Horse Color Genetics

A (Agouti)

restricts black to points (with E)

allows black over entire body (with E)

B (Chocolate)

chocolate brown body color (not found in most breeds)

non-chocolate brown body color

non-dilute

c (Cr Cremello)

cream dilution (chestnut to palomino, bay to buckskin)

D (Dun)

dun dilution (bay to dun, chestnut to red dun, black to grulla)

non-dilute

E (Extension)

black color (entire body or points only, depending on A)

red body color

F (Flaxen)

red mane and tail (on ee horses)

flaxen mane and tail (on ee horses)

G (Grey)

grey

non-grey

O (Overo Paint Pattern)

overo spotting

non-overo

R (Roan)

roan

non-roan

Sb (Sabino Paint pattern)

sabino spotting

non-sabino

Spl (Splashed White Paint pattern)

splashed white spotting

spl

non-splashed white

T (Tobiano Paint Pattern)

tobiano spotting

non-tobiano

dominant white, causes pigment loss in hair and skin, but not eyes.

non-white

silver dapple dilution (black to silver dapple, bay to silver dapple bay)

non-dilute

A horse's color is determined by its genetic makeup, or Genotype. The visual appearance of the horse is its Phenotype. The genotype designates the alleles present to determines the phenotype.

A Horse having a ee will always have a red color phenotype.

A Horse having genotypes EE, and Ee may be Black or Bay.....color. (Among these colors other genes control the differences)

Chromosomes are threadlike structures located within the cells of an animal. Each horse has 32 pairs of chromosomes within each cell. The genes, which control the animal's traits, are located on the chromosomes. Genes exist in matching pairs, with one of each type of gene on each pair of chromosomes. Each member of a pair of genes is called an Allele. Each allele exists at a certain spot on the chromosomes, called a locus (the plural is loci).

Genes can be Dominant or Recessive. The dominant form of a gene is identified with a capital letter, and the recessive form with a lower-case letter. The Red color allele e is recessive to black, and the Black allele E is dominant to red. A dominant allele will be expressed even if it is carried by only one member on the chromosone pair EE, and Ee. The recessive allele will be expressed only when a dominant member on the chromosone pair is absent ee. When both allele in a pair are the same EE, or ee, the horse is Homozygous; when the chromosome pairs have unlike alleles Ee, the horse is Heterozygous.

Genotype Phenotype

EE Black Hair Homozygous

Ee Black Hair Heterozygous

ee Red Hair Homozygous

For another example, let's use a simple gene, T, which controls Tobiano color pattern. T is the dominant form of the gene, causing a horse to show tobiano color pattern, and t is the recessive (solid) form. Since chromosomes exist in pairs, one of the chromosomes in the pair will be T or t, and the other one will also be T or t. Of course, they do not necessarily match. Each horse has one of three combinations of T genes at the T loci: TT, Tt, or tt. Horses that are TT or tt are homozygous, while horses that are Tt are heterozygous.

T is a dominant gene, so the horse only needs to possess one copy of T to display the tobiano pattern. Therefore, a horse with TT or Tt will be tobiano, and a horse with tt will not be tobiano, it will be a solid. (its color and markings are determined by genes at other loci). The presence of T masks the expression of t, so T is dominant over t. The non-tobiano horse is said to be "recessive at the T locus," and the tobiano is dominant at that locus.

A tobiano horse is usually described as T-. The dash (-) means that the gene present at that locus has no effect on the horse's visual appearance. TT and Tt horses exhibit the same phenotype tobiano color pattern.