coded by extension, to block the signal for black pigment production. The signal for black pigment comes from a melanocyte-stimulating hormone, which is present throughout the horse. ASIP is not present everywhere, which allows some areas to be black while others are red. ASIP can also be limited by the phase of hair growth, allowing the tips of the hairs to be black while the base is red. This can be observed in horses which have their winter coats clipped. When shaved close, the black tip is shorn off leaving the phaeomelanic bottom of the shaft. This produces a dull, orange-gold appearance on the body coat which is lost with the spring shed. This is not usually seen in dark bays, which have little red in the hair shaft. A mutation to agouti removes the ability to block the black signal, resulting in a fully black horse. The dominant, wildtype, allele of agouti is called A, and the non-agouti mutation is called a. Phenotypes Extension Extension is found on equine chromosome 3 as part of a linkage group with roan, tobiano, and the KIT gene. Extension is also sometimes called "red factor" and can be identified through DNA testing. Horses with the genotype E/E are sometimes called "homozygous black", however depending on the agouti genotype there is no guarantee that any offspring can be black coated, only that no offspring will be "red". A study that compared horse genotypes to their coat color phenotypes did find a statistically significant connection that suggested that lighter bay shades were heterozygous for the Extension mutation (E/e) and darker bay shades were homozygous. Mutations that break protein function generally lead to recessively inherited lighter or redder coat colors in various mammals, while mutations that cause MC1R to be constantly active result in dominantly inherited black coats. In horses, both known mutations break the protein and therefore result in red coats. Various mutations in the human MC1R gene result in red hair, blond hair, fair skin, and susceptibility to sunburnt skin and melanoma. Polymorphisms of MC1R also lead to light or red coats in mice, cattle, and dogs, among others. The Extension locus was first suggested to have a role in horse coat color determination in 1974 by Stefan Adalsteinsson. Researchers at Uppsala University, Sweden, identified a missense mutation in the MC1R gene that resulted in a loss-of-function of the MC1R protein. Without the ability