♘امیرحسین♞
♘ مدیریت انجمن اسب ایران ♞
An Introduction to Genetics:
Horses, like most animals, have two sets of chromosomes. The location of a gene on a particular chromosome is called a gene locus. Because there are two of each chromosome, there are two loci for every type of gene. This means a horse has two genes for each trait (two for eye color, etc.). There are multiple varieties, called alleles, of each gene, so a given horse may have two different genes for a particular trait. A horse with two different alleles for a trait is termed heterozygous for that trait, while a horse with identical alleles is termed homozygous.
There are usually two possible alleles for a gene, though there can be more (for example, there are three alleles for human blood type). Generally one allele is dominant over the other, meaning that if an individual is heterozygous for a trait (has two different alleles), the dominant gene will be expressed and the recessive gene will not be expressed. When making genetic notations, a dominant allele is represented by a capital letter, and a recessive allele is represented by a lower-case letter. So a horse homozygous for a dominant allele would be noted "AA", a horse homozygous for a recessive allele would be noted "aa", and a heterozygous horse would be noted "Aa".
Point Given, chestnut Thoroughbred
Basic Horse Color:
Surprisingly, there are only two possible "base colors" for horses - red (chestnut) and black. Every other color is the result of some modifier gene acting on top of one of these bases. Black is the dominant allele, and chestnut is the recessive. The gene that codes for coat color is usually represented by the letter E (because it is located at the extension locus on the chromosomes) - black is is noted as "E" and chestnut as "e".
Because chestnut is recessive, the color is only expressed in horses that are homozygous for the recessive allele (if a dominant allele were present, it would override the chestnut and the horse would be black). Therefore, all chestnut horses are homozygous, or "ee". Breeding two chestnut horses together can result in nothing but a chestnut foal. Another common term for the color chestnut is "sorrel."
Sunday Silence, black Thoroughbred
Black horses can be either homozygous dominant (EE), or heterozygous (Ee), because only one copy of the dominant allele is necessary for the black color to be expressed. A heterozygous black horse does carry the recessive chestnut allele, however, and can produce chestnut foals. It is even possible to get a chestnut foal from two black parents, provided both are heterozygous. A homozygous black horse does not have the chestnut allele to pass on, and therefore can only produce black-based foals no matter what color the other parent is.
Shades of Chestnut:
The chestnut color can come in many different shades, which is often confusing. The Thoroughbred colt Point Given, pictured above, is a typical expression of chestnut. Below are some examples of variations.
medium chestnut
(Affirmed - Thoroughbred
stallion)
red chestnut
(Thoroughbred stallion)
blond sorrel
(Cindy - Belgian mare)
liver chestnut
(Unescoba - Dutch
Warmblood colt)
bright flaxen chestnut
(Ibn Alih - Egyptian
Arabian stallion)
dark flaxen chestnut
(Rowfantina High Hopes -
Welsh Pony stallion)
Flaxen Chestnut:
It should be noted that blond sorrel and flaxen chestnut are NOT palomino, which is caused by the cream gene and will be discussed further on. Flaxen manes and tails are thought to be caused by a recessive gene, usually denoted by the letter "f", which acts only on red-pigmented manes (i.e. a black-based horse carrying this gene would not be visually affected). Because the flaxen allele is recessive, it is only expressed when in its homozygous (ff) state. Heterozygotes (Ff) and dominant homozygotes (FF) will not have flaxen manes and tails.
Horses, like most animals, have two sets of chromosomes. The location of a gene on a particular chromosome is called a gene locus. Because there are two of each chromosome, there are two loci for every type of gene. This means a horse has two genes for each trait (two for eye color, etc.). There are multiple varieties, called alleles, of each gene, so a given horse may have two different genes for a particular trait. A horse with two different alleles for a trait is termed heterozygous for that trait, while a horse with identical alleles is termed homozygous.
There are usually two possible alleles for a gene, though there can be more (for example, there are three alleles for human blood type). Generally one allele is dominant over the other, meaning that if an individual is heterozygous for a trait (has two different alleles), the dominant gene will be expressed and the recessive gene will not be expressed. When making genetic notations, a dominant allele is represented by a capital letter, and a recessive allele is represented by a lower-case letter. So a horse homozygous for a dominant allele would be noted "AA", a horse homozygous for a recessive allele would be noted "aa", and a heterozygous horse would be noted "Aa".
Point Given, chestnut Thoroughbred
Basic Horse Color:
Surprisingly, there are only two possible "base colors" for horses - red (chestnut) and black. Every other color is the result of some modifier gene acting on top of one of these bases. Black is the dominant allele, and chestnut is the recessive. The gene that codes for coat color is usually represented by the letter E (because it is located at the extension locus on the chromosomes) - black is is noted as "E" and chestnut as "e".
Because chestnut is recessive, the color is only expressed in horses that are homozygous for the recessive allele (if a dominant allele were present, it would override the chestnut and the horse would be black). Therefore, all chestnut horses are homozygous, or "ee". Breeding two chestnut horses together can result in nothing but a chestnut foal. Another common term for the color chestnut is "sorrel."
Sunday Silence, black Thoroughbred
Black horses can be either homozygous dominant (EE), or heterozygous (Ee), because only one copy of the dominant allele is necessary for the black color to be expressed. A heterozygous black horse does carry the recessive chestnut allele, however, and can produce chestnut foals. It is even possible to get a chestnut foal from two black parents, provided both are heterozygous. A homozygous black horse does not have the chestnut allele to pass on, and therefore can only produce black-based foals no matter what color the other parent is.
Shades of Chestnut:
The chestnut color can come in many different shades, which is often confusing. The Thoroughbred colt Point Given, pictured above, is a typical expression of chestnut. Below are some examples of variations.
medium chestnut
(Affirmed - Thoroughbred
stallion)
red chestnut
(Thoroughbred stallion)
blond sorrel
(Cindy - Belgian mare)
liver chestnut
(Unescoba - Dutch
Warmblood colt)
bright flaxen chestnut
(Ibn Alih - Egyptian
Arabian stallion)
dark flaxen chestnut
(Rowfantina High Hopes -
Welsh Pony stallion)
Flaxen Chestnut:
It should be noted that blond sorrel and flaxen chestnut are NOT palomino, which is caused by the cream gene and will be discussed further on. Flaxen manes and tails are thought to be caused by a recessive gene, usually denoted by the letter "f", which acts only on red-pigmented manes (i.e. a black-based horse carrying this gene would not be visually affected). Because the flaxen allele is recessive, it is only expressed when in its homozygous (ff) state. Heterozygotes (Ff) and dominant homozygotes (FF) will not have flaxen manes and tails.
