Retriever dogs have three distinct phenotypes. There is the black phenotype, the brown phenotype, and the yellow phenotype. The difference in expression between the black and brown phenotype comes down to the production of two enzymes: TRP-1 and TRP-2. All retrievers produce tyrosine, which when reacted with tyrosinase forms dopaquinone. TRP-2 then reacts with the dopaquinone to create the brown pigment, which then reacts with TRP-1 to form the black pigment. Black is the dominant color of the two because it takes only one signal to produce both TRP-1 and TRP-2. There is a mutation in the recessive allele that causes only TRP-2 to be produced, causing the pigmentation to halt once it reaches the brown pigment.
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