Cat Coat Genetics Glossary: Key Terms and Definitions
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Full bio →Why Genetics Terms Matter for Coat Color Prediction
Reading cat genetics results — from a DNA test report, a breeder’s pedigree, or the output of the Cat Coat Calculator — requires knowing what the terms mean. This glossary defines every term used in cat coat color genetics in plain English.
A
Allele One version of a gene at a specific locus. Most cats carry two alleles at each autosomal locus (one inherited from each parent). The combination of alleles determines which trait is expressed.
Example: At the B locus, a cat may carry alleles B and b — one for black, one for chocolate.
Agouti A coat pattern in which each individual hair has alternating bands of light and dark pigment. Cats with at least one A allele (A_) show a tabby pattern. Cats that are aa (non-agouti) have solid, uniformly pigmented coats. See also: A locus, non-agouti, tabby.
A Locus (Agouti locus) The gene controlling tabby vs. solid coat expression. A (agouti) is dominant over a (non-agouti). A cat must inherit a from both parents (aa) to have a solid coat. Exception: orange cats appear tabby regardless of A locus because the O allele overrides agouti suppression in phaeomelanin.
Autosomal Located on a non-sex chromosome. The B, D, and A loci are all autosomal — they are inherited equally from both parents regardless of sex. Contrast with X-linked.
B
B Locus (Brown locus) Controls the type of eumelanin (dark pigment) a cat produces. Three alleles in order of dominance: B (black) > b (chocolate) > bl (cinnamon). A cat needs two copies of b to appear chocolate, and two copies of bl to appear cinnamon.
Bicolor A coat pattern with two colors — usually white combined with another color. Controlled by the S locus (white spotting gene), which is separate from the B, D, O, and A loci covered by the Cat Coat Calculator.
C
Carrier A cat that carries a recessive allele without expressing it. Example: a black cat with genotype Bb appears black but carries the chocolate allele (b). It can pass b to offspring. DNA testing is the only reliable way to confirm carrier status in visually ambiguous cats.
Calico A coat pattern with orange, black, and white patches. Genetically identical to a tortoiseshell at the O locus (X^O X^o), but also carries the white spotting gene (S locus). See the Tortoiseshell and Calico Genetics article for detail.
Cinnamon A warm reddish-brown coat color produced when a cat is homozygous for the bl allele (blbl) at the B locus. Rarer than chocolate. Cinnamon diluted by dd becomes fawn.
Chocolate A warm brown coat produced when a cat is homozygous recessive (bb) at the B locus. Chocolate + dd (dilute) = lilac.
D
D Locus (Dense/Dilute locus) Controls pigment granule distribution in the hair shaft. D (dense) is dominant; d (dilute) is recessive. A cat must be homozygous dd to show dilution. One or two copies of D always produce full, saturated color.
Dense Full, saturated color expression. A cat with genotype DD or Dd shows dense color at the D locus. Contrast with dilute.
Dilute A lighter, washed-out version of a dense color. Requires homozygous dd. Dilute effects:
| Dense | Dilute (dd) |
|---|---|
| Black | Blue (grey) |
| Chocolate | Lilac |
| Cinnamon | Fawn |
| Red/orange | Cream |
Dominant An allele that is expressed even when paired with a different allele. A single copy is sufficient to produce the trait. Example: B (black) is dominant over b (chocolate) — a Bb cat appears black, not chocolate.
E
Eumelanin The dark pigment responsible for black, chocolate, cinnamon, blue, lilac, and fawn coat colors. The B locus and D locus both act on eumelanin. Contrast with phaeomelanin (orange/red pigment). For a full breakdown of how loci interact, see the Cat Genetics Loci Reference.
F
Fawn A warm buff/beige coat color produced when a cat is homozygous for cinnamon (blbl) AND homozygous dilute (dd). Rare — requires two recessive cinnamon alleles and two recessive dilute alleles.
