The chestnut color, called "red" by geneticists, is created by an allele that is a mutation from the wildtype and is genetically the most recessive coat color that exists in modern horses. The gene for "red" color is designated as "e". This is because the presence or absence of red color in horses is determined by the equine melanocortin 1 receptor (MC1R), a protein positioned on chromosome 3 (ECA3) at the Extension locus. The wild type version of the gene encoding MC1R is the "E" allele (colloquially, though imprecisely, called the "Extension gene"), and is part of the genetic pathway that allows melanocytes to produce eumelanin, or black pigment. When the "E" allele is not present, no eumelanin is produced, but the "e" allele still allows melanin to be produced in the form of pheomelanin, or red pigment, creating a chestnut or red-based coat color. 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").