelors and band stallions.

A dark chestnut Frame Overo Paint from Sand Wash Basin with very little white on his belly (the rest is dried mud)- referred to as a minimal Frame Overo. This stallion is called Spyder. 

A Minimal Dun Frame Overo Paint from Sand Wash Basin.

This beautiful grey colt is a minimally marked Frame Overo. All four legs are solid and there is no white mark on either side. According to Sponenberg (2013), this horse will sire horses with varying amounts of white. Becuase of the lethal white syndrome, any Frame Overo owner should test both horses prior to breeding to avoid the chance of a foal that will not survive. Sand Wash Basin, Colorado

A minimally marked Black Frame Overo from Sand Wash Basin.

A beautiful Black Frame Overo from Sand Wash Basin name Lightning. He was believed to be over 35 years of age at his death one year ago. He is one of the stallions responsible for the Frame Overo trait becoming so prevalent at Sand Wash Basin. He lived all his years free running amongst the Colorado sage.

A minimally marked Bay Frame Overo from Sand Wash Basin with very little white and four solid legs.

Another Minimal Frame Overo Chestnut. This is Mimi, she is a foal by Picasso. Sh has white socks but they are normal height one normally associates with solid coloured horses. The first is her right side, the second image, her left.

A lovely Palomino Frame Overo colt named Meteor. He has a lot of white on both hinds legs (the white goes up the front of the leg to the stifle) and this indicates there is probably another paint gene such as Sabino, is present. Sand Wash Basin, Colorado

Kokomo, a Bay stallion by Picasso and a minimally marked Chestnut Frame Overo mare. Although they are muddy, the mare has solid legs and Kokomo has both white legs marked with white including a thin strip almost reaching his chest. Although he is heavily marked with white, like his sire, they both have solid coloured backs. Sand Wash Basin, Colorado

A very Minimally Marked Frame Overo. The other side is also solid. However, there is a white spot on this stallion's tail evident by the light colour at the end. The facial marking (apron and pigmented lip) are good indicators of a Frame Overo. Sand Wash Basin, Colorado

Lethal White

The Frame Overo (Ff or Oo) trait is found on the equine chromosome #17 at the same locus (location on a chromosome) that controls EDNRB (Endothelin Receptor B) (Sponenberg, 20009). The change in the dinucleotide that occurs in Frame Overos changes an amino acid from isoleucine to lysine which disrupts the function of the EDNRB. In homozygous Frame Overo horses (Ff), the functional inability of Receptor Type B proteins (EDNRB) causes loss of gastric ganglia precursor cell migration and loss of melanocyte migration (Bailey & Brooks, 2013).

The loss of function in EDNRB prevents the embryologic migration of:
  • Gastric ganglia precursor cells from migrating, which means a loss of enervation in the digestive tracts. No nerves ending exist in the colon of these horses and function is completely disrupted. Rarely the loss of EDNRB function results in an incomplete colon (ileocolonic aganglionosis). Foals with either gastric malformation die within a few days of birth and it cannot be surgically corrected.
  • Melanocyte migration means loss of pigment, resulting in white colouration. 

In heterozygous horses, the presence of one "f" results in partial solid colour, but the digestive system is normal. In lethal white, the presence of two "ff" results in a pure white horse (no melanocyte migration) and a non-functional colon, or a blocked, atrophied, or dead-end colon. The loss of gastric enervation has a similar aetiology to Hirschsprung's Disease in humans. Overo Lethal White Syndrome is found in Frame Overo horses as well as highly white calico overo, and frame blend overo (>94%) (Santschi, Vrotsos, Purdy & Mickelson, 2001)

Mingo X Picasso
©Nancy Roberts

©Nancy Roberts

Picasso X Fleck
 ©Danielle M. Williams

 ©Danielle M. Williams

 ©Danielle M. Williams

Here is the link for more information on the Lethal White Syndrome including a handy Punnett Square


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.

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

Santschi, E. M., Vrotsos, P. D., Purdy, A. K., & Mickelson, J. R. (2001). Incidence of the endothelin receptor B mutation that causes lethal white foal syndrome in white-patterned horses. Am J Vet Res, 62(1), 97-103.

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

***A special thanks to Heather Robson & Nancy Kerson for identification and inspiration, respectively

About the Author & Photographers


Dr. Meredith Hudes-Lowder,
Meredith received a Bachelor of Science Degree from Binghamton University with an emphasis in ethology and genetics. She received a Bachelor of Science in Nursing also from Binghamton and a Masters of Nursing in Perinatal/Women's Health from Stony Brook University. She has a Research Doctorate of Nursing Practice from Stony Brook University. Her doctoral thesis was a research study on cervical cancer screening intervals. She was invited to present her research findings at the podium for the Nurse Practitioners in Women's Health Annual Conference in New Orleans, October 2016. She is a member of several professional organisations and was inducted into Sigma Theta Tau- the Nursing Honor Society in 2007.


Karen McLain: Painter, artist, photographer

Karen McLain is a third generation Arizona native. Growing up in Arizona, she developed a deep appreciation for the outdoors, and for the rural and ranching lifestyle. Karen graduated from Arizona State University with a B.A. in Studio Art. She went on to pursue more traditional and realistic styles, and to create a style of her own. A number of commissioned works are accepted from collectors. The rest of the time, Karen can be found drawing or painting en plein air. These landscapes and life studies of wild horses are then developed into larger works in her studio.

McLain states: ”Painting from life not only reveals natures beauty first hand, but it also challenges me to focus and see clearly the light, form, and wonderful color present.  Time spent in the saddle, and painting en Plein air, results in an outlook that McLain describes as “Drawn from life, and inspired by life”, which is reflected in her work. See Karen’s “studio tour” here


Sunday, January 15, 2017


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).


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


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.

Why we do what we do.... Ben's Chase

The air is beginning to cool after 95°F (35°C); dusk is settling over the Basin. Karen and I saw horses heading to the waterhole just...