Base Coat Colors
Every horse has one of three main base coat colors: Black, Bay, or Chestnut. These form the foundation of a horse's coat color, even if other genes eventually change or completely hide its appearance.
A horse's base color is determined by the Extension (E) and Agouti (A) genes. Extension and Agouti may be referred to as "E" and "A" in this text.
Extension determines whether the horse can produce black pigment in its coat, while Agouti determines where that black pigment appears.
Extension (E)
The Extension gene determines whether a horse has a red or black base coat.
Extension has two alleles:
- E - This is the dominant allele that allows black pigment to be produced.
- e - This is the recessive allele that prevents black pigment from being produced in the coat.
The possible combinations are:
- E/E - The horse can produce black pigment and cannot pass down the recessive e allele.
- E/e - The horse can produce black pigment but carries one copy of recessive e.
- e/e - The horse cannot produce black pigment in its coat and will have a chestnut base color.
Because chestnut is recessive, a horse must inherit e from both parents to be chestnut.
This means two chestnut horses (e/e × e/e) will always produce a chestnut-based foal, regardless of any other color genes they carry, as Agouti (if present) cannot be expressed due to a lack of black pigment.
However, two black-based horses can still produce a chestnut foal if both carry the recessive e allele.
A good example of this in-game would be two black Friesians carrying the recessive e (E/e), bred together to produce a chestnut (e/e) Friesian, despite both parents visually appearing black.
Agouti (A)
While Extension determines whether black pigment can be produced, Agouti controls where that pigment appears.
In our mod, Agouti has four possible alleles: A, A+, At, and a.
These alleles determine whether a horse has a Bay, Wild Bay, Seal Brown, or Black base color, provided the horse can produce black pigment.
The four alleles are:
- A (Bay) - The dominant allele that restricts black pigment primarily to the mane, tail, and lower legs.
- A+ (Wild Bay) - Restricts black pigment more strongly, typically producing less extensive black points.
- At (Seal Brown) - Produces a very dark brown coat, often with lighter brown or reddish areas around the muzzle, eyes, and flanks.
- a (Black) - The recessive allele that allows black pigment to spread across the entire coat.
Agouti Dominance
Our dominance order is as follows:
A > A+ > At > a
(This will be corrected to A+ > A > At > a in future as we released the variations of bay after the main genetics release)
This means A is the most dominant allele, while a is the most recessive.
When a horse inherits two different Agouti alleles, the more dominant allele determines which base color is expressed.
For example:
- A/A+ - The horse will be Bay but carries Wild Bay.
- A/At - The horse will be Bay but carries Seal Brown.
- A+/At - The horse will be Wild Bay but carries Seal Brown.
- At/a - The horse will be Seal Brown but carries Black.
- a/a - The horse has two copies of recessive Agouti and will have a Black base color.
These outcomes assume the horse has at least one dominant Extension (E) allele.
Agouti only affects the appearance of horses capable of producing black pigment. This means a chestnut horse (e/e) can carry any combination of Agouti alleles without them affecting its visible base color.
Even though Agouti is hidden on chestnut horses, those alleles can still be inherited by their foals. So an e/e A/A parent bred to another horse could produce a Bay base color foal.
Visually, a chestnut horse carrying dominant Agouti alleles and one carrying only recessive Agouti alleles will look the same, so it's important to genetically test horses to work out possible genetic outcomes.
Agouti Breeding Example
For example, in-game, when breeding a Bay horse (A/A+ - carrying the Wild Bay Agouti allele but not expressing it) with a Seal Brown horse (At/a), the pairing could produce different foals.
The possible Agouti combinations for their potential foals are:
- A/At - 25% chance of Bay
- A/a - 25% chance of Bay
- A+/At - 25% chance of Wild Bay
- A+/a - 25% chance of Wild Bay
The foal's appearance is determined by whichever of its two inherited Agouti alleles is more dominant.
Despite one parent carrying the Seal Brown Agouti allele, Seal Brown is not a possible expression from this pairing, as the Bay parent can only pass down A or A+, both of which are dominant over At (Seal Brown).
This means the pairing has a 50% chance of producing Bay and a 50% chance of producing Wild Bay, assuming the foal inherits at least one dominant Extension (E) allele.
How the Genes Work Together
A horse's final base colour depends on the combination of Extension and Agouti it inherits.
- E/E or E/e + at least one A = Bay
- E/E or E/e + A+ (without A) = Wild Bay
- E/E or E/e + At (without A or A+) = Seal Brown
- E/E or E/e + a/a = Black
- e/e + any combination of Agouti = Chestnut
Because Extension determines whether black pigment can be produced, a horse with e/e will always have a Chestnut base color, regardless of its Agouti combination.
Base Color Variations
Although there are only three main base colors, horses can display a wide range of shades within them.
Chestnut horses can range from light orange to deep liver (almost black) chestnut.
Bay horses can range from lighter reddish shades to very dark brown, while black horses may appear jet black or become lighter due to sun bleaching.
In game, we have several variations for bay and chestnut, so it's possible two horses may carry the exact same genetics but look different due to the natural variation in their base color.
Dilutions, modifiers, and white patterns can further change how these base colors appear, creating the wide variety of horse colors. There is no "one size fits all" in terms of genetics.