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TYRP1 (aka B-locus) is a gene in dogs that determines whether black or brown eumelanin is produced (assuming eumelanin is produced at all which is determined by the MC1R gene). I've looked at many papers and flow charts and now i have a contradiction: DHI seems to be the molecule that gives eumelanin a black color and DHICA a brown color. According to many flow charts, TYRP1 seems to be involved in the polymerisation of DHICA (not DHI). Mutations that cause a loss-of-function of TYRP1 will cause dogs that carry these mutations (homozygous) to have brown fur. But if TYRP1 is important for polymerisation of DHICA to brown, why does a loss-of-function of this very protein cause dogs to have brown coat color?

Where did I get it wrong?

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The actual coat color is determined by the ratio of the products of two genes: Tyr and TyrP1. The first oxidizes DHI to Indole-5,6-quinone and forms brown pigment, TyrP1 on the other hand oxidizes DHICA to Indole-5,6-quinone carbolic acid, which forms black pigment. See this pathway diagram (from here):

enter image description here

So loss of TyrP1 activity is not harmful to general pigmentation but prevents the formation of black color, while the inhibition (or loss of function) for Tyr will lead to albinism, since no pigment precursors (from Tyrosine to DOPA can be formed.

You may also be interested in this paper: "TYRP1 and MC1R genotypes and their effects on coat color in dogs."

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  • $\begingroup$ Thank you! Then, my error was that DHICA forms black eumlanin and not brown. $\endgroup$
    – Lukeception
    Commented Jul 30, 2016 at 19:36

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