Coat colours in dogs are determined by two types of pigment: pheomelanin and eumelanin. Pheomelanin has a brownish colour and shows up in different shades ranging from blond in the Golden Retriever til red-brown in the Irish Setter. Eumelanin is black or brown and can be diluted to grey (blue) or liver.
Relations between coat colour genes
Both pigments eumelanin and pheomelanin share a part of the production path. This means that they compete in their production; if pheomalanin production is increased then the production of eumelanin is automatically reduced.
Which of the two pigments eumelanin and pheomelanin is produced is controled through a receptor in melanocytes (pigment cells): MC1R (the MelanoCortin1Receptor). The E-locus contains the code for this receptor. The e allele produces a receptor that is 'locked' in the position to produce pheomelanin.
The A-locus (Agouti) produces a protein that binds to the MC1R receptor and controls the switch between production of eumelanin and pheomelanin. The switch can occur both in the location in the skin as well as in time.
The K-locus (Dominant black) produces a peptide that also binds to the MC1R receptor and locks it in the position to produce eumelanin.
Eumelanin can come in two colours: black and brown. Which type is produced is controled by the B-locus (TYRP1).
The shape of the pigment particles in melanocytes depends on a matrix of filaments. This is controled by the D-locus (MLPH).
The matrix of filaments which are needed for the production of pigment and to maintain the structure of a melanocyte is also influenced by the M-locus (SILV/PMEL).
Pheomelanin can also show a range of intensities. This is still subject of research but in some breeds a couple of intensity-related genes have been identified.