The Genetics behind Chestnuts

All horse color is based on black, red, or a dilution or absence of these colours.

The Melanocortin 1 Receptor (MC1R) and Agouti Signaling Protein (ASIP) are both important genes involved in determining coat color in mammals, including horses.

1. Melanocortin 1 Receptor (MC1R): Red Factor

1. MC1R is a gene that plays a crucial role in determining the distribution of pigments in the coat.
2. The MC1R gene is associated with the production of melanin, the pigment responsible for coloration in hair, skin, and eyes.
3. In horses, the MC1R gene has various alleles that influence coat color. The two main alleles are the “E” (extension) and “e” (non-extension) alleles.
4. Horses with the E allele (dominant) have a black coat, while those with the e allele (recessive) have a chestnut coat. The interactions between these alleles contribute to various coat colors.

2. Agouti Signaling Protein (ASIP):

2. ASIP is another gene involved in coat color regulation, and it acts by regulating the distribution of pigments in individual hairs.
3. The ASIP gene has different alleles that affect the distribution of eumelanin (black pigment) and pheomelanin (red/yellow pigment) in the coat.
4. The interaction between MC1R and ASIP genes determines whether a horse’s coat will have a uniform color or if there will be areas of different pigmentation such as points on a bay horse. Points refer to black located on the lowder legs, mane, tail, and ear rims.

In summary, while MC1R determines the base color of the coat (black or chestnut), ASIP influences the distribution of pigments within individual hairs, leading to variations in coat patterns. The combination of alleles at both MC1R and ASIP loci contributes to the diversity of horse coat colors observed in different breeds. Other genes and modifiers can further influence coat color such as:

• Dilution Genes UC Davis: Champagne, Mushroom Dilution, Pearl, Silver
  Dilution Genes Equus ferus: Cream, Dun
  
• White Spotting Pattern UC Davis: Appaloosa Spotting, Appaloosa Pattern-1, Camarillo White, Dominant White,, Sabino 1, Splashed White,
  White Spotting Pattern Genes Equus ferus: Frame Overo, Gray, Lethal White Overo, Roan, Tobiano

The chestnut coat color in horses is determined by genetic factors. The main gene responsible for chestnut color is the extension (E) gene, specifically the e allele. Horses with at least one copy of the e allele will exhibit chestnut color.

There are two variations of the extension gene: E and e. The E allele is dominant and suppresses the expression of other color genes, resulting in a black coat. The e allele, on the other hand, allows the expression of other color genes, leading to chestnut color.

The possible combinations for chestnut color are:

• ee: Homozygous for the recessive e allele, resulting in a chestnut coat.
• Ee: Heterozygous, with one copy of the dominant E allele and one copy of the recessive e allele. The horse will have a black coat but can pass the chestnut gene to its offspring.
• EE: Homozygous for the dominant E allele, resulting in a black coat.

It’s important to note that chestnut color can vary in shades, from light to dark, and may be influenced by other genetic factors and modifiers. Additionally, there are different variations of the chestnut color, such as sorrel and liver chestnut, which may involve additional genetic elements.

Breeding two chestnut horses together (ee x ee) will always result in a chestnut foal, as both parents carry only the recessive e allele. Breeding two black horses together (EE x EE) will always result in a black foal, as both parents carry only the dominant E allele.

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Chestnut (No mmodifier)

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THE DILUTES

PALOMINO

The Cream Dilution can either be expressed as a single dilution, or a double dilution. Every chromosome has two alleles that represent the way in which each chromosome is inherited and you receive one allele from each parent. Simply put, the chromosomes (usually represented by letters) appear in pairs. To review high school biology, these pairs are generally dominant or recessive. Dominant genes are represented by two capital letters or one capital and one lower-case. The animal appears the same (phenotype) whether they are EE or Ee. The recessive form is represented by two lower-case letters ee.

The cream gene in the single form acts upon chestnut, bay and black by diluting the red colour to cream. The Cream Colour may be light enough to appear almost white to a dark chocolate tan colour. The black is generally unaffected so bay horses horses retain the black points, and mane/tail. Black horses appear somewhat diluted- a mousey chocolate. Horses with a single Cream dilution generally have dark eyes (unless blue from paint patterns) and black skin except where there are white markings (paint markings, facial markings, and leg markings).

A better view of Bolder & his brother, Echo (Pryor Mountain, ©equus ferus- wild horse photography)

CREMELLO/PERLINO

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DUNS

The Dun Dilution Factor works on all horse colours. It can appear in black, bay, chestnut, palomino, buckskin, cremello, roan, and even appears in grey horses prior to the horse greying out. Like the Cream Gene, the Dun Dilution lightens the base coat colour, adds primitive markings, but leaves black colour alone. There are so many different shades of dun and the nomenclature varies between horse breeds and regionally. We will describe them related to their base color. Black horses are unique because the resulting dun horses have a specific name called grullo/grulla. Dark bays and blacks can be almost impossible to distinguish.

A black horse with a Dun Dilution turns the horse a lighter shade, with primitive markings, and guard hairs (lighter hairs on the outside of the mane and tail). The head is usually darker. The resulting horses have a slate-blue with less red present although some can have a warm tone. They generally have a very dark head. Without genetic testing, we are only guessing at the actual chromosomes of these horses. .

Primitive markings are named for markings found on ancient horse breeds such as the Przewalski or Tappan horse. They include a continuous line from the withers to the dock (top of the tail). Other markings include wither and/or neck stripes, cobwebbing: faint stripes on the face, zebra stripes on the legs, chest stripes, and darker ear tips. Zebra stripes are the colour of the underlying coat color, so a chestnut horse has darker chestnut stripes and a bay has black zebra stripes. Lighter hair on either side of the mane and each side of the tail, called ‘guard hairs’ may also be present.

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WHITE SPOTTING PATTERNS:

ROANS

(Rock Springs/Pilot Butte, ©equus ferus- wild horse photography)

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PINTOS

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