G
Genotype The actual genetic makeup of a cat at a given locus or across multiple loci. May differ from the phenotype (visible appearance). Example: a black cat may have genotype Bb (carrier) or BB — both look black.
H
Hemizygous Having only one allele at a locus, rather than the typical two. Male cats are hemizygous at the O locus because they have only one X chromosome. A hemizygous orange male is X^O Y — he has one O allele, which is fully expressed (no second allele to interact with).
Heterozygous Carrying two different alleles at a locus (e.g., Bb, Dd, or X^O X^o). Heterozygous at the O locus in females produces tortoiseshell coloring.
Homozygous Carrying two identical alleles at a locus (e.g., BB, bb, DD, dd). Required for recessive traits: dd produces dilution; bb produces chocolate; blbl produces cinnamon.
L
Lilac A pale grey-lavender coat produced when a cat is chocolate (bb) AND dilute (dd). Often called “lavender” in some breed standards.
Locus (plural: loci) A specific location on a chromosome where a gene sits. Cat coat color involves four main loci: B, D, O, and A. Each locus operates independently (Mendel’s law of independent assortment applies to autosomal loci).
N
Non-agouti Genotype aa at the A locus. Suppresses tabby banding so the coat appears solid. Contrast with agouti (A_) cats, which show tabby striping. Note: orange cats with genotype aa still show tabby stripes in orange areas due to O locus override.
O
O Locus (Orange locus) An X-linked gene controlling whether a cat produces orange/red phaeomelanin (O allele) or non-orange eumelanin (o allele). The O allele is epistatic to the B locus — it overrides black/chocolate/cinnamon and replaces it with orange.
Epistasis: When one gene overrides or masks the effect of another gene.
P
Phaeomelanin The orange/red pigment responsible for ginger, red, cream, and tortoiseshell orange areas. Produced when the O allele is active at the O locus. The D locus dilutes phaeomelanin: red/orange → cream.
Phenotype The observable physical trait — what a cat actually looks like. A cat’s phenotype is determined by its genotype, but one phenotype (e.g., “black”) can correspond to multiple genotypes (BB, Bb, Bbl).
Punnett Square A grid used to map all possible allele combinations from a specific cross between two parents. Each parent’s gametes are listed on each axis; the inner cells show all possible offspring genotypes. The Cat Coat Calculator runs multi-locus Punnett squares automatically.
R
Recessive An allele that is only expressed when two copies are present (homozygous). A single recessive allele is hidden by a dominant allele. Examples: b (chocolate), d (dilute), a (non-agouti) are all recessive.
S
S Locus (White spotting) Controls the presence and amount of white in the coat. Not covered by the Cat Coat Calculator. SS = high white (mostly white); Ss = moderate white spotting; ss = no white. Determines whether a tortoiseshell becomes a calico.
T
Tabby A coat with a repeating pattern of darker stripes, spots, or swirls on a lighter background. Produced when a cat has at least one A (agouti) allele. The specific pattern (mackerel stripes, blotched, spotted, ticked) is controlled by the T locus, which is separate from the color loci.
Tortoiseshell A coat combining orange and non-orange patches in mosaic form. Produced in female cats that are heterozygous at the O locus (X^O X^o). The patches arise from random X-chromosome inactivation in each skin cell during fetal development.
X
X-inactivation (Lyonization) The process by which one X chromosome in each cell of a female cat is permanently inactivated early in fetal development. In tortoiseshell cats, cells where X^O is active produce orange pigment and cells where X^o is active produce dark pigment — creating the characteristic mosaic patch pattern. Named after geneticist Mary Lyon.
X-linked Located on the X chromosome. The O locus is X-linked. Because males have only one X chromosome (hemizygous), they cannot be tortoiseshell — that requires two X chromosomes with different O alleles.
To see these terms applied in practice, use the Cat Coat Calculator to run a full Punnett square cross. For step-by-step instructions on predicting kitten colors using these genetic concepts, see How to Predict Kitten Coat Colors. For breeders using genotype data to plan litters, see Cat Genetics for Breeders.