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Some Basic Principles of Color Genetics
By Mary Graybeal

Horses come in a rainbow of exotic colors and coat patterns. A mare owner shopping for a stallion for his or her mare may see stallions described as “homozygous tobiano", “recessive", or “heterozygous black” or may see codings such “Black = Ee” or “Tobiano = TT”. The genetics of horse color and pattern are complex; what follows is a very basic introduction to some aspects of it.

Basic Facts about Genetics

  1. With the exception of some sex-linked genes, an individual possesses two copies of a gene, one inherited from the mother and one from the father.
  2. The genes we’ll be discussing will have two or more versions or variations. If both of an individual’s copies of a gene are the same version of that gene, then the individual is said to be homozygous for that version. If his copies are two different versions of that gene, then he is said to be heterozygous for that version.
  3. Gene versions may be recessive, dominant, or semi-dominant. A recessive version is one that has no effect on the individual’s appearance unless both copies of that gene are the same version. A dominant version is one which renders any other versions present inactive if at least one copy of the dominant version is present.
  4. A horse’s coat can be described in terms of color and of pattern. The genes which determine pattern are different from those which determine color.

Piebald Coat Pattern

Let’s look at the piebald pattern, the black and white pattern for which the Gypsy Horse, for instance, is best known. Probably the most commonly seen version of the piebald pattern is tobiano. In the tobiano pattern, the white part of the pattern crosses the horse’s spine. The tobiano pattern is as if someone poured white paint over a nonwhite, solid colored horse from above and allowed it to dribble down his sides. Like paint, the edges of the white parts of the pattern will be smooth and not serrated.

There are two genes involved here, the Extension Gene, which determines a horse’s base body color, and the Tobiano Gene, which determines whether the horse’s coat displays a tobiano pattern or not.

The Extension (or Black) Gene. Let’s look at the Extension Gene first. Although these colors may be modified or masked by the effects of other color-determining genes, a horse’s coat has only two possible base colors—red and black. The Extension Gene, sometimes also known as the Black Gene, has two versions, one which codes for red (chestnut) and the other which codes for black. Typically, the red version of this gene is denoted by e and the black by E. E is dominant, meaning that if at least one copy of E is present, the horse will have a base body color of black.

The Tobiano Gene. The Tobiano Gene also has two versions, a tobiano version and a nontobiano version. The former codes for the tobiano pattern whereas the latter is simply an “off” switch for tobiano. The tobiano version of this gene is seen represented by T and the nontobinao version by n or, alternatively, by TO and to, respectively.  The tobiano version is dominant, meaning that, if the horse has at least one copy of this version, the horse will display the tobiano pattern.

Let’s look at all the possible genetic configurations of the Extension and Tobiano genes. The symbols for these are given in parentheses.

          Extension Gene                     Tobiano Gene
1.      black, black (E, E)                  tobiano, tobiano (T, T)
2.      black, black (E, E)                  tobiano, nontobiano (T, n)
3.      black, black (E, E)                  nontobiano, nontobiano (n, n)
4.      black, red   (E, e)                    tobiano, tobiano (T, T)
5.      black, red   (E, e)                    tobiano, nontobiano (T, n)
6.      black, red   (E, e)                    nontobiano, nontobiano (n, n)
7.      red, red (e, e)                          tobiano, tobiano (T, T)
8.      red, red (e, e)                          tobiano, nontobiano (T, n)
9.      red, red (e, e)                          nontobiano, nontobiano (n, n)                        

Knowing that the “black” (E) version of the Extension Gene and the “tobiano” version (T) of the Tobiano Gene are dominant, can you determine which of these nine combinations will, in the absence of other color- or pattern-modifying genes, produce a black and white tobiano?

If you said combinations 1, 2, 4, and 5, you were correct. In the absence of any other active color- or pattern-modifying genes, what will the other configurations produce? Number 3 has two copies of E and no copies of T, and so this horse will display a solid black body color. Since E is dominant, so will Number 6.

What about Configurations 7, 8, and 9? Since the horse is homozygous red, that is, has two copies of e, his coat will possess a red base color. The dominant T is present in Numbers 7 and 8, and so these will display a tobiano pattern, but a red and white tobiano pattern as opposed to a black and white one. This horse’s color would be known as skewbald.

From a breeder’s perspective, there are some important implications from all this. One, if you see a chestnut (or red-based) horse, you know that that horse is homozygous “red” (ee), because e is recessive.

Second, since T is dominant and since an offspring inherits one version of a gene from each parent, the offspring of a homozygous tobiano (TT) must express the tobiano coat pattern. This has profound implications for those wishing to breed tobiano horses. Stallions homozygous for tobiano are much in demand because their offspring are guaranteed to display the tobiano coat pattern even if the mare is homozygous nontobiano (nn).

