to produce a functional MC1R protein, eumelanin production could not be initiated in the melanocyte, resulting in coats devoid of true black pigment. Since horses with only one copy of the defective gene were normal, the mutation was labeled e. Extension alleles There are three known alleles of extension, the wildtype E, and two recessive alleles e and ea which cause chestnut color. The E allele can also be called E+ or EE, and the e allele may also be called Ee. Of the two known mutations, the first to be discovered was e, and is a change of a single cytosine to thymine at base pair 901 which results in the serine in position 83 being changed to a phenylalanine. In 2000 ea was found, which is a change of a single guanine to adenine at base pair 903, resulting in aspartate being changed to asparagine at position 84 in the polypeptide. Visually there is no difference between the two. Agouti In many species, successive pulses of ASIP block contact between α-MSH and MC1R, resulting in alternating production of eumelanin and pheomelanin; hairs are banded light and dark as a result. In other species, ASIP is regulated such that it only occurs in certain parts of the body. The light undersides of most mammals are due to the carefully controlled action of ASIP. In mice, two mutations on Agouti are responsible for yellow coats and marked obesity, with other health defects. Additionally, the Agouti locus is the site of mutations in several species that result in black-and-tan pigmentations. One genetics testing lab began offering a test for another allele At, thought to be responsible for seal brown, but it was later found to be inaccurate and is no longer offered. Dun Dun is one of several genes that control the saturation or intensity of pigment in the coat. Dun is unique in that it is simple dominant, affects eumelanin and pheomelanin equally, and does not affect the eyes or skin. Horses with the dominant D allele (D/D or D/d genotype) exhibit hypomelanism of the body coat, while d/d horses have otherwise intense, saturated coat colors. The mane, tail, head, legs, and primitive markings are not diluted. Zygosity for Dun can be determined with a DNA test. The Dun locus is TBX3 on equine chromosome 8. The molecular cause behind the dun coat colors is not entirely understood, but the