The chocolate plumage color in chickens is due to a sex-linked recessive mutation, choc, which dilutes eumelanin pigmentation. Because TYRP1 is sex-linked in chickens, and TYRP1 mutations determine brown coat color in mammals, TYRP1 appeared as the obvious candidate gene for the choc mutation. By combining gene mapping with gene capture, a complete association was identified between the chocolate phenotype and a missense mutation leading to a His214Asn change in the ZnA zinc-binding domain of the protein. A diagnostic test confirmed complete association by screening 428 non-chocolate chickens of various origins. This is the first TYRP1 mutation described in the chicken. Electron microscopy analysis showed that melanosomes were more numerous in feather follicles of chocolate chickens but exhibited an abnormal structure characterized by a granular content and an irregular shape. A similar altered morphology was observed on melanosomes of another TYRP1 mutant in birds, the roux mutation of the quail.

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