Coat Color Genetics

Have you ever wondered why your horse is the color he is? Why it is that gray horses are born black and gray with age? Why it is that bay horses have black manes and tails with black leg markings? The mystery behind it all is a very interesting story once you understand the basics. It is then very easy to understand how a number of “genes” are mixed together to form the variety of coat colors we see today.

Where it all starts

It is important to have some understanding of basic genetics in order for the rest of this blog to make sense, but don't worry, you don't have to be a genius either. 

Here is all you need to know:
Genes come in sets of two, with each parent contributing one gene. When the offspring has two identical genes ( one from the dam and one from the sire) the gene is said to be homozygous. When the offspring has two different genes the dominant gene is said to be heterozygous.
A dominant gene is a gene that will express its characteristic whether it is heterozygous or homozygous. A recessive gene, however, can only express its characteristic in its homozygous form. In other words, the offspring needs to have two genes of the gene in question for it to show that particular characteristic.

Base Colours

All horses will have one of two base coat colors: black or red (chestnut). A black gene is always dominant over the red gene and may be either homozygous or heterozygous. For example a horse that has both a black gene and a red gene will have a black base coat. The only difference will be this horse can pass on the red gene to its offspring.

Variations in coat colors

There are many variations in coat colors. These variations are caused by dilution genes and modifiers acting upon the base coats. Dilutions genes will lighten the coat color. The degree to which the dilution gene will lighten the coat depends on the gene in question, as well, as whether it is in its heterozygous or homozygous state. Here are four different types of dilution genes:

1.Cream gene
2.Champagne gene
3.Silver gene 
4.Dun gene

Modifiers modifies or alters the base coat. These modifiers can either affect the base coat or certain parts of the body or both, depending on the modifier. Here are 6 modifiers:

1.Agouti (bay) 
2.Sooty
3.Brindle
4.Gray
5.Flaxen
6.Mealy (Pangare)

Chestnut horses

Chestnut horses will always have a red base coat and will always be homozygous for the red gene. The coat will vary in shade depending on the other genes or modifiers acting on it. The coat color can range from a light, golden red to coppery red to a dark liver color. Black chestnut is the darkest chestnut color; it is so dark that the horse can easily be mistaken for a black horse. This color is very common in Morgans. As mentioned, other factors will have an affect on the shade of the coat color. For example if you combine the red (chestnut) gene with a cream gene you will end up with a palomino coat color. Or if you combine the red gene with a dun gene you will end up with a red dun coat color. 

Black horses

Black horses will have at least one black gene. The coat color will vary from a blue black (non -fading black) to a brown (fading black) to a smoky color. Factors can act on the base coat to lighten the coat color. For example If you combine a black gene with a silver gene you will end up with a chocolate coat color (known as chocolate silver dapple). Or if you combine a black gene with a dun gene you will end up with a smoky (grulla) coat color. 

Bay horses

Bay horses always have black base coats with an agouti modifier acting on it. This agouti modifier restricts the black coat to the legs, mane and tail allowing the red base coat to come through. The extent to which the agouti restricts the black coat depends on the type of agouti. It is theorized that there are three different types of agouti. One type causes the regular bay color called bright bay. The second type causes a “wild type” of bay called wild bay (the horse has a very light bay coat with the black leg markings restricted to the fetlocks). The third type causes a brown or seal brown color where the horse is nearly black with brown/red points ( at the muzzle, eyes, elbow and flank). 
Like chestnut and black, other factors can act on a bay coat to change its shade. For example if a bay horse has a dun gene it will have a yellow or tan coat color, called buckskin. Or if a bay horse has a cream gene it will have a golden brown coat color.

Gray horses

Gray horses can have either a black or a red base coat. They are born bay, black or chestnut, but the gray modifier causes depigmentation of the coat, resulting in the coat becoming lighter as the horse ages. Gray is dominant over the other colors and can be both homozygous and heterozygous.