Horse Coat Colors

Coat color in horses is an interesting and complicated topic.  To begin, it is best to review some basic genetics terms.  The phenotype refers to the characteristics of an animal that can be seen or measured such as color, birth weight, etc.  Genotype refers to the genetic makeup of an animal.  A dominant gene is expressed when carried by only one of a pair of chromosomes.  A recessive gene is expressed in the absence of the dominant gene.  In horses, the black color is dominant and represented by E while chestnut is recessive and represented by e.  A black horse will have EE or Ee while a chestnut horse will have ee.  Homozygous refers to having the same genes for a particular trait (EE or ee) and heterozygous refers to having different genes for a particular trait (Ee). 

The base coat color of any horse may be either black with a genotype of EE or Ee or chestnut (sorrel) with a genotype of ee.  So why are there horses of many colors?   There are diluting genes that restrict or dilute the base coat color.  Some examples include the bay or agouti gene, dun gene, champagne gene, and pearl gene.  For example, the bay gene restricts the black color to the legs, ear rims, mane and tail.  Other color modifications include the grey gene, roan gene, white gene, tobiano, overo, sabino, and  appaloosa.  Horses will have a pair of genes for every color gene.  So a chestnut horse would have the color genotype of ee dd CC gg ww zz toto oo lplp rnrn.

See the full article from Alabama Cooperative Extension at Basic Horse Genetics to learn more on coat color genetics.

Some other references for coat color include the American Quarter Horse Association which has a Color and Markings Reference Chart and a Color Coat Genetic Book that has great pictures and descriptions of coat colors.  eXtension has a lesson with Color Descriptions.