The A Locus has three different alleles in it: ‘A’, ‘a(t)’, and ‘a’. ‘A’ is the most dominant, ‘a(t)’ is the next most dominant, and ‘a’ is the most recessive.

A: Agouti pattern
aᵗ: tan pattern
a: self pattern

Gene involved: ASIP (agouti signaling protein) gene (Fontanesi, 2021).

AGOUTI
The ‘A’ allele is the Agouti allele. All agoutis must have at least one of these alleles. If they have two of these alleles, then they are homozygous (Meaning it is “pure” for that allele, so that it cannot produce anything recessive to it no mater what it is bred to.) for Agouti. The following varieties have the Agouti allele:

Chestnut, Castor, Sandy Gray, Copper, Gray, Opal, Chocolate Agouti, Cinnamon, Lynx, Red, Fawn, Cream, Orange, Chinchilla, Blue Chinchilla, Squirrel, Chocolate Chinchilla, Lilac Chinchilla, Sable Chinchilla, Smoke Pearl Chinchilla, Tricolour*, Harlequin*

* A properly-colored Tricolor or Harlequin will typically but not always have the Agouti allele. A Tricolor or a Harlequin with Tortoise shading has the Self allele as well as Non-Extension (see E Locus).

Agouti can carry either Tan Pattern or Self.

TAN PATTERN
The 'aᵗ' allele is the Tan Pattern allele. All Tan Pattern animals must have at least one of these alleles. If they have two of these alleles, then they are homozygous for Tan Pattern. The following varieties have the Tan Pattern alleles:

Tan (Black, Blue, Chocolate, Lilac), Otter ((Black, Blue, Chocolate, Lilac), Silver Marten (Black, Blue, Chocolate, Lilac), Tortoise Marten (Black, Blue, Chocolate, Lilac), Fox (Black, Blue, Chocolate, Lilac), Martenized Himalayan/Pointed White (Black, Blue, Chocolate, Lilac)

Tan Pattern can carry Self.

SELF
The ‘a’ allele is the Self allele. All selfs must have two of these alleles, and are automatically homozygous for self. The following varieties have the Self allele:

Black, Blue, Chocolate, Lilac, Himalayan/Pointed White*, Tortoise (Black, Blue, Chocolate, Lilac), Sable, Seal, Smoke Pearl, Siamese Sable, Frosted Pearl (Black, Blue, Chocolate, Lilac), Sable Point.

* Frosted Pearl Himalayan/Pointed White and Agouti (Chinchilla) Point Himalayan/Pointed White animals have the Agouti allele, Martenized Himalayan/Pointed White animals have the Tan Pattern allele.

Note: Self group (black, blue, chocolate, lilac, REW, BEW) is not the same as genetically self! There is some overlap (black, blue, chocolate, and lilac are both in the self group and are genetically self). REW and BEW may not be genetically self. Many genetically self colors are not in the self group!

References PDFs