MatHalTed
Well-Known Member
I have a deep interest in Equine Colour Genetics and recently did a presentation on it for my Uni degree, so thought I would share the guide I made here. Most information is from UC Davis, combined with what I've learned from my studying. Enjoy lol:
The Base Coats:
The two basic coat colours of horses include chestnut, and black. Bay is created due to interactions between these 2 genes, so is not necessarily considered to be a base coat. These are controlled by the interaction between 2 genes: Melanocortin 1 Receptor (MC1R) and Agouti Signaling Protein (ASIP).
MC1R, more commonly referred to as the Extension or red factor locus, is responsible for determining whether a horse will have a chestnut base coat colour or a black or a bay base coat colour. The Extension gene has an autosomal recessive mode of inheritance, meaning that the gene causing the trait is located on a non-sex chromosome & that 2 copies of a variant allele are needed to express the trait.
Mode of Inheritance: Autosomal recessive
Alleles:
E = Dominant Extension (red factor) allele
e = Recessive Extension (red factor) allele
Horses with E/E genotype will be capable of producing black pigment and cannot transmit the recessive Extension allele to their offspring, so cannot produce Chestnut offspring. They will be Bay or Black, depending on their Agouti status, and if there are any other colour modifying genes present. This is sometimes referred to as 'Homozygous Black', however a horse can be Bay and have Homozygous Extension alleles, so this is a slightly inaccurate term.
Horses with E/e genotype will be capable of producing black pigment. They will be Bay or Black, depending on their Agouti status, and if there are any other colour modifying genes present. They may transmit the dominant Extension allele (E) to 50% of their offspring, and may transmit the recessive Extension allele (e) to 50% of their offspring.
Horses with e/e genotype will not produce black pigment in their coat. They will be Chestnut, regardless of their Agouti status, unless there are any other colour modifying genes present. They will transmit a recessive Extension allele to all of their offspring.
ASIP, more commonly referred to as the Agouti gene, controls the distribution of black pigment and determines whether a horse will have a bay or black base coat colour. The Agouti gene has an autosomal dominant mode of inheritance, meaning that the gene causing the trait is located on a non-sex chromosome & that only 1 copy of the variant allele is needed to express the trait.
Mode of Inheritance: Autosomal dominant
Alleles:
A = Dominant Agouti allele (responsible for bay)
a = Recessive Agouti allele (causes black)
If black pigment is present in the form of at least 1 copy of the dominant Extension allele, horses with A/A genotype will have black pigment restricted to the ‘points’. As long as the horse is not homozygous recessive for Extension, they will be Bay, unless other colour modifying genes are present. They will transmit the dominant Agouti allele to all of their offspring.
If black pigment is present, in the form of at least 1 copy of the dominant Extension allele, horses with A/a genotype have black pigment restricted to the ‘points’. As long as the horse is not homozygous recessive for Extension, they will be Bay, unless other colour modifying genes are present.
They will transmit the dominant Agouti allele to 50% of their offspring and the recessive Agouti allele to 50% of their offspring.
If black pigment is present, in the form of at least 1 copy of the dominant Extension allele, horses with a/a genotype will have black pigment distributed evenly uniformly over the body. They will be Black, unless other colour modifying genes are present, and will transmit the recessive Agouti allele to all of their offspring.
Bay: Bay horses have a reddish-brown coat colour, with black points (mane, ear tips, tail and lower legs) and dark skin. Bay is caused by the Agouti Gene in combination with the Red Factor Gene meaning that all bay horses have a black base coat affected by one copy of the Agouti gene. Bay is the most common coat colour present in domestic equines and there are multiple colour variations of it, such as dark bay, red bay & light bay. Some consider it to be a base coat colour alongside chestnut and black, however this is debatable as bay is derived from black and not a colour in of itself. Bay can be genetically expressed as EE/AA, Ee/AA, EE/Aa & Ee/Aa
Black: Black horses have a solid black base coat spread uniformly over the entire body, mane & tail, dark brown eyes, and black skin. Sometimes if black horses are exposed to the elements ‘sun-bleaching’ will occur, resulting in a faded-reddish tinge to the coat, mane & tail. Black coats are caused by the Red Factor gene alongside an unrestricted Agouti gene. Black can be genetically expressed as EE/aa & Ee/aa.
Chestnut: Chestnut horses have a reddish-brown coat colour with no black pigment present. The mane and tail can be the same shade of chestnut, or a lighter shade, as the body coat. There are multiple colour variations of chestnut, such as liver chestnut, which is a very dark red coat, and flaxen chestnut, which can be any shade of chestnut with a significantly lighter mane and tail. Chestnut is caused by two copies of the recessive red factor gene. Chestnut horses can be genetically expressed as ee/AA, ee/Aa & ee/aa. The Agouti gene will always be present on chestnut horses, but has no effect on the coat colour as it only controls black pigment.