The Gray Gene

The effects of the Tobiano and Extension Genes may be masked or modified by other genes. The Gray Gene, the ultimate dominant gene, masks all other colors when present. Like the Tobiano Gene, this can have an “on” or dominant version and an “off” or recessive version. Only the “on” version has an effect, to make the horse gray, and the presence of this version, denoted G, masks the influence of all other color genes. The “off” or recessive version, denoted by g, simply accedes control to other genes. Therefore, a horse which inherits at least one copy of G will be gray.

This fact has an important implication. When you see a gray horse, you know that that horse must have had at least one gray parent, and that the parent must also have had at least one gray parent, and so on. The gray in front of you is part of an unbroken chain of gray horses stretching back through time.

Breeding a gray to a nongray will not guarantee that you obtain a gray offspring however. If the nongray parent is heterozygous gray, that is, has both G and g, then the offspring has a 50% chance of inheriting the G version of the Gray Gene. However, if the gray parent is homozygous, that is, has two copies of G, then all foals produced by that horse will be gray.

Remember that a horse’s base body color is either red or black. If present, G simply masks these body colors, which are controlled by a separate gene, the Extension Gene. The gray and white tobiano pattern is also possible and will be produced by the following color codings:

Gray           Tobiano
G, G           T, T
G, g            T, t

Modifying Genes

The major modifying genes are the Agouti (bay), Cream, and Silver Genes. We’ll quickly look at these.

The Agouti Gene restricts black to the points of the horse—legs, mane, and tail. The dominant form is usually denoted A and the recessive form a.  A horse displays bay coloration only if he has at least one copy of A and at least one copy of E. If the horse is chestnut, the presence of A is considered to have no effect on his color, but it can be passed on to offspring.

The Cream Gene, whose presence is denoted by Cr and absence by n, is known as a dilution gene because it dilutes the effects of the Extension Gene. It is semi-dominant, meaning that the effects of homozygous Cream on the Extension Gene are stronger than the effects of heterozygous Cream.

A chestnut (ee) horse having one copy of Cr will display palomino coloring but with two copies of Cr, will display a Cremello. A palomino’s coat may range from a pale yellow to a rich golden color or even to a chocolate color. A Cremello has pink skin and hair ranging from a pale cream to almost white. The mane and tail are white, and blue eye coloring is typical.

The presence of one copy of the Cream Gene has little effect on a black horse, a horse having either Ee or EE at the Extension Gene. However a black horse having two copies of the Cream Gene (CrCr) typically has blue eyes, pink skin, and a cream coat color having an orange cast.

The presence of either one or two copies of the Cream Gene also affects the bay. A bay having one copy of Cr is a buckskin, whose color range may range from a pale cream to a darker, smutty color but who retains the black points of the bay. A bay having two copies of Cr is a perlino, which typically has blue eyes, a coat ranging from pale cream to almost white, and a darker mane and tail.

The Silver Gene, like bay, affects only black (Ee or EE) horses. This gene is also a dilution gene in that it dilutes areas of black pigment, making them a chocolate color, and lightens the mane and tail. Dappling may be present. The Rocky Mountain Horse is the breed most identified with this color. A bay horse having the Silver Gene will have light colored mane and tail and light legs.

As with black, chestnut, and gray, any of the above colors can be combined with the tobiano gene or with any other coat pattern.

 

This was a very basic explanation of some aspects of color genetics. Jeanette Gower’s Horse Color Explained: A Breeder’s Perspective makes the subject very approachable.

 

Among the organizations which offer color testing are
The University of Kentucky & Animal Genetics, which also has an offspring coat color calculator on its web site.

 

GHA member, Danni Showers of Kintara Gypsy Cobs in Australia, is a known expert on horse color genetics, and specializes in the many variations seen in the Gypsy Horse breed. See her informational page on Horse Colour Genetics, which includes explanations and photos of the colors discussed here and MORE!

 
6147L
Gray tobiano Gypsy Horse, Silver Phantom. Photo (c) Mark J. Barrett

 

"VICTOR TANGO" Mary Graybeal
Bay tobiano gelding Victor Tango owned by Sherry Evertson.
He possesses an Agouti Gene which modifies his base body color
of Black to Bay. Photo (c) Mark J. Barrett.

 

8565-13
Villa Vanners' handsome Gypsy Horse, Romeo, owes his lovely coloration to a single copy of the Cream Gene, which dilutes the chestnut coat color to Palomino. Photo (c) Mark J. Barrett and Courtesy of Villa Vanners.

 

1401-5
Villa Vanner's Sinead, a Gypsy Horse, displays the chocolate dapple coloration. She possesses at least one copy of the Silver Gene which dilutes her base body color of black to the chocolaty brown visible here. Photo (c) Mark J. Barrett and Courtesy of Villa Vanners.

COLOR CHART

Gypsy Horse Colors - View Large