Dark brown (cocoa) coat colour in French Bulldogs

The brown coat colour in most dog breeds is caused by a mutation in TYRP1 gene. However, some French Bulldogs do not carry any of the known variants of the mutant TYRP1alleles, and yet they have visibly darker brown colour, so-called cocoa colour.

This cocoa variant in French Bulldogs is caused by a nonsense mutation in the HPS3 gene (HPS3: c.2420G> A). The HPS3 gene encodes a protein required for the correct biogenesis of lysosome-related organelles, incl. melanosomes. Due to incorrect function of HPS3, melanocytes are formed that have smaller than normal portion of fully pigmented melanosomes. The synthesis of the mature black eumelanin is partially blocked in comparison to TYRP1 mutation (chocolate colour) and so the resulting colour shade is dark brown (cocoa).

Puppies with HPS3 mutation are born brown and become dark brown with the age. Adult dogs are slightly darker than adult dogs with TYRP1 mutation, however the resulting shade is also influenced by the genetic background and it is probably not possible to reliably distinguish these coat colours without genetic testing.

The mode of inheritance of this mutation is autosomal recessive i.e. dogs that inherit the mutant allele from both parents (recessive homozygote) will display the cocoa phenotype. A heterozygote is an individuum that inherits the mutant allele only from one parent and will not display the cocoa colour but can pass the mutant allele to its offspring.

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Reference:

Kiener, S., Kehl, A., Loechel, R., Langbein-Detsch, I., Müller, E., Bannasch, D., Jagannathan, V., Leeb, T. : Novel Brown Coat Color (Cocoa) in French Bulldogs Results from a Nonsense Variant in HPS3. Genes (Basel) 11:, 2020.