The Base Coats:
The two basic coat colours of horses include chestnut, and black. Bay is created due to interactions between these 2 genes, so is not necessarily considered to be a base coat. These are controlled by the interaction between 2 genes: Melanocortin 1 Receptor (MC1R) and Agouti Signaling Protein (ASIP).
MC1R, more commonly referred to as the Extension or red factor locus, is responsible for determining whether a horse will have a chestnut base coat colour or a black or a bay base coat colour. The Extension gene has an autosomal recessive mode of inheritance, meaning that the gene causing the trait is located on a non-sex chromosome & that 2 copies of a variant allele are needed to express the trait.
Mode of Inheritance: Autosomal recessive
Alleles:
E = Dominant Extension (red factor) allele
e = Recessive Extension (red factor) allele
Horses with E/E genotype will be capable of producing black pigment and cannot transmit the recessive Extension allele to their offspring, so cannot produce Chestnut offspring. They will be Bay or Black, depending on their Agouti status, and if there are any other colour modifying genes present. This is sometimes referred to as 'Homozygous Black', however a horse can be Bay and have Homozygous Extension alleles, so this is a slightly inaccurate term.
Horses with E/e genotype will be capable of producing black pigment. They will be Bay or Black, depending on their Agouti status, and if there are any other colour modifying genes present. They may transmit the dominant Extension allele (E) to 50% of their offspring, and may transmit the recessive Extension allele (e) to 50% of their offspring.
Horses with e/e genotype will not produce black pigment in their coat. They will be Chestnut, regardless of their Agouti status, unless there are any other colour modifying genes present. They will transmit a recessive Extension allele to all of their offspring.
ASIP, more commonly referred to as the Agouti gene, controls the distribution of black pigment and determines whether a horse will have a bay or black base coat colour. The Agouti gene has an autosomal dominant mode of inheritance, meaning that the gene causing the trait is located on a non-sex chromosome & that only 1 copy of the variant allele is needed to express the trait.
Mode of Inheritance: Autosomal dominant
Alleles:
A = Dominant Agouti allele (responsible for bay)
a = Recessive Agouti allele (causes black)
If black pigment is present in the form of at least 1 copy of the dominant Extension allele, horses with A/A genotype will have black pigment restricted to the ‘points’. As long as the horse is not homozygous recessive for Extension, they will be Bay, unless other colour modifying genes are present. They will transmit the dominant Agouti allele to all of their offspring.
If black pigment is present, in the form of at least 1 copy of the dominant Extension allele, horses with A/a genotype have black pigment restricted to the ‘points’. As long as the horse is not homozygous recessive for Extension, they will be Bay, unless other colour modifying genes are present.
They will transmit the dominant Agouti allele to 50% of their offspring and the recessive Agouti allele to 50% of their offspring.
If black pigment is present, in the form of at least 1 copy of the dominant Extension allele, horses with a/a genotype will have black pigment distributed evenly uniformly over the body. They will be Black, unless other colour modifying genes are present, and will transmit the recessive Agouti allele to all of their offspring.
Bay: Bay horses have a reddish-brown coat colour, with black points (mane, ear tips, tail and lower legs) and dark skin. Bay is caused by the Agouti Gene in combination with the Red Factor Gene meaning that all bay horses have a black base coat affected by one copy of the Agouti gene. Bay is the most common coat colour present in domestic equines and there are multiple colour variations of it, such as dark bay, red bay & light bay. Some consider it to be a base coat colour alongside chestnut and black, however this is debatable as bay is derived from black and not a colour in of itself. Bay can be genetically expressed as EE/AA, Ee/AA, EE/Aa & Ee/Aa
Black: Black horses have a solid black base coat spread uniformly over the entire body, mane & tail, dark brown eyes, and black skin. Sometimes if black horses are exposed to the elements ‘sun-bleaching’ will occur, resulting in a faded-reddish tinge to the coat, mane & tail. Black coats are caused by the Red Factor gene alongside an unrestricted Agouti gene. Black can be genetically expressed as EE/aa & Ee/aa.
Chestnut: Chestnut horses have a reddish-brown coat colour with no black pigment present. The mane and tail can be the same shade of chestnut, or a lighter shade, as the body coat. There are multiple colour variations of chestnut, such as liver chestnut, which is a very dark red coat, and flaxen chestnut, which can be any shade of chestnut with a significantly lighter mane and tail. Chestnut is caused by two copies of the recessive red factor gene. Chestnut horses can be genetically expressed as ee/AA, ee/Aa & ee/aa. The Agouti gene will always be present on chestnut horses, but has no effect on the coat colour as it only controls black pigment.
some have more extensive roan and some ‘frosty’ roans have it concentrated over the dorsal/top of tail only (and test positive for Rn)
