Just a little information
Chestnut and sorrel are essentially the same color, genetically speaking. These horses are red, yellowish red, or reddish brown, and they do not have black points (the points being the mane, tail, and lower legs). Chestnut and sorrel are determined by genes at the E locus. Horses that are ee at that locus are chestnut/sorrel; horses with an E gene at the locus are black (absent other modifiers). Chestnut is recessive to black, meaning that a chestnut bred to a chestnut will always produce a chestnut foal. Two blacks bred together can produce a chestnut foal if both blacks are heterozygous (Ee). In that mating, there's a 25% chance of a chestnut foal, and 75% chance of a black foal. A black horse who is heterozygous (EE) will only produce black foals.
Also known as sorrel, chestnut is the most recessive equine color. Different shades of chestnut may be given different names in different parts of the world or in different breeds, but since every red horse has the same genetic makeup, we will stick to one term: Chestnut.
Every chestnut has these two genes: ee. This allele of the Extension gene makes the horse produce red pigments, instead of black (E produces black). All chestnuts are homozygous-- they have two e genes. This is why a chestnut crossed with a chestnut will always produce a chestnut.