Siamese Colour Genetics Course · Lesson 3 of 8
Ask a new breeder what colour a seal point is and they’ll say “dark brown.” Ask what’s underneath it and you often get a blank look. Yet the Siamese base colour — seal, chocolate or cinnamon — is the single most misread thing in a pedigree, because a seal point routinely hides a chocolate or cinnamon gene it never shows. Get this one gene straight and half the surprises in your litters stop being surprises.
In Lesson 2 we saw how the colourpoint gene decides where colour appears. This lesson is about which colour appears on those points — the base colour, set by a single gene with three versions.
In this lesson
- What the B locus is and the three base colours it produces
- The dominance order: seal over chocolate over cinnamon
- How a seal point can secretly carry chocolate or cinnamon
- How to read base colour from a pedigree
- Why cinnamon is comparatively rare in the breed
The B locus — one gene, three versions
Base colour is controlled by a single gene at the B locus, and it comes in three versions (alleles):
- B — produces black pigment, which on a pointed cat shows as seal, the rich dark brown everyone pictures first.
- b — produces chocolate, a warm milk-chocolate brown.
- b1 — produces cinnamon, a lighter, warmer reddish-brown.
They sit in a strict pecking order: B is dominant over b, and b is dominant over b1. Black beats chocolate, and chocolate beats cinnamon. A cat carries two copies of this gene (one from each parent), and the more dominant of the two is the colour you see.
What each genotype looks like
Because there are three alleles in a dominance ladder, the combinations map neatly onto the coat:
- Seal point — any cat with at least one B copy: B/B, B/b or B/b1. It looks identical in all three cases.
- Chocolate point — b/b, or b/b1 (chocolate carrying cinnamon). No B copy present.
- Cinnamon point — only b1/b1, two copies of the lowest allele.
This is the same dominant/recessive machinery from Lesson 1, just with three cards in the deck instead of two.
Why a seal so often carries a hidden colour
Here’s the crux. A seal point can be B/B (carrying nothing but black), B/b (carrying chocolate) or B/b1 (carrying cinnamon). All three are seal to the eye — there is no visible difference. The recessive colour rides along underneath, invisible, until it meets a matching recessive from the other parent.
This is why two seal points can produce a chocolate kitten, and why breeders talk about a line “throwing chocolate.” The chocolate was there all along, carried quietly by seals for generations, waiting for two carriers to be mated together. A chocolate point, similarly, can be b/b (carrying nothing lower) or b/b1 (carrying cinnamon) — so two chocolates can occasionally produce a cinnamon.
Reading base colour from a pedigree
You can’t see a carrier, but a pedigree often reveals one. A few reliable rules:
- If a cat has produced a chocolate kitten, it carries chocolate — regardless of its own colour. That’s proven, not guessed.
- If a cat has a chocolate or cinnamon parent, it inherited that recessive allele and carries it.
- A cinnamon point (b1/b1) passes b1 to every one of its kittens — so all its offspring carry cinnamon at minimum.
This is exactly where good records earn their keep. A pedigree that notes the colours of ancestors and the colours they produced lets you track which recessives are travelling through your lines — the difference between guessing at a pairing and knowing what it can throw.
GCCF colour names
A quick note on naming, because it trips people up. Geneticists call the dominant allele “black,” but the GCCF breed standard — and every show catalogue you’ll read — calls the pointed version seal. Use seal, chocolate and cinnamon for Siamese; “black,” “brown” and “sorrel” belong to other breeds and registries. Getting the names right matters when you register a litter or fill in a show entry.
Worked example: two seals carrying chocolate
Mate a seal point carrying chocolate (B/b) to another seal point carrying chocolate (B/b). Each kitten draws one base-colour allele from each parent, giving four equally likely combinations:
- B + B → B/B — seal, carries nothing
- B + b → B/b — seal, carries chocolate
- b + B → B/b — seal, carries chocolate
- b + b → b/b — chocolate
Expected across many litters: three seal to one chocolate — and two of those three seals are themselves chocolate carriers. As always, that’s a probability averaged over many kittens, not a guarantee in any one litter of four. But it explains, precisely, how two seal parents produce a chocolate: each quietly carried the b allele, and one kitten drew it from both.
Why cinnamon is rare
Cinnamon (b1/b1) asks a lot of a pairing. Both parents must carry the b1 allele, and each must pass it to the same kitten — and because b1 sits at the bottom of the dominance ladder, it’s easily masked by both B and b for generations without ever being seen. On top of that, b1 is simply less common in the breed’s gene pool than b. Fewer carriers, and a recessive that hides beneath two colours rather than one, means cinnamon points (and their dilute partner, fawn) turn up far less often than chocolate. When you do breed one, it’s a genuine event — and every kitten it produces will carry cinnamon onward.
Want to see how base colour combines with everything else? The Siamese colour-point predictor lets you set each parent’s base colour and what they carry, and returns every possible kitten colour with its probability.
Key takeaways
- Base colour is one gene, the B locus, with three alleles: B (seal), b (chocolate) and b1 (cinnamon).
- Dominance runs seal > chocolate > cinnamon — the more dominant of a cat’s two copies is the colour you see.
- A seal can carry chocolate or cinnamon invisibly; a chocolate can carry cinnamon — which is how “surprise” colours appear.
- A proven produced kitten, or a chocolate/cinnamon parent, reveals a carrier in the pedigree.
- Cinnamon is rare because b1 is the lowest allele and less common in the gene pool.
In Lesson 4 we add the gene that pales every one of these colours down — dilution, which turns seal into blue, chocolate into lilac and cinnamon into fawn.
See these base colours in detail: seal point, chocolate point and cinnamon point.
