Tobiano Gene
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Pinto coloured horses have a variety of colour patterns. One of them is known as tobiano and the genetics for this particular gene are quite simple. Although there are many other "patchy" colour patterns in horses such as overo and splash, we will just be looking at tobiano for the sake of simplicity.
So what exactly constitutes a tobiano coloured coat? According to the APHA website, it includes the dark color usually covering one or both flanks, four white legs below the hocks or knees, and regular, distinct spots that are round or oval. The head is also usually a solid colour.
This mutation is caused by a large chromosomal inversion, meaning that a large part of the chromosome is the wrong way. This trait is common to all tobiano horses and is considered to be the source of the colour pattern.
The tobiano pattern is a classic example of Mendelian genetics. The allele for the tobiano pattern is dominant (T) and solid coloured (t) is recessive. This means that if a horse carries atleast one tobiano alllele, it will display the tobiano colour pattern. It is important to note that the tobiano pattern can be expressed on any solid coloured coat. A chestnut(brown) horse with the tobiano expression will have chestnut and white coat. A black horse will be splashed with white like a holstien cow. If you were to breed a homozygous(TT) stallion with a solid mare(tt), you would get 100% heterozygous(tt) tobiano coloured horses. Breeding two heterozygous horses will result in a 1:2:1 genotypic ratio where 1 would be tt, 2 would be Tt, and one would be TT. Out of 4 foals, only one would be solid coloured, but 3 would be tobiano. Out of these three, only one would be homozygous(TT) for tobiano.
Grey also follow this classic pattern of inheritance.
So what exactly constitutes a tobiano coloured coat? According to the APHA website, it includes the dark color usually covering one or both flanks, four white legs below the hocks or knees, and regular, distinct spots that are round or oval. The head is also usually a solid colour.
This mutation is caused by a large chromosomal inversion, meaning that a large part of the chromosome is the wrong way. This trait is common to all tobiano horses and is considered to be the source of the colour pattern.
The tobiano pattern is a classic example of Mendelian genetics. The allele for the tobiano pattern is dominant (T) and solid coloured (t) is recessive. This means that if a horse carries atleast one tobiano alllele, it will display the tobiano colour pattern. It is important to note that the tobiano pattern can be expressed on any solid coloured coat. A chestnut(brown) horse with the tobiano expression will have chestnut and white coat. A black horse will be splashed with white like a holstien cow. If you were to breed a homozygous(TT) stallion with a solid mare(tt), you would get 100% heterozygous(tt) tobiano coloured horses. Breeding two heterozygous horses will result in a 1:2:1 genotypic ratio where 1 would be tt, 2 would be Tt, and one would be TT. Out of 4 foals, only one would be solid coloured, but 3 would be tobiano. Out of these three, only one would be homozygous(TT) for tobiano.
Grey also follow this classic pattern of inheritance.
The Cream Dilution Gene
Golden coloured horses have always been prized for their flashy colour. A horse can increase in value just for being this flashy colour so alot of breeders center their program around a diluted coloured horses.
The dilution factor follows the incomplete dominance pattern of inheritance. This means that both alleles for non-cream and cream are dominant. This is the typical example we have all seen in school where red and white flowers produce pink flowers. The allele acts on the 3 base coats of horses which are chestnut, black, and bay. Any other colours are affected by certain genes. Grey, for example, follows the classic Mendelian pattern and is dominant. The genetics for roan are co-dominant.
For the sake of simplicity, we will be looking at how the cream dilution gene affects the 3 base coats even if there can be a combination of several colour genes.
The cream gene mutes some of the melanin in the coat.
One copy of the cream gene will dilute a chestnit coat to a palomino colour. This is a golden base with a white or cream mane and tail. The eyes will also lighten from a chocolatey brown colour to hazel. Two copies of the allele will dilute the coat to a creamy white colour and the eyes will be lightened to blue because of the decrease in melanin. Most people confuse this colour with albino, but there are no true albinos in horses. Also, an albino horse would have pink eyes due to the complete lack of melanin.
It is intresting to note that a single copy of the cream gene will not dilute black. As seen in the picture below, bay horses are diluted to buckskin. This is a golden coat with black mane and tail and black points(ear tips,lower legs, ect..). A black horse will stay black. There is a very subtle difference in the black coat that is very hard to detect. The coat of a black horse with one copy of the dilution gene will have a very suble golden reflection, but sunbleaching also does this to black horses. The best way to determine if a black coloured horse is a smokey black(one copy of the cream gene) is through genetic testing which is simply done by sending a hair sample to a university.
