COLOUR GENETICS - A locus

The A locus is named AGOUTI. This is the gene which controls dark pigment patterning and determines where and how the dark pigment made by E will be deposited. Agouti and Saddle variations are tentatively shown here; the correct order of dominance in unclear, but it is theorized that they may both belong between [ A^y ] and [ a^t ]. Dominance appears to be [ A^y ] >[ a^w ] > [ a^st ] > [ a^t ] > [a]. To some extent the A series alleles appear to be incompletely dominant over each other. For instance; [ A^y ] appears to be incompletely dominant over [ a^t ], which may help to explain why some sable dogs appear to have tan points, or a tan mask. An [ A^y a^t ] dog may appear as a sable with tan points. The sable coat in a sable/tan Havanese is usually darker, having more black overlay and tips than a pure [ Ay Ay ] dog. In all likelihood, its not that simple and there may be other genes in play. To further complicate things, the A gene appears to have two separate and distinct parameters controlling its expression. One seems to control the colour of the head, neck and back (dorsal areas) and the other seems to control the chest, belly and legs (ventral areas). Remember. Phenotype is what you see and genotype is the possible genetic codes behind it.

[A^y] SABLE patterning is considered the most dominant allele on the A locus. Sable is a predominantly light coat with dark tipping or dark overlay. The light coat colour can be two or more shadings of colour.
Phenotype - dog displays sable
Genotype - [ A^yA^y ] or [ A^ya^w ] or [ A^ya^st ] or [ A^ya^t ]

[a^w] AGOUTI or WILD patterning is a banding of colour throughout each hair(dark/light/dark/light)
The appearance is similar to the fur of a wild animal such as a rabbit or wolf.
Phenotype - dog displays agouti (wild)
Genotype - [ a^wa^w ] or [ a^wa^st ] or [ a^wa^t ]

[a^st] SADDLE TAN and CREEPING TAN Starting out typical black and tan, the tan points gradually creep back to form larger areas of tan. In Creeping Tan, the tan expands through the face and ears. In Saddle Tan, the colour expands to the withers. The final result gives an appearance similar to a Yorkie. Creeping Tan / Saddle Tan are considered variations of Black and Tan.
Phenotype - dog displays saddle
Genotype - [a^sta^st ] or [ a^sta^t ]

[a^t] BLACK & TAN recessive [ a^t ], needing two copies to be expressed, produces the black and tan or point pattern.
It can be completely hidden or sometimes just partially obscured by other A patterns.
Phenotype - dog displays black and tan
Genotype - [a^ta^t ]