Cats have 19 pairs of chromosomes. Chromosomes are located in the nucleus of each cell, chromosomes make up the blueprint of animals. The chromosomes are made up of genes. Genes come in pairs, one from each parent. Each gene is responsible for a single feature, or a group of features. One pair of chromosomes determines the cat's sex. A female has two X chromosomes, and a male will have one X and one Y chromosome.
Siamese cats carry a gene known as the Himalayan gene. This gene is seen in other species, such as the rabbit and the mouse. It is a mutation at the C locus and it causes partial albinism. This gene is recessive to the full colour C gene. This means you need two doses of it (homozygous) for the Siamese colour to show up. If you mate a Siamese to a Siamese, you will get Siamese offspring. If you mate a Siamese to a black cat, you will get black offspring which will carry one dose of the Siamese (cs) gene at the C locus.
The Burmese also shares the same type of gene, which is known as cb.
The cs and cb genes are co-dominant and hence if you mate a Siamese (cs) to a Burmese (cb) you will get a Tonkinese (cs/cb), which has "mink" colouring.
This gene is heat sensitive, the cooler the area, the darker the colour. Which explains why a Siamese has dark extremities such as the face, tail and legs. The body being the warmest part of the cat remains lighter in colour. You will notice your Siamese get darker in the winter months, especially if your Siamese is an indoor/outdoor cat. Siamese cats are white at birth, this is due to being in the constant warmth of the mothers womb. This colouring varies from Siamese to Siamese.
Genetically, a seal point Siamese is a black cat. But the Himalayan gene inhibits the full expression of the pigment.
The albino allele has another effect on the Siamese cat. Strabismus (crossed eyes) is sometimes seen in the Siamese cat. This is thought to be due to a misrouting of the nerve fibres from the eye to the brain. Instead of the fibres splitting left and right, they criss-cross, this is believed to be contributory to strabismus. This condition is not only seen in Siamese cats, but other species of true albino animals. Breeders have worked hard to breed this problem out of Siamese cats.
Please see our colours page for more details on the colours Siamese cats come in, Siamese cats come in four main colours.
(aa BB DD oo cscs) (aa=non agouti, BB=black, DD=dense, oo=non orange, cscs=Siamese)
Note how the dd (dilute) gene changes the darker Seal and Chocolate colours into the paler Blue and Lilac.
The outward appearance of a cat is known as the Phenotype. The internally coded, inheritable information is known as the Genotype. So, while a Seal Point Siamese will carry the gene for Seal Point colouration, it may also carry a gene for "Blue Point".
By looking at a cat's pedigree we will get a good idea of the cat's "genotype". We can also predict what colour kittens to expect from a mating between Siamese cats.
We know that Blue Point is a dilute of Seal Point and that Lilac Point is a dilute of Chocolate. Dilution is recessive to non dilution. The dilution gene is represented by the letter "d". The mutant form of dilution is Dm (dilute modifier). This is dominant to the dilute gene. Dm only works where there is already a dilute colour.
Below is a table which shows the effects of the Dilute and Dilute Modifier genes on the dense colours.