Colour Genetics 3: Dilution Genes
This is the third post in the Colour Genetics series! If you haven't checked out the first two, I suggest you go check them out, or at least the first post in this series, as it will provide some basic information on genetics if you need some review!
Today's post consists of the five dilution genes! Dilutions lighten certain pigments, or "dilute" them. The presence of one or more dilution genes, or alleles of the same gene, will affect how diluted the horses coat will be. We will be looking at base colours and the Agouti gene only in this post, so the following tables that show what affect the various dilution genes have on a horses coat will be limited to these genetic interactions, and we will disregard more complicated gene interactions and other genes that have not yet been discussed in this series. (More complicated genetic interactions will be discussed in a future post).
The first dilution gene we will look at is the Champagne Dilution gene! This is a dominant gene meaning that a horse needs to carry only one copy in order for it to affect the horses phenotype. Because it is a dominant gene, there is little visual difference between a horse that is heterozygous and a horse that is homozygous Champagne. Champagne horses have mottled skin, hazel eyes (although usually born with blue eyes that darken as they mature), and their coat tends to darken over the winter. Homozygous horses tend to retain their sky blue eyes as adults and their coat colour is even lighter. This dilution affects both pheomelanin (red pigments) and eumelanin (black pigments). Seal brown horses (a form of bay) carrying this gene are known as "Sable Champagne".
The second dilution is the Dun dilution. This gene also affects pheomelanin and eumelanin and is a dominant gene as well. This gene is visually very recognizable, as horses that carry the gene display so-called "primitive markings", which consist of a dorsal stripe down the horses spine, and (not always) zebra stripes (or bands) on the lower legs, and sometimes shoulder bars (very rarely). The points of the horse also tend to be darker than the main body. Note that the difference between Duns and Buckskin horses (as these often get confused for each other) is that Dun horses have the Dorsal stripe! A good example of a Dun horse is to look at the Przewalski's Horse; a wild horse that displays this primitive colouration thought to be displayed by all prehistoric wild horses (along with maybe appaloosas which will be expanded on in a future post!).
The third dilution gene is the Silver or Silver Dapple dilution. Most prominent in Scandinavian breeds, this is the last dilution that is dominant, however, it only affects eumelanin, and for some reason it especially affects the mane and tail hairs. For black horses, it will dilute the body to a chocolate colour (often with dapples) and turn the mane and tail a bright silver colour. For bay horses, the lower legs will be lightened with a flaxen mane and tail. These horses are just known as "black silver" and "bay silver", not to be confused with flaxen chestnut or other similar colourations, as Silver horses have more cool tones and usually their coat hints as to where the black pigments would sit. They also tend to darken with age unlike grey horses. Notably, horses carrying the silver gene tend to have eye disorders such as lesions, general malformations of the anterior region of the eyes, cataracts, and sometimes even retinal detachment. These disorders are more extreme in homozygous horses, and can make a horse appear to have bulging eyes.
The fourth dilution gene is the Cream gene. A horse only needs one copy of this gene for it to noticeably affect the horses coat colour, and two copies of the gene drastically lighten and change the coat colour. This gene affects both pheomelanin and eumelanin. However, it tends to affect red pigments the most, so some dark coloured horses may carry the cream gene without displaying it and owners won't know until genetic testing is performed. Because the cream gene affects both of the two pigments, any horse can be affected by this gene. Due to the fact that you can still tell (for the most part) what a horses base colour is even with this gene in play, it means that this gene is an incompletely dominant gene. Cream horses have very pale blue eyes and little mottling of the skin (distinguishes a Cream carrier from a Champagne carrier, although it is common for a cream horse to carry a copy of both genes). Due to their pale eyes and skin, breeders used to ban Cremello horses from breeding programs, mistaking the colouration for a form of albinism until further study clarified that it wasn't.
The fifth and final dilution is the Pearl Dilution gene. Pearl, which used to be called Apricot, is usually seen in only breeds of Iberian origin (Andalusion etc.), however, it is present in Quarter Horses where it is known as the "Barlink Factor". This is probably due to the fact that a lot of Iberian breeds were mixed with Thoroughbreds when the Thoroughbreds were bred with native stocks of horses, which were descendants from Spain and other European lineages, to give rise to the Quarter Horse. We used to think that the Pearl and the Barlink were two different genes, but we now know that it is the same gene responsible for the colouration in both Iberian breeds and Quarter Horses. Unlike the other dilution genes, the Pearl gene is recessive, meaning that a horse must carry two copies in order for it to be expressed. However, if a horse is heterozygous for this gene but also carries one copy of the Cream gene, it has been found that the Cream gene will activate the lone Pearl, and the horse will exhibit the Pearl phenotype. Pearl horses have an apricot-ish colour, hence why the gene used to be called apricot.
And those are the 5 dilution genes responsible for altering horse coat colours! The next post in this series will cover grey horses. Grey is an extreme example of dilution and will require it's own post to fully understand it. We will go into coat pattern genetics after.
Today's post consists of the five dilution genes! Dilutions lighten certain pigments, or "dilute" them. The presence of one or more dilution genes, or alleles of the same gene, will affect how diluted the horses coat will be. We will be looking at base colours and the Agouti gene only in this post, so the following tables that show what affect the various dilution genes have on a horses coat will be limited to these genetic interactions, and we will disregard more complicated gene interactions and other genes that have not yet been discussed in this series. (More complicated genetic interactions will be discussed in a future post).
