Siamese Colour Genetics Course · Lesson 1 of 8
At a show a couple of seasons ago, I stood with a new breeder in front of a litter of four kittens. Both parents were seal points. The litter had one seal, one blue, one chocolate and one lilac — not a colour that matched either parent. The breeder’s face said: how? This lesson answers that question.
Cat coat colour inheritance isn’t complicated once you understand a handful of rules. By the end of this lesson you’ll know what it means for a gene to be dominant or recessive, what it means for a cat to “carry” a colour it doesn’t show, and why — with a simple mental model — you can predict the range of colours a pairing can produce before a single kitten is born.
In this lesson
- What genotype and phenotype mean — and why they differ
- What makes a gene dominant or recessive
- What it means for a cat to “carry” a colour it doesn’t show
- The 1-in-4 rule when both parents carry the same recessive
- How to work out the range of colours a pairing can produce
What a cat carries vs. what it shows
Every cat has two copies of each colour gene — one inherited from its mother, one from its father. The combination of copies it actually holds is called its genotype. What you see in the coat — the colour visible to the eye — is the phenotype. The gap between those two things is where all the surprises in a litter come from.
A seal point looks seal. But whether it’s carrying a hidden chocolate gene underneath that seal coat is invisible to the eye. Two cats with identical phenotypes can have very different genotypes — and that difference only reveals itself in the kittens.
The gene lottery — how inheritance actually happens
When a kitten is conceived, it doesn’t inherit either parent’s full genetic profile. It draws one randomly selected copy of each gene from each parent. Each parent has two copies of the gene; the kitten picks one of those two at random — independently, for each gene, for each parent.
This means that for any given gene, there are exactly four possible combinations a kitten can inherit from two parents:
- Copy 1 from Mum + Copy 1 from Dad
- Copy 1 from Mum + Copy 2 from Dad
- Copy 2 from Mum + Copy 1 from Dad
- Copy 2 from Mum + Copy 2 from Dad
Each of these is equally probable. Each kitten draws independently — what the previous kitten inherited has no influence on the next one. This is the foundation of the Punnett square (which we’ll use in full in Lesson 8). For now, the key is: four equally likely outcomes, every kitten a fresh draw.
Dominant and recessive — when one copy is enough
Not all gene copies behave equally. Some are dominant: a single copy is enough to express in the coat, regardless of what the other copy is. Some are recessive: the colour only shows when the cat has two copies of the recessive gene — one from each parent.
Take the B locus, which sets base colour. The dominant version, written B, produces black pigment — seen on a Siamese as the dark, rich brown we call seal. The recessive version, b, gives chocolate, a warm milk-brown. There’s a third, even more recessive version, b1, which gives cinnamon — but we’ll come back to that in Lesson 3.
For now, focus on seal vs. chocolate. A cat with two dominant copies (B/B) is seal. A cat with one of each (B/b) is also seal — because the dominant B masks the recessive b completely. Only a cat with two recessive copies (b/b) can actually show the chocolate colour.
This is the rule that explains everything: a recessive colour can only appear when both copies of that gene are recessive. One dominant copy is enough to override it — and to hide it.
Carrying — the hidden gene
What happens to a B/b cat? It looks seal — indistinguishable, coat-to-coat, from a B/B seal. But it has one hidden chocolate gene. When it breeds, it will pass that chocolate copy to roughly half its kittens. If one of those kittens also receives a chocolate copy from the other parent, that kitten will be chocolate — even though neither parent showed the colour.
This is what breeders mean when they say a cat “carries” chocolate. The cat doesn’t express the colour itself. It carries one copy of the recessive gene, invisible in the coat, and passes it on.
Carrying is the norm in Siamese breeding, not the exception. Seal is the dominant base colour, and many lines carry chocolate or cinnamon layered beneath it — sometimes unknowingly, for generations. A seal cat can carry chocolate (B/b), carry cinnamon (B/b1), or carry neither (B/B). You cannot tell which by looking at the cat.
Why you can’t see a carrier — and what to do about it
Visual assessment of coat colour tells you the phenotype. It tells you nothing definitive about the genotype. A B/B seal and a B/b seal look identical. A D/D (dense, non-carrier of dilute) seal and a D/d (carrying dilute) seal look identical. There is no visible marker for carrying.
