The seven types of genes can interact among them, so it is a little complex to explain their function.
Fortunately Jane A kelley did it very well in this post on catster:
- 1) B gene-> Black versus non Black fur.
B gene has three alleles, B the dominant, and b and b' that are recesives. Cats with at least one copy of B are black. b produces chocolate color, and b' cynamon color.
- 2) A gene-> Agouti versus Non Agouti.
This gene controls the coat’s “ticking,” or banding of colors on a cat’s fur. The dominant allele, A, produces banded furs which help to create the tabby pattern. The recessive allele, a, makes the cat’s fur a solid color from tip to root.
- 3) T gene-> Tabby versus Non-Tabby
If a cat inherited the A allele, the T gene determines what kind of tabby pattern will be produced. The dominant allele, T, produces the mackerel or striped tabby coat; the Ta allele produces an all-agouti tabby like the Abyssinian; and the recessive tb allele produces the classic, or blotched, tabby.
-4) D gene-> Dense versus Dilute color
The dominant D, or full-pigmented, allele produces cats that are black, brown, or orange. The recessive d allele produces paler colors like gray (known as blue to breeders), tan, or cream.
-5) O-> Orange versus non Orange
This gene actually can mask other coat colors, including black. Although the o, or non-orange, allele is much more common, if a cat gets the dominant O allele, any other colors will be covered up and the cat will be orange.
-6) S-> Spots versus No Spots
The dominant S allele produces white spotting that masks the cat’s true color in the areas where the spots occur. This produces “cow kitties,” and cats with white “lockets” or white boots and mittens. It even produces cats that are entirely white because their fur is just one giant white spot. The recessive s allele is actually the normal expression of this gene; it produces no white fur at all.
7. W-> White versus Non-White
The W gene is known as a masking gene. That means if the dominant W allele is present, the cat will be white, no matter what the other coat color and pattern alleles say. The recessive w allele produces full expression of any of the other color traits.
With all these ingredients, we have that to get the beautiful solid brown color which is characteristic of Havana breed, and that it is a true brown color, not just a fadded black, the cat has to carry two copies of the recesive b allele of the B gene that produces chocolate color and also two copies of the recesive a alelle of A gene (non aguti color), that produces a solid color, and at least one D alelle that also acts on color to obtain dense color. Besides this, the cat has not carry copies of the genes that masks colors, such as S, and O, both dominant alelles.
This explains how difficult is to obtain a solid brown color cat, in fact they are very rare.