When I saw my first "chocolate" horse I was intrigued by the color. I was also confused by the various color terms and what was the big deal about "no red gene"? Did you ever wonder how you could have a RED chocolate horse if it didn't have a red gene? I was determined to find answers and educate myself - and I plunged into the fascinating realm of color genetics.
This page is meant to be a simplified color gene crash course for the average Joe (like me!) who is curious about the basics. Hopefully it will be helpful and you will find this as interesting as I do. There are many different genes that control colors in many various breeds, but I will only address here a few of the basics in the Rocky Mountain horse (if you want to learn what makes a paint horse you will have to do a little extra reading on your own!) So here goes:
There are two basic color pigments in all breeds - Black and Red. Black is dominant over red.
There are some genes that dilute colors and other genes that restrict where the pigment is able to form on the body.
Horses have a series of "color genes" within their DNA. They have two copies of each color gene and each horse parent will pass on one copy to their offspring. So a foal gets one copy from mom & one copy from dad for each gene pair on their color chromosome.
The simplest way it has been explained to me is that each color pair might have one or both genes turned ON or OFF. If the gene is ON (or dominant) then it can affect the color of the horse. If the gene is OFF (or recessive) it does not affect the color. If both genes are OFF - the horse color will not be affected and that color will not be passed onto the offspring from this parent. If one of the pair is ON, and the other is OFF then the horse color will be affected - so it only takes one of the pair being ON to work on the color. However, since the horse will only pass on one of the pair to its offspring you may or may not get a foal with the ON gene. A horse with both an ON and an OFF gene is considered to be "heterozygous" for that color. If both genes are ON the horse is considered to be "homozygous" for that color and you are guaranteed that they will pass on that color to their foal.
The first gene to consider is called the "Extension" gene and is represented by the letter "e". This gene when ON (E) allows black pigment to form on the body - this is what is referred to as a black gene. When this gene is OFF (e) it blocks black pigment from forming - leaving only red pigment left to form - this is what is referred to as a red gene. Because black is dominant over red, if a horse has a single black gene and a red gene (Ee) the horse will appear black. This is what is considered a black horse with a red gene - he may or may not throw a red foal. If both genes are ON (EE) he is homozygous for black, meaning he has no red gene and cannot produce a red foal - even if the foal receives a red gene from the other parent it is guaranteed a black gene from this parent and so cannot be red even though it carries the gene.
The next gene to consider is the Agouti (or Bay) gene and is symbolized by the letter "a". This gene is a restrictive gene and only affects the black pigment, restricting the black pigment to the points of the horse (legs, mane, tail and around the mouth/nostrils/ears). A red (ee) horse with the Agouti gene would not be visibly affected. A black horse (EE or Ee) with the Agouti gene would have the black pigment restricted to the points, leaving the body of the horse some shade of red/brown and the horse would be called a Bay. This is how you can have a horse with no red gene (EE) that appears to have red hair - it's simply a case of the black pigment not being able to form leaving a red appearance.
Chocolate Horses: The Silver Dapple gene (Z) is a dilution gene that only affects black hair, and it has a more intense dilution effect on the coarse hair of the mane & tail than it does on the body hair. This is what gives the Rocky Mountain Horse the chocolate color. A red horse with the silver gene will not show any effect. A black horse with the silver gene will have a chocolate body and the mane & tail will be a shade of flaxen to white. This means a horse that has two copies of both the black and the silver gene (such as our Maple) will always throw a chocolate foal if no other dilution genes are added to affect the color. A chocolate horse can also be called a silver black.
Red Chocolate Horses: A red chocolate horse is a black horse that carries the Bay (Agouti) gene and the Silver gene. The black is restricted to the points by the Agouti gene and the black points are then diluted to a shade of chocolate/flax by the Silver Dapple gene. A red chocolate horse can also be called a silver bay.
The Cream gene is another dilution gene and is symbolized by the letters "Cr". This gene is a little different in that it acts differently if one gene is ON than it does if both genes are ON. When only one Cream gene is ON it dilutes only red hair - so a red horse with a cream gene would be a palomino, a bay horse with a cream gene would be a buckskin. A black horse with a cream gene is considered a smoky black and doesn't look any different than regular black but some say they are sometimes are more faded or are easily sunburned to a lighter shade. If both Cream genes are ON it affects both red and black hair as well as skin and eye color. Perlinos (Bay/double Cream) and Cremellos (Red/double Cream) have two Cream genes. A black with two Cream Genes is a smoky cream. Horses with two Cream genes are light colored and can be difficult to tell exactly what they are (Perlino, Cremello, or smoky cream) without DNA testing.
Our Maple has a cream gene to go with her two black & two silver genes - she is called a chocolate cream (she can also be called a silver smokey black). If bred to a black or a red stallion she will ALWAYS throw chocolate foals. If bred to an Agouti gene she could also produce a red chocolate or a silver buckskin.
While your top priorities in breeding Rocky Mountain Horses are temperament, conformation and gait - it's fun to study the color genetics and be able to predict what each cross will throw.
If you'd like to read more about horse color genetics and find out about other color genes (such as Roan, Dun, Champagne, Grullo, Gray, etc.) I've listed a couple of sites below that I think are pretty informative and easy to understand.