This is where records, pedigrees and test matings earn their value. Three practical approaches:
- Pedigree analysis. If a stud cat has produced chocolate kittens from any queen, he carries chocolate — that’s established regardless of his own colour. If a line has produced a recessive colour reliably across generations, assume its descendants carry it until you have evidence otherwise.
- Test matings. Breed the cat to a known recessive (e.g. a confirmed chocolate queen) and look at the litter. If any kitten is chocolate, the cat carries it. A clean litter — all seal — gives you better confidence but doesn’t prove the cat is clear: with only one gene in question, a B/B cat and a B/b cat can both produce an all-seal litter from a single mating by chance.
- DNA testing. Currently available for some colour genes in some breeds. Worth checking with the Siamese and Oriental breed health advisors for the current picture. Where a reliable test exists, it resolves the question definitively.
For most breeders working with established lines, pedigree analysis and careful record-keeping do most of the work. This is one reason why a complete pedigree — tracking not just names and titles but the colours of ancestors and their offspring — is one of the most useful tools in a breeding programme.
The 1-in-4 rule — a mental model for recessive pairings
When both parents carry the same recessive gene, the expected outcome is that roughly one in four kittens shows the recessive colour. Here’s why.
Say both parents carry chocolate: each is B/b. Each kitten independently draws one copy from each parent. The four equally likely combinations are:
- B from Mum + B from Dad → B/B — seal, does not carry chocolate
- B from Mum + b from Dad → B/b — seal, carries chocolate
- b from Mum + B from Dad → B/b — seal, carries chocolate
- b from Mum + b from Dad → b/b — chocolate
Expected ratio: one chocolate to three seals — but two of those three seals are themselves carriers, which matters for future generations. The carrying doesn’t disappear from the line when you don’t see chocolate in a litter; it just waits for a second recessive to meet it.
One important caveat: these are probabilities averaged over many litters, not guarantees in any single one. A litter of four from this pairing might produce no chocolate kittens at all, or it might produce two. Over hundreds of litters, the ratio approaches 1 chocolate to 3 seals. In any given litter, you’re rolling the dice four times with a one-in-four chance each time.
Worked example: what two seals can produce
Let’s combine two genes. Both parents are seal carrying chocolate (B/b) and seal carrying dilute (D/d). Dilute works identically to chocolate in terms of dominance — D is dominant, d is recessive, and dd gives you dilution (seal becomes blue, chocolate becomes lilac). We now have two independent recessives in play.
From one gene working alone: 1-in-4 kittens show the recessive. With two independent genes, those probabilities multiply. A kitten that draws b/b and d/d is both chocolate and dilute — which gives lilac. The full range of possible colours from this one pairing:
- Seal (B/- D/-) — looks seal, may carry one or both recessives
- Chocolate (b/b D/-) — shows chocolate, may carry dilute
- Blue (B/- d/d) — shows dilution on a seal base
- Lilac (b/b d/d) — dilution on a chocolate base; the palest of the four
Four colours from two seal parents. Not magic — just Mendel, applied. This is exactly the litter I described at the start of this lesson, and exactly what the breeder I was watching didn’t yet know to expect. Now you do.
Use the predictor to check your own pairing
The Siamese colour-point predictor applies these same rules to any pairing you choose. Select each parent’s colour and what they carry, and it returns every possible kitten colour with its probability — base colour, dilution, caramel, red, tortie and tabby all handled. It’s the fastest way to sanity-check a mating before committing to it.
In Lesson 2, we’ll look at the gene that makes a Siamese a Siamese — the colourpoint mechanism, how temperature controls where pigment develops, and why every Siamese kitten is born almost white.
Key takeaways
- Every cat has two copies of each colour gene — one from each parent.
- Dominant genes show with a single copy; recessive colours need two.
- A cat can carry a recessive colour invisibly and pass it on.
- When both parents carry the same recessive, about one in four kittens shows it.
- What a cat shows (phenotype) can differ from what it carries (genotype) — which is how two seals produce four colours.
