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Extension Gene
Notation: E
Alleles: E, e
Dominance: E > e
Basic Function: controls production of black pigment (eumelanin) and red/yellow pigment (pheomelanin)
Genotypes & Phenotypes:
EE or Ee = Black** (left)
ee = Chestnut (right)
**when agouti is not present
Description:
- When E is present, both black pigment (eumelanin) and red/yellow pigment (pheomelanin) can be produced. When E is present, as long as there are no other modifiers (such as agouti) the horse will be black (left). If a horse is homozygous for e it cannot produce black pigment in the hair, and the horse will be chestnut (right). The skin and hooves of chestnut horses are still black or grey.
Variations:
- -Sun-faded black (bottom left) are genetically black, but have some lighter areas, usually along the barrel and flank. In extreme cases, the horse may appear completely brown. As the name suggests, this is caused by sunlight "bleaching" the coat.
-Liver Chestnut (bottom right) are chestnut horses that are very dark in colour. You can tell them apart from black or seal bay horses because they typically have lighter hair on their lower legs, as well as on the flank and belly. Liver chestnut appears to be at least somewhat heritable, but it is unknown what gene(s) are involved.
Genetic Cause:
- See basic Mammalian Colour Genetics - MC1R.
