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Horse Color Genes: Introduction

By Ann Streett-Joslin

The field of horse color genetics is an active and interesting area of research. New information is being discovered and the color genes in the DNA macromolecules are being better understood. This article will present some introductory information on horse colors for a better understanding of the science behind the colors that we see.

First there is some vocabulary to understand:
• Gene - This is the control for an individual trait in the genetic instructions. They appear in pairs.
• Allele - This is one member of a gene pair. Each gene has two alleles, one from each parent.
• Heterozygous - If the alleles of an individual gene pair are different, the horse is said to be heterozygous for that trait.
• Homozygous - If the alleles of an individual gene pair are the same, the horse is said to be homozygous for that trait.
• Genotype - This is the genetic makeup of the horse, with the allele pairs for every gene in the genetic makeup.
• Phenotype - This is the visual appearance of the horse.

Color begins with the pigment or melanin that is in the horse’s skin. Horses have only two colors of melanin. They are eumelanin which is black pigment and phaeomelanin which is orange-red pigment. All horse colors and color combinations start with these two pigments, black and red. Modifier genes may dilute these colors, cover or mix them with white, or cause the pigment to fade out, but it all starts with black and red pigment.

Now we’ll look at two genes that are the basic controls for the black and red pigment. First is the E or Extension gene. The Extension gene is dominant which means that if the Extension allele from either parent is turned on (represented by a capital E), then the horse has the characteristic. If a horse is “ee” then no Extension characteristic is present and the horse will appear red or what we call sorrel or chestnut. If the horse has “Ee” or “EE” then the Extension characteristic is present and the horse will appear black. There are some variations of the E allele that have characteristics for brown or different shades of black.

The second gene we will look at here is the A or Agouti gene. This gene is also dominant and controls the distribution of black on the body. If the horse has a dominant A allele from either parent, then the Agouti characteristic is present and the black will be restricted the points—mane, tail, legs, ear tips—while the rest of the body stays red or may be darkened somewhat. Again, there are variations of the A allele that may cause different shading.

The two genes, Extension and Agouti, account for the three basic horse colors of black, bay, and sorrel/chestnut. Here are their genotypes:

Black: aaE-    
Has at least one dominant Extension allele.
Bay: A-E-
Also has at least one dominant Agouti allele.
Sorrel/Chestnut: --ee
Has no dominant Extension or Agouti allele. This is the most common makeup for these genes.

Stay tuned for more articles on horse colors in future issue of The Instructor.
About the Author: Ann Streett-Joslin manages Rancho Vista near Dolores, Colorado, and teaches both there and at client’s homes. Visit her at www.RanchoVistaLLC.com