The mane, tail, and legs may be lighter or darker than the body coat, but unlike the bay they are never truly black.

This is especially apparent in breeds like the Friesian horse and Ariegeois pony which have been selected for many years to be uniformly black, but on rare occasions still produce chestnut foals.

[5][6] Because the red color is recessive, two bay or black parents can produce a chestnut foal if both carry "e" or "ea".

The extension locus (genetics) is found on chromosome 3 (ECA3) and is part of the gene that codes for the equine melanocortin 1 receptor (MC1R).

[7] The two mutant alleles "e" and "ea" code for dysfunctional receptors unable to activate this pathway, so absent "E", only red pigment can be produced.

In general, alleles that create fully functional MC1R proteins are inherited dominantly and result in a black-based coat color ("E"), while mutated alleles that create "dysfunctional" MC1R are recessive and result in a lighter coat color ("e").

Red hair color in horses ("e") is created by a missense mutation in the code for MC1R,[8] which results in a protein that cannot bind to MSH.

Some chestnut foals are also born with lighter eyes and lightened skin, which darken not long after birth.