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
- 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.
- 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.
- 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.
- 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).