The first dilution gene we will look at is the Champagne Dilution gene! This is a dominant gene meaning that a horse needs to carry only one copy in order for it to affect the horses phenotype. Because it is a dominant gene, there is little visual difference between a horse that is heterozygous and a horse that is homozygous Champagne. Champagne horses have mottled skin, hazel eyes (although usually born with blue eyes that darken as they mature), and their coat tends to darken over the winter. Homozygous horses tend to retain their sky blue eyes as adults and their coat colour is even lighter. This dilution affects both pheomelanin (red pigments) and eumelanin (black pigments). Seal brown horses (a form of bay) carrying this gene are known as "Sable Champagne".
Champagne
Base colour
|
Heterozygous / Homozygous
|
Chestnut
|
Gold Champagne
|
Bay
|
Amber Champagne / Amber Dun
|
Black
|
Classic Champagne
|
The second dilution is the Dun dilution. This gene also affects pheomelanin and eumelanin and is a dominant gene as well. This gene is visually very recognizable, as horses that carry the gene display so-called "primitive markings", which consist of a dorsal stripe down the horses spine, and (not always) zebra stripes (or bands) on the lower legs, and sometimes shoulder bars (very rarely). The points of the horse also tend to be darker than the main body. Note that the difference between Duns and Buckskin horses (as these often get confused for each other) is that Dun horses have the Dorsal stripe! A good example of a Dun horse is to look at the Przewalski's Horse; a wild horse that displays this primitive colouration thought to be displayed by all prehistoric wild horses (along with maybe appaloosas which will be expanded on in a future post!).
Dun
Base colour
|
Heterozygous / Homozygous
|
Chestnut
|
Red Dun
|
Bay
|
Classic Dun / Bay Dun / Zebra Dun
|
Black
|
Blue Dun / Mouse Dun / Grulla / Grullo
|
The third dilution gene is the Silver or Silver Dapple dilution. Most prominent in Scandinavian breeds, this is the last dilution that is dominant, however, it only affects eumelanin, and for some reason it especially affects the mane and tail hairs. For black horses, it will dilute the body to a chocolate colour (often with dapples) and turn the mane and tail a bright silver colour. For bay horses, the lower legs will be lightened with a flaxen mane and tail. These horses are just known as "black silver" and "bay silver", not to be confused with flaxen chestnut or other similar colourations, as Silver horses have more cool tones and usually their coat hints as to where the black pigments would sit. They also tend to darken with age unlike grey horses. Notably, horses carrying the silver gene tend to have eye disorders such as lesions, general malformations of the anterior region of the eyes, cataracts, and sometimes even retinal detachment. These disorders are more extreme in homozygous horses, and can make a horse appear to have bulging eyes.
The fourth dilution gene is the Cream gene. A horse only needs one copy of this gene for it to noticeably affect the horses coat colour, and two copies of the gene drastically lighten and change the coat colour. This gene affects both pheomelanin and eumelanin. However, it tends to affect red pigments the most, so some dark coloured horses may carry the cream gene without displaying it and owners won't know until genetic testing is performed. Because the cream gene affects both of the two pigments, any horse can be affected by this gene. Due to the fact that you can still tell (for the most part) what a horses base colour is even with this gene in play, it means that this gene is an incompletely dominant gene. Cream horses have very pale blue eyes and little mottling of the skin (distinguishes a Cream carrier from a Champagne carrier, although it is common for a cream horse to carry a copy of both genes). Due to their pale eyes and skin, breeders used to ban Cremello horses from breeding programs, mistaking the colouration for a form of albinism until further study clarified that it wasn't.
Cream
Base colour
|
Heterozygous
|
Homozygous
|
Chestnut
|
Palomino
|
Cremello
|
Bay
|
Buckskin
|
Perlino
|
Black
|
Smokey Black
|
Smokey Cream
|
The fifth and final dilution is the Pearl Dilution gene. Pearl, which used to be called Apricot, is usually seen in only breeds of Iberian origin (Andalusion etc.), however, it is present in Quarter Horses where it is known as the "Barlink Factor". This is probably due to the fact that a lot of Iberian breeds were mixed with Thoroughbreds when the Thoroughbreds were bred with native stocks of horses, which were descendants from Spain and other European lineages, to give rise to the Quarter Horse. We used to think that the Pearl and the Barlink were two different genes, but we now know that it is the same gene responsible for the colouration in both Iberian breeds and Quarter Horses. Unlike the other dilution genes, the Pearl gene is recessive, meaning that a horse must carry two copies in order for it to be expressed. However, if a horse is heterozygous for this gene but also carries one copy of the Cream gene, it has been found that the Cream gene will activate the lone Pearl, and the horse will exhibit the Pearl phenotype. Pearl horses have an apricot-ish colour, hence why the gene used to be called apricot.
And those are the 5 dilution genes responsible for altering horse coat colours! The next post in this series will cover grey horses. Grey is an extreme example of dilution and will require it's own post to fully understand it. We will go into coat pattern genetics after.
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