Two copies of the cream gene will dilute black. Both bay and black horses will be diluted to a creamy colour with blue eyes. Again, genetic testing is the best way to tell which base colour(bay,chestnut, and black) a horse has which is covered by the cream gene. Also, most of the time the black that has been diluted will have an orange-yellow tinge, but not always.
The dilution factor follows the incomplete dominance pattern of inheritance. This means that both alleles for non-cream and cream are dominant. This is the typical example we have all seen in school where red and white flowers produce pink flowers. The allele acts on the 3 base coats of horses which are chestnut, black, and bay. Any other colours are affected by certain genes. Grey, for example, follows the classic Mendelian pattern and is dominant. The genetics for roan are co-dominant.
For the sake of simplicity, we will be looking at how the cream dilution gene affects the 3 base coats even if there can be a combination of several colour genes.
The cream gene mutes some of the melanin in the coat.
One copy of the cream gene will dilute a chestnit coat to a palomino colour. This is a golden base with a white or cream mane and tail. The eyes will also lighten from a chocolatey brown colour to hazel. Two copies of the allele will dilute the coat to a creamy white colour and the eyes will be lightened to blue because of the decrease in melanin. Most people confuse this colour with albino, but there are no true albinos in horses. Also, an albino horse would have pink eyes due to the complete lack of melanin.
It is intresting to note that a single copy of the cream gene will not dilute black. As seen in the picture below, bay horses are diluted to buckskin. This is a golden coat with black mane and tail and black points(ear tips,lower legs, ect..). A black horse will stay black. There is a very subtle difference in the black coat that is very hard to detect. The coat of a black horse with one copy of the dilution gene will have a very suble golden reflection, but sunbleaching also does this to black horses. The best way to determine if a black coloured horse is a smokey black(one copy of the cream gene) is through genetic testing which is simply done by sending a hair sample to a university.
Two copies of the cream gene will dilute black. Both bay and black horses will be diluted to a creamy colour with blue eyes. Again, genetic testing is the best way to tell which base colour(bay,chestnut, and black) a horse has which is covered by the cream gene. Also, most of the time the black that has been diluted will have an orange-yellow tinge, but not always.
Lethal White Overo Syndrome
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In the last decade alot of research has been done about lethal white overo
syndrome(LWOS). In the past, paint horse breeders knew that a completely white foal would have to be euthanized
shortly after birth because they would suffer and die a few hours or days later.
It is important to note that grey(white) horses are never born white. Their coat
fades to grey by the time they are five and fades to white in their teens. Most
breeders did not know why their white foals would die soon after birth. Through
autopsies, it was found that these foals all had an incomplete colon.
The overo colour pattern is a very desirable coat colour in paint horses. It is characterized by white jig-saw puzzle patches on a base coat. There are many genes that can produce this colour, but the frame overo allele which is linked to other genes is the one responsible for both LWOS and the pretty overo pattern. The gene is recessive so one copy of the gene (heterozygous) does not prove fatal. It only produces the pretty colour. Now, what happened is breeders started breeding overos together thinking that they could produce a homozygous overo horse that breeds true, but that is when LWOS starting emerging. The foals that are born white and die have two copies of the frame overo gene which is fatal. The APHA association encourages breeders to test their solid and overo coloured horses to for this allele. The tricky part is that a solid coloured horse could easily carry the frame overo allele. A solid horse may not look overo, but might be phenotypically. All that is needed is a small patch of white or a few white
hairs mixed into the coat. A lot of horses have white in their coat and are not frame overos.
syndrome(LWOS). In the past, paint horse breeders knew that a completely white foal would have to be euthanized
shortly after birth because they would suffer and die a few hours or days later.
It is important to note that grey(white) horses are never born white. Their coat
fades to grey by the time they are five and fades to white in their teens. Most
breeders did not know why their white foals would die soon after birth. Through
autopsies, it was found that these foals all had an incomplete colon.
The overo colour pattern is a very desirable coat colour in paint horses. It is characterized by white jig-saw puzzle patches on a base coat. There are many genes that can produce this colour, but the frame overo allele which is linked to other genes is the one responsible for both LWOS and the pretty overo pattern. The gene is recessive so one copy of the gene (heterozygous) does not prove fatal. It only produces the pretty colour. Now, what happened is breeders started breeding overos together thinking that they could produce a homozygous overo horse that breeds true, but that is when LWOS starting emerging. The foals that are born white and die have two copies of the frame overo gene which is fatal. The APHA association encourages breeders to test their solid and overo coloured horses to for this allele. The tricky part is that a solid coloured horse could easily carry the frame overo allele. A solid horse may not look overo, but might be phenotypically. All that is needed is a small patch of white or a few white
hairs mixed into the coat. A lot of horses have white in their coat and are not frame overos.