brown
Chestnut (including red, brown, and flaxen chestnut)
DILUTED COLORS
Colors determined by cream dilution:
Buckskin
Smoky black
Smoky seal brown
Palomino
Double cream
“Wild” colors determined by the Dun gene:
Bay dun
Grullo
Brown dun
Red dun
Silver colors:
Silver bay
Silver seal brown
Silver black (including silver dapple)
Champagne colors:
Amber champagne
Classic champagne
Sable champagne
Gold champagne
Pearl colors:
Bay pearl
Black pearl
Seal brown pearl
Chestnut pearl
Mixed diluted colors (colors determined simultaneously by combinations of different dilution genes):
Dun and cream
Dun and silver
Cream and silver
Cream and champagne
Cream and pearl
And others
COLORS DUE TO ADMIXTURE OF WHITE HAIR
Gray
Roan
Spotted
Appaloosa-spotting patterns (Leopard complex)
To summarize, there are four base colors, plus genes controlling all remaining colors. Figuratively speaking their “actions” are superimposed on the four base colors. I also introduce a separate category I call color phenomena: countershading, rabicano, pangaré, brindle, gloss of body hair, false dun, and giraffe marks, as well as characteristics of the horse’s coat where the genetic nature is not proven or the characteristics do not remain stable over time. Examples include dapples, the phenomenon of “Catch A Bird,” and frosty.
A Very Basic Introduction to Genetics
You can’t learn about horse color without learning at least a little bit about equine genetics.
HOW IT ALL BEGINS
Genetic information that determines the traits of a living organism is stored in the nucleus of a cell. Using a microscope, it is possible to observe oblong structures in the nucleus during certain time periods in the life cycle of a cell. These structures, called chromosomes, contain deoxyribonucleic acid (DNA), which is the direct carrier of hereditary information. The molecular building blocks of DNA are the four nucleotides known as adenine, thymine, cytosine, and guanine. A DNA molecule has two strands aligned with each other (like the tracks of a train), forming the familiar double helix. Segments of DNA are called genes. They are essentially the ingredients for the recipe to make a living thing and determine the kind of “products” the cell will manufacture and the characteristics they determine—for example, eye or hair color.
SO…WHAT’S SO IMPORTANT ABOUT GENES?
The DNA sequence of an organism and the presence or absence of proteins that code for certain traits is called genotype; their external manifestation (the organism’s observable traits and characteristics) is phenotype. Most multicellular organisms obtain half of their chromosomes and genes from each parent. Horses have 32 pairs of chromosomes for 64 in total, and each gene normally exists in the same place on the same chromosome—this location is called the locus. The locus is the physical site on the chromosome where one form of a gene can be found. The different forms of genes that determine possible phenotypes are known as alleles. The alleles for a gene normally will be found in the same place on a chromosome from one parent as on the same chromosome from the other parent. As one locus is paired with another locus, the horse has two allelic genes matched from the series of alleles possible for a specific characteristic.
Alleles may be represented by a pair of letters: one letter denoting the allele donated by one parent and a second letter indicating the allele contributed by the other parent. If both alleles from both parents are identical, the animal has a homozygous genotype with two identical alleles at this locus (for example, AA or aa), but if they are different, then the individual is heterozygous (Aa or aA). When one of the alleles in a heterozygote completely determines the phenotype even when the second allele is present, the suppressing allele is said to be dominant and the suppressed allele is recessive. This phenomenon is called complete dominance. The completely dominant allele is designated by a capital letter (A), and the corresponding recessive allele is designated by a lowercase letter (a). In this scenario, the individual with the genotype “AA” has the same phenotype as individual with the genotype “Aa”—the recessive gene is hidden. But, while it doesn’t “show” in the individual, it could emerge as observable in offspring. This plays an important role in the probabilities of the inheritance of any trait.
When individuals with the genotype “AA” and “Aa” are phenotypically different, and the trait encoded by allele “A” is more weakly expressed in the heterozygous “Aa” individual than in the homozygous “AA” individual, then the allele “A” is designated as incompletely dominant. In this case the heterozygous individual has a phenotype that can be an intermediate phenotype (that is, an “average”) between individuals with the genotype “AA” and “aa.”
A LITTLE ABOUT TRAITS
Now let us discuss genes that can amplify, weaken, or otherwise modify the action of other genes. Such genes are called genetic modifiers. An organism’s phenotype can be formed by the action of two or more of these non-allelic or complementary genes, which in combination create an effect other than what the genes would on their own. Examples in horses are the genes Extension and Agouti, the first of which codes for
