Siamese cat genetics: How colorpoint cats get their points

Level: High school · You’ll need: a monohybrid cross and dominant and recessive alleles · Part of cat genetics

Colorpoint cat genetics comes down to one gene and one heat-sensitive enzyme. Siamese cats carry two copies of the colorpoint allele cs of the TYR gene. It makes a version of the pigment enzyme tyrosinase that only works in cooler skin, so dark color develops on the face, ears, paws and tail (the points) while the warmer body stays pale.

Assumptions for the crosses on this page: one gene (TYR, the “C” locus) with the alleles below, every other gene held the same, and enough kittens for the ratios to show. Probabilities describe each kitten’s chances, not a promise for a particular litter.

The colorpoint gene and its alleles

Tyrosinase is the enzyme that starts making melanin pigment. Cat geneticists call its gene the C locus. The three alleles taught in most courses are:

C is dominant to both other alleles, so a C/cs cat looks fully colored but carries Siamese. The cb and cs alleles are different: a cb/cs cat shows an in-between look called mink (the Tonkinese coat). Neither allele hides the other, so this pair is often taught as codominance or incomplete dominance.

A rarer fourth allele at the same gene gives a blue-eyed albino cat. The cat coat color calculator includes C, cb and cs.

Why the points are dark: temperature and the enzyme

The cs enzyme folds properly and works only below body temperature. The skin on the ears, nose, paws and tail is a few degrees cooler than the trunk, so pigment is made there and hardly anywhere else.

This explains three things owners notice:

• Kittens are born almost white. The womb is evenly warm, so there’s nowhere cool enough for pigment. Points darken over the first weeks of life.
• Pointed cats often darken with age or in cold climates, as more of the body surface runs cooler.
• Fur can regrow darker after a patch is shaved, because bare skin runs cooler while the hair grows back.

The same gene and the same environment make a gene–environment interaction: the genotype sets what’s possible, and temperature decides where it shows. It’s one of the clearest examples in beyond Mendelian inheritance.

Pointed cats also have blue eyes. The iris has very little pigment, and blue is how a low-pigment iris scatters light.

Point colors come from other genes

The colorpoint allele decides where pigment goes. Other genes decide which color the points are, using the registry names (with the everyday word in brackets):

The dilute gene and the agouti gene work the same way on pointed cats as on any other cat.

Worked crosses

Each cross below uses black-based cats with no other differences, so you can check every ratio in the calculator.

Two carriers: C/cs × C/cs

¾ full color, ¼ seal point. Two non-pointed cats can have a Siamese-pointed kitten if both carry cs. Check it in the calculator.

Two pointed cats: cs/cs × cs/cs

Every kitten is cs/cs, so all kittens are pointed. Pointed parents can’t have a fully colored kitten through this gene, because neither carries C.

Mink and point: cb/cs × cs/cs

½ mink, ½ seal point. Check it.

Adding a second gene: cs/cs A/a × cs/cs a/a

All kittens are pointed. Half get A/a and show tabby stripes on the points, so the result is ½ seal lynx (tabby) point, ½ seal point. That’s the product rule: 1 (pointed) × ½ (agouti). Check it.

Common questions

Are pointed cats albino?

No. Pointed cats make normal pigment wherever the skin is cool enough. A true albino allele is a different, much rarer allele.

Is cb recessive to cs?

No. A cb/cs cat is mink, not pointed or sepia.

Can a C/C parent have a pointed kitten?

No. A C/C cat passes C to every kitten, so none can be cs/cs.

Does temperature change a Siamese cat’s genes?

No. Temperature changes where the enzyme works, not the DNA. A pointed cat’s kittens inherit cs, not its dark points.

Practice

1. A seal point cat (cs/cs) is crossed with a full-colored cat that carries Siamese (C/cs). What fraction of kittens are pointed?
2. Two mink cats (cb/cs × cb/cs) have kittens. What are the expected proportions of sepia, mink and pointed kittens?
3. A pointed cat is kept indoors in a warm house and another in an outdoor enclosure in winter. Why might the second look darker, even with the same genotype?

Where the model stops

• Shading varies. How dark the body gets depends on age, climate and the individual cat. Coat names describe typical cats.
• Breeds share the gene but differ in other ways. Siamese, Himalayan (colorpoint Persian), Birman, Ragdoll and Balinese cats are all cs/cs. Their coat length, white markings and body type come from other genes.
• More alleles exist than the three in this table, including the albino allele.

Related

• Cat coat color calculator: set the colorpoint gene for both parents
• Dilute cat genetics: why blue and lilac points look pale
• Beyond Mendelian inheritance: gene–environment interactions and more
• Monohybrid cross calculator: the 3:1 logic behind carrier crosses

Sources

• Lyons LA, et al. “Tyrosinase mutations associated with Siamese and Burmese patterns in the domestic cat (Felis catus).” Animal Genetics 36, 119–126 (2005). doi:10.1111/j.1365-2052.2005.01253.x
• Schmidt-Küntzel A, et al. “Tyrosinase and tyrosinase related protein 1 alleles specify domestic cat coat color phenotypes of the albino and brown loci.” Journal of Heredity 96, 289–301 (2005). doi:10.1093/jhered/esi066
• UC Davis Veterinary Genetics Laboratory, Feline coat color.
• King RA, et al. “Temperature-sensitive tyrosinase associated with peripheral pigmentation in oculocutaneous albinism.” Journal of Clinical Investigation 87, 1046–1053 (1991). doi:10.1172/JCI115064
• Imes DL, et al. “Albinism in the domestic cat (Felis catus) is associated with a tyrosinase (TYR) mutation.” Animal Genetics 37, 175–178 (2006). doi:10.1111/j.1365-2052.2005.01409.x