A black horse needs at least one copy of the E-allele, which is responsible for the black pigment eumelanin. A common black horse is therefore black all over, including the coat, mane, tail and other points. Due to diet, genetic and environmental factors, black can sometimes appear 'sun-bleached' with red or other coloured hairs throughout the coat.
Black is caused by the dominant E-allele of the extension gene. The recessive e-allele is responsible for chestnut -based horses. Together with the bay/brown versions of agouti, these two colours form the basis for every other horse colour.
All black horses have the E-allele. As this allele is dominant, it requires only one copy of it to be expressed. When a horse has no copies of E, it automatically has to have ee, resulting in a horse with only red pigment, which is called a chestnut.
- EE = black
- Ee = black
- eE = black (due to preferences, we usually just say Ee)
- ee = chestnut
When we see a black horse, we can already tell it has at least one copy of E. But can we say whether it has EE or Ee? No, we can't, not by looking at it alone. All horses with EE and with Ee are black. If we do want to write down a horse's genetics based on this information alone, we simply say the horse's genotype is E_. Here, the _ may be read for both E and e.
When we add the agouti gene, we get the third base colour bay/brown. For a horse to be bay or brown, it not only requires a dominant copy A, but also at least one copy of E. (keep in mind, a bay horse has black points, therefore it needs to have black pigment (= E-allele) to function!)
- EEaa = black
- Eeaa = black
- EEAa = bay
- EeAa = bay
- EEAA = bay
- EeAA = bay
- eeaa = chestnut
- eeAa = chestnut
- eeAA = chestnut