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| Most of the color genes are dominant. "Dominant" means simply that a horse needs only one gene (in a pair) for a particular trait in order to exhibit it. If the horse has the gene, it will exhibit that trait. (The extent to which the trait is exhibited can be modified by the presence of other dominant genes - but it WILL express itself!) Technically, Genes themselves are not dominant or recessive, although we speak of them as such. Alleles are dominant or recessive. For example, the MC1R gene (called Extension) codes for the production of eumelanin pigment, causing a horse to be black. A mutation occurred in that gene at some time long ago, such that the gene now coded for the production of phaeomelanin, which produces red pigment. If a horse carries one copy of the unmutated gene, and a second copy of the mutated gene, the unmutated gene creates enough eumelanin to mask the other copy. Therefore, we say that the unmutated copy, or allele (E) is dominant to the mutated copy, or allele (e). The same is true for all genes: cream, tobiano, overo, dun...is the mutated allele dominant or recessive? But the gene is just a gene. So you can't say that chestnut is caused by a recessive gene. Red is caused by the same gene that causes black--it's just caused by carrying two recessive alleles of the Extension gene, whereas black is caused by carrying at least one dominant allele of the Extension gene. It is generally not possible to tell from appearances alone whether the horse is homozygous (both genes are the same for a trait) or heterozygous (only one gene for the trait). Exceptions to this are the Lethals, where being homozygous for a trait (Frame and White) is lethal. In these cases, the foal either dies before birth, or soon after. The other exception occurs with the Creme dilution gene, which acts as an "Incomplete Dominant." With an "incomplete dominant", you get one effect from one gene, and an exaggerated effect from a double dose. Thus, one creme gene on a red base creates Palomino; Two creme genes create Cremello (a further diluted state)
Both Appaloosa and Tobiano horses sometimes show visual clues to their homozygous state, although genetic testing is the only sure way to know (if you care) A horse bearing multiple dominant genes will exhibit multiple traits. Red and Flaxen are two of the very few Recessive Genes. Flaxen only affects Red. A black horse with the Flaxen gene will show no flaxen effects. Because both are Recessive (their trait is only expressed if both genes on the pair are the same) Chestnut and Flaxen Chestnut horses are able to breed true. Because Red can "hide" for generations, until bred to another horse with a hidden recessive red gene - breeders (Freisian, Percheron, etc.) who desire only black horses have a problem. UC Davis has a simple test for the red gene, which is a useful tool.
Other Pages In the Color Section of this Website:Major Headings: The Single Dominant Genes: Agouti | Appaloosa | The Pinto Patterns: Tobiano | The Overo Complex: Frame | Sabino | Splash | Tovero The Incomplete Dominant Genes: Champagne | Creme The Recessive Genes: Red | Flaxen Colors with multiple genetic bases: Blue | Brown | White | Roan-like Effects |
