Siamese Colour Genetics Course · Lesson 4 of 8
Let me settle an argument I hear at shows every season: there is no such thing as a grey Siamese. What people are looking at when they say “grey” is a blue point — and that soft, cool colour comes from Siamese dilution, a single recessive gene that pales blue, lilac and fawn out of the base colours you met in the last lesson. Once you understand dilution, half the Siamese colour chart falls into place.
In Lesson 3 we set the base colour — seal, chocolate or cinnamon. This lesson adds the gene that decides how densely that colour is laid down.
In this lesson
- What dilution is and how the D locus works
- How dilution turns seal into blue, chocolate into lilac and cinnamon into fawn
- Why a “grey” Siamese is really a blue point
- How dense-coated cats carry dilute without showing it
- Why lilac is a dilute chocolate, not a pale blue
The D locus — dense or dilute
Dilution is controlled by a single gene at the D locus, with two versions. D (dense) is dominant and lays pigment down thickly along the hair, giving the full, deep base colour. d (dilute) is recessive and clumps the pigment unevenly, so the hair reflects more light and the colour reads as a softer, cooler, paler version of itself.
Because d is recessive, a cat needs two copies — dd — to show dilution. One D is enough to keep the coat dense. It’s the same recessive rule as chocolate: the pale version only appears when both copies agree.
Every base colour has a dilute partner
Dilution doesn’t invent a new colour — it softens whatever base colour is already there. Each base has a fixed dilute partner:
- Seal → blue — a cool, even grey.
- Chocolate → lilac — a pinkish, dove-grey (also spelled “lilac” in the GCCF standard).
- Cinnamon → fawn — a soft, warm mushroom.
So a cat’s final colour is really two decisions stacked together: the base-colour gene picks seal, chocolate or cinnamon, and the dilution gene decides whether you see it at full strength or in its paler partner. A blue point is genetically a seal that happens to be dd. A lilac is a chocolate that happens to be dd.
Why lilac is a dilute chocolate, not a pale blue
This one catches people out, because a blue and a lilac can look similar in a photo. But they come from different base colours. Blue is dilute seal (a black-based colour), so it reads cool and grey. Lilac is dilute chocolate (a brown-based colour), so it keeps a warm, pinkish cast — a dove-grey with a hint of taupe rather than a true cool grey. Genetically a lilac is b/b dd: chocolate at the B locus, dilute at the D locus. It is not “a paler blue” — it’s a different base colour that has also been diluted. Reading a colour photo can be tricky, so when the shade is ambiguous, the pedigree (which base colours are in the line) settles it.
Carrying dilute — very common in the breed
A dense-coated cat can be D/D (carrying no dilute) or D/d (carrying dilute), and the two look identical — both are full, dense colour. Dilute is widespread in Siamese lines, so a great many seals and chocolates quietly carry it. That’s why blue and lilac kittens appear in litters from two dense parents: both parents carried the d allele, unseen, and passed it to the same kitten.
Worked example: two seals carrying dilute
Mate a seal point carrying dilute (D/d) to another seal point carrying dilute (D/d). Each kitten draws one D-locus allele from each parent:
- D + D → D/D — seal, carries no dilute
- D + d → D/d — seal, carries dilute
- d + D → D/d — seal, carries dilute
- d + d → dd — blue
Expected across many litters: three dense (seal) to one dilute (blue), with two of the three seals carrying dilute themselves. If you’re getting the occasional unexpected blue in a seal litter, this is your answer — both parents carry dilute. And because base colour and dilution are separate genes, a seal that carries both chocolate and dilute can, with the right mate, produce seal, chocolate, blue and lilac in a single litter — exactly the four-colour litter from Lesson 1.
The Siamese colour-point predictor handles base colour and dilution together — tell it what each parent is and carries, and it works out how many blues and lilacs a pairing can throw.
Key takeaways
- Dilution is one recessive gene at the D locus; a cat must be dd to show it.
- It pales each base colour into a fixed partner: seal→blue, chocolate→lilac, cinnamon→fawn.
- There is no “grey” Siamese — what looks grey is a blue point (dilute seal).
- Dense cats (D/d) commonly carry dilute unseen; two carriers can produce dilute kittens.
- Lilac is a dilute chocolate (warm, pinkish) — not a pale blue.
In Lesson 5 we meet a gene most guides skip entirely — the caramel modifier, which acts only on dilute cats and produces the cool, metallic caramels and apricots that confuse so many breeders.
See the dilute colours in detail: blue point, lilac point and fawn point.
