The Agouti Locus - A
The Agouti locus controls the formation of the Agouti protein, that in turn is one of the mechanisms that controls the replacement of eumelanin with phaeomelanin in the growing hair. The alleles of the Agouti locus can affect not just whether or not the eumelanin->phaeomelanin shift occurs, but also where on the dog's body this happens.
The probable alleles at the Agouti locus, in order of decreasing dominance, are: Ay, aw, as, at and a.
Dominant Black. Dogs certainly do have a dominant form of black that is indeed very dominant: completely obliterating all formation of phaeomelanin pigment. Traditionally, dominant black has been placed at the head of the Agouti series (symbol As). It is now believed to be part of a separate series (the K series - see below) and not at the Agouti locus at all. This is in keeping with the operation of the Agouti locus in all other mammals that have it: increasing dominance of Agouti locus alleles results in increasing production of phaeomelanin without exception. We mention it here simply because it has long been thought, mistakenly, to be part of the Agouti locus.
So at the top of the Agouti series then we have Ay, Sable - also known as 'dominant yellow' or 'golden sable'. This results in an essentially phaeomelanic phenotype, but the hair tips are eumelanin (black). The extent of the eumelanin tip varies considerably from lighter sables (where just the ear tips are black) to darker sables - where much of the body is dark. It is possible that Ay is not completely dominant over the lower Agouti series alleles, with an Ay heterozygote e.g. Ayat having a darker body. AyAy may be called 'clear red' whereas Ayat can be 'sabled red'. Sable is a very common colour in many breeds of dog, e.g. German Shepherd.
Next we have aw, 'wolf' colour. This is like Ay but the tan is replaced with a pale grey/cream colour and the hairs usually have several bands of light and dark colour, not just the black tip of sable. Seen in the Keeshond, Siberian and Norwegian Elkhound.
Moving down the series we next come to as, 'saddle tan'. This is somewhat like the black+tan allele (below), except that eumelanin is restricted to the back and side regions, hence the name 'saddle'. It is also possible that this is due to another gene interacting with at/at genotypes.
Allele at, 'black+tan' is next. This is a primarily black dog but with tan (phaeomelanin) markings around the eyes, muzzle, chest, stomach and lower legs. Commonly seen in hounds, Dobermann's and Rottweilers.
Finally, at the bottom of the Agouti series is recessive black, symbol a. When a dog is homozygous for recessive black (aa), it will have no phaeomelanin in its coat (unless it is also ee, which is epistatic to the Agouti series - see below). Examples of breeds that exhibit recessive black are German Shepherd and Shetland Sheepdog. Whilst some breeders discount the presence of a recessive black at the bottom of the Agouti series is is consistent with the behaviour of this locus in many other mammals.
The existence of all these alleles in the Agouti series is not certain, nor is the precise order of dominance of the intermediate alleles aw as and at.