First of all, I would love to thank the wonderful owner of the site Colours of the Rainbow. Without them, this post would not be possible! They were able to confirm this information for us, making an accurate explanation of the C Locus possible.
The C Locus is extremely important in terms of genotyping, as it modifies a number of different genes and alleles. This locus controls the expression of phaeomelanin (also spelled pheomelanin), or red pigment, which is seen in Sable (Ay), Agouti (aw), Tan Points/Black and Tan (at), Recessive red (ee) and Brindle (kbr).
The image above illustrates the C Locus well. These ten Golden Retrievers range from the English Cream variety which is nearly white to the deep red seen in setters. It is known that Goldens do not carry sable, so the dogs are recessive red. From numbers one to ten, here is the likely C Locus.
1: cece, or extreme white filtering
2: chce, or pale cream
3, 4, and 5: Cce/chch, or champagne
6, 7, and 8: Cch, or gold
9 and 10: CC, or red/deep red
This image is one of the best explanations of the C locus there is.
In a future post, I will go into the effect as seen on other genes, but for now, this is to explain in depth how it works.
Image property of Buckeye Kennels; used with permission.