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Equine Color Part 1 - Base Colors

FILED UNDER: Equine Color

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Wed

Feb 2010

10

Author: Katie

Site: www.gofreshdesign.com

This is the first in a series of entries on the rainbow of horse coat colors. Behind every flashy paint, every appaloosa spotted pony and every dunalino, is a base coat of either Red or Black. From these bases of red (chestnut) and black, modifying genes are layered to create these fancier colors. Once you understand how the base colors work you can figure out the more complex genetics that cause the rarer coat colors.

The first step to understanding horse color genetics is to know what simple dominant and recessive genes are. Genes are indicated by letters. When indicating a dominant gene the letter in question is capitalized, when it is recessive it will be lowercase. Each horse receives one copy of the base coat gene from their Sire and one from their Dam. When these genes combine they battle it out, and whichever gene is stronger (dominant) is expressed in the physical appearance of the horse.

The gene that causes a horse's base coat to have black pigment is called eumelanin. When a horse lacks eumelanin there will be no black pigment and the base coat will be red. Every horse is either red or black, regardless of what they look like. Black is dominant, red is recessive. Black is indicated by the capital letter "E" and Chesnut is indicated by the lowercase letter "e". Eumelanin is a simple dominant which means that in order for it to be expressed (visible) only one copy of the gene is necessary. The black gene will always win the battle of expression over the red gene. Let's explore a couple of breeding scenarios...

What Happens if a Black Horse is Crossed with a Chestnut Horse?

Let's imagine there are two horses who have no modifying genes whatsoever. The sire is simply black and the dam is simply chestnut. There are two possible scenarios. Because the dam is chestnut and chestnut is recessive you know that she only carries the gene for red. If she had even one copy of the gene for black it would express itself and she would be black. The sire could either be heterozygous for Black: Ee (meaning he has one copy of the Black gene) or homozygous for Black: EE (meaning he has two copies of the Black gene.)

Black crossed with Chestnut Horse Possibilities Black crossed with Chestnut Horse Possibilities

Do you see the difference in the two breeding scenarios above? Why does the first breeding result in all black foals but the second breeding results in half black foals and half chestnut foals? The answer is in the first breeding the sire was Homozygous for black; he was "EE" and could only pass on black. In the second breeding the sire was only Heterozygous for black; he was "Ee". He had one copy of the chestnut gene to pass on, but it was hidden by his dominant black gene in his physical appearance. So he passed on half the time his black gene, and the other half the chestnut gene. So when crossed with the chestnut mare half of the foals were chestnut and the other half were black.

Because black is a simple dominant there is no way to know if the sire has two copies or one of Black without genetic testing or test breeding. If a black horse crossed with a chestnut horse ever produces a red-based foal, then you know it is heterozygous for Black. Or you could send off for a genetic test.

What Happens if a Chestnut Horse is Crossed with a Chestnut Horse?

Black crossed with Chestnut Horse Possibilities

Because chestnut is a recessive the resulting cross will always create more chestnut horses. Neither the sire or dam have any black to pass on.

Next, Agouti the Modifying Gene...

You now have a fair understanding of the base coats: Red and Black. Next up Agouti.

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