The B Locus has two different alleles in it: ‘B’, and ‘b’. ‘B’ is the most dominant and ‘b’ is the most recessive.

B: Black-based
b: Chocolate-based

Gene involved: TYRP1 (tyrosinase related protein 1) gene (Fontanesi, 2021)

BLACK
The ‘B’ allele is the Black allele. All varieties that are not Chocolate or Lilac based 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 matter what it is bred to.) for Black. The following varieties have the Black allele:
Chestnut, Castor, Sandy Gray, Copper, Gray, Opal, Re.d, Fawn (Blue), Cream (Blue), Orange, Chinchilla, Blue Chinchilla, Squirrel, Sable Chinchilla, Smoke Pearl Chinchilla, Black/Orange Tricolor, Blue/Fawn Tricolor, Black Japanese, Blue Japanese, Black Magpie, Blue Magpie, Black, Blue, Tortoise, Blue Tortoise, Sable, Seal, Smoke Pearl, Black Silver Marten, Blue Silver Marten, Black Otter, Blue Otter, Black Fox, Blue Fox.
Black can carry Chocolate.

CHOCOLATE
The ‘b’ allele is the Chocolate allele. All Chocolate or Lilac based colors must have two of these alleles, and are automatically homozygous for self. The following varieties are examples of varieties with the Chocolate allele:
Cinnamon (Chocolate Agouti), Lynx, Red (Chocolate), Fawn (Lilac), Cream (Lilac), Orange (Chocolate), Chocolate Chinchilla, Lilac Chinchilla, Chocolate/Orange Tricolor, Lilac/Fawn Tricolor, Chocolate Japanese, Lilac Japanese, Chocolate Magpie, Lilac Magpie, Chocolate, Lilac, Chocolate Tortoise, Lilac Tortoise, Chocolate Silver Marten, Lilac Silver Marten, Chocolate Otter, Lilac Otter, Chocolate Fox, Lilac Fox.

References PDFs