Cat Coat Genetics Reference: B, D, O, and A Loci Explained
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Full bio →Overview: The Four Main Color Loci
Cat coat color is determined by four independent gene loci, each controlling a different aspect of pigmentation. The complete color of a cat is the combined result of all four loci simultaneously.
| Locus | Gene Symbol | Controls | Chromosome |
|---|---|---|---|
| B (Brown) | B/b/bl | Base pigment type: black, chocolate, or cinnamon | Autosomal |
| D (Dense) | D/d | Full (dense) or diluted pigment expression | Autosomal |
| O (Orange) | O/o | Orange (phaeomelanin) vs black/brown (eumelanin) | X-linked |
| A (Agouti) | A/a | Tabby banding vs solid (non-agouti) | Autosomal |
Use the Cat Coat Calculator to run full Punnett square crosses across all four loci.
The B Locus: Black, Chocolate, and Cinnamon
The B locus determines the form of eumelanin (dark pigment) produced in the hair:
| Genotype | Phenotype | Notes |
|---|---|---|
| BB | Black | Homozygous dominant |
| Bb | Black (chocolate carrier) | Appears black; carries chocolate |
| Bbl | Black (cinnamon carrier) | Appears black; carries cinnamon |
| bb | Chocolate (brown) | Homozygous recessive |
| bbl | Chocolate (cinnamon carrier) | Appears chocolate; carries cinnamon |
| blbl | Cinnamon | Requires two cinnamon alleles |
Dominance hierarchy: B > b > bl (black is dominant over chocolate, which is dominant over cinnamon)
What this means visually:
- Black: dense dark pigment in the eumelanic range
- Chocolate: warm brown, visually distinct from black
- Cinnamon: a reddish-brown, distinctly lighter and warmer than chocolate
Combined with dilution (D locus):
- Dilute black = Blue (grey)
- Dilute chocolate = Lilac (lavender-grey)
- Dilute cinnamon = Fawn (buff)
The D Locus: Dense vs. Dilute
The D locus controls how pigment granules are distributed in the hair shaft:
| Genotype | Phenotype | Notes |
|---|---|---|
| DD | Dense (full color) | Homozygous dominant |
| Dd | Dense (dilute carrier) | Appears full color; carries dilute |
| dd | Dilute | Both alleles required for dilution |
Dilution effect: The dilute allele (d) causes melanin granules to clump and be distributed unevenly, resulting in a lighter, washed-out appearance:
| Dense | Dilute version |
|---|---|
| Black | Blue (grey) |
| Chocolate | Lilac |
| Cinnamon | Fawn |
| Red/orange | Cream |
Important: dd is required to show dilution. A Dd cat appears fully saturated — it is a dilute carrier but does not appear dilute.
The O Locus: Orange (X-linked)
The O locus is unique because it is X-linked — located on the X chromosome. This means:
- Males (XY) have only one X chromosome and express whatever O allele they carry
- Females (XX) have two X chromosomes and can carry one of each allele
| Sex | Genotype | Phenotype |
|---|---|---|
| Male | X^O Y | Orange/red |
| Male | X^o Y | Non-orange (black/brown by B locus) |
| Female | X^O X^O | Orange/red |
| Female | X^O X^o | Tortoiseshell (orange AND non-orange patches) |
| Female | X^o X^o | Non-orange |
Why tortoiseshell cats are almost always female: Tortoiseshell requires one O and one o allele on separate X chromosomes — a condition only females can have (males have only one X). The rare male tortoiseshell is almost always XXY (Klinefelter syndrome) and is typically sterile.
X-inactivation: In each cell of a female cat’s body, one X chromosome is randomly inactivated. The orange/non-orange patchwork of a tortoiseshell is the visual expression of this cellular-level X-inactivation — each patch of color represents a clone of cells where the same X chromosome is active.
The A Locus: Agouti (Tabby vs. Solid)
The A locus controls whether the tabby banding pattern is expressed in the coat:
| Genotype | Phenotype | Notes |
|---|---|---|
| AA | Tabby (agouti) | Dominant; hair shafts have alternating dark/light bands |
| Aa | Tabby (solid carrier) | Appears tabby |
| aa | Solid (non-agouti) | Pigment is uniform throughout the hair shaft |
What solid means: An aa cat has uniform pigment throughout each hair — no banding, no tabby striping (in the eumelanic color range).
Exception — orange cats: The O allele (orange) overrides the agouti suppression in orange eumelanin. Even if a cat is aa (non-agouti), its orange areas will show tabby striping. This is why all-orange cats appear striped regardless of their A locus genotype.
How the Loci Interact: Color Result Table
The final coat color is the combined result of all four loci:
| O locus | B locus | D locus | A locus | Color |
|---|---|---|---|---|
| o/o (fem) or o (male) | BB/Bb | DD/Dd | AA/Aa | Black tabby |
| o/o or o | BB/Bb | DD/Dd | aa | Black solid |
| o/o or o | bb | DD/Dd | — | Chocolate |
| o/o or o | BB/Bb | dd | — | Blue (dilute black) |
| o/o or o | bb | dd | — | Lilac |
| O/O (fem) or O (male) | any | DD/Dd | — | Red/orange (tabby) |
| O/O (fem) or O (male) | any | dd | — | Cream |
| O/o (fem only) | BB/Bb | DD/Dd | — | Tortoiseshell (black + orange) |
| O/o (fem only) | BB/Bb | dd | — | Blue-cream tortoiseshell |
| O/o (fem only) | bb | DD/Dd | — | Chocolate tortoiseshell |
Loci NOT Covered by the Calculator
The calculator models the four main loci above. Other genes affecting coat color/pattern are not modeled:
| Gene | Effect | Why Excluded |
|---|---|---|
| W (White masking) | Dominant white masks all color loci | Requires veterinary genetic testing to identify true genotype |
| S (White spotting) | Produces calico/bicolor patterns | Complex graded expression; not Mendelian simple |
| T (Tabby type) | Controls pattern within tabby (mackerel, blotched, spotted) | Separate from color |
| Ly (Colorpoint) | Siamese/Burmese point coloring | Requires additional locus |
| Dm (Dilute modifier) | Caramel/apricot variants | Rare; complex expression |
For testing these additional loci, UC Davis Veterinary Genetics Laboratory offers cat coat color DNA panels.
For plain-English definitions of every term used above (eumelanin, hemizygous, agouti, carrier, etc.), see the Cat Coat Genetics Glossary. To walk through Punnett square crosses step by step, see How to Predict Kitten Coat Colors. For breeders applying these loci to real pairing decisions, see Cat Genetics for Breeders.