The intention of this page is to provide some genetic background of this fascinating breed, the Tonkinese, and to share some facts about the genetics of Coat Colour inheritance in domestic cats as a whole - This is not a complete analysis of coat colour genetics (see Links page) but is primarily focused on the genetics that underpin theTonkinese breed. 

I try not to blind with science, all too often I'm met with a blank stare from other breeders when the subject of genetics arises, yet with a little effort it is easy to understand the genetics that underpin the breed and the resulting patterns of inheritance.......

..... so let's start with the basics;

The expression of Coat Colour in all cats is mainly determined by two key factors;
These two factors are;

In terms of an identification system for colour and markings, most cats are described in terms of the colour name first and then pattern name second - ie; blue-tabby, lilac-point etc.

Adding to the potential confusion with identifying colours, different breeds utilise different naming systems for the same colours, ie; black may be called 'ebony' in some breeds and is expressed as Seal in Tonkinese. Brown may be called 'chocolate' and thus it can be very confusing to determine which of the 4 basic colours a cat possesses, but regardless of the colour name, all cat coat colours fall under one of the 4 basic colours that may have been altered further by with modifications genes, and all have a pattern (even if the pattern is 'plain' ie 'self/solid-colours')

The main colours bred in Australia

In Australia the main four colours Seal, Chocolate, Blue & Lilac have been bred since the breed was first developed. At Anniesong we have now introduced another 2 colours with the addition of Cinnamon & Fawn to the Australian repertoir of colours available in the Tonkinese. 

Until recent times the pattern associated with the breed in Australia has been almost exclusively been the 'self' pattern that has been modified by the Himalayan gene. And although the 'self' pattern combined with the Himalayan gene is most prevalent and popular, there are other lesser known patterns that also are permitted  within different breed registries.

Also referred to as the ‘Colourpoint gene’, the Himalayan gene was originally a mutation of the ‘self’ or ‘solid’ patterns, and results in the colour of the cat to only be expressed fully on the extremities, fading to a lighter tone on the rest of the body. The effect is referred to as 'Colour-Restriction'

and what is meant by COLOUR RESTRICTION ?

The patterns that define Burmese, Siamese & Tonkinese are referred to as; ‘colour restriction patterns’ and all are controlled and determined by the ‘Himalayan gene’

Colour restriction – or the limiting of colour expression to the extremities of the cat’s body is caused by the Himalayan gene. Genetically, all self-coloured Tonkinese regardless of colour and pattern, are actually black cats. However the Himalayan gene inhibits the full expression of the pigment on most of the cat's body. The effect of the gene is that most of the colour is 'bleached' or 'faded' and the deteremining factor is body temperature (see below)

• The Himalayan generally occurs most usually within domestic rodent species, and is most commonly seen in rabbits, guinea pigs and mice.
• The melanin in their hair and skin is temperature sensitive, controlling the points on the cat's body.
• In addition to limiting where the colour will be expressed, the Himalayan gene tends to "bleach out" the colour to varying degrees, depending on the form of colour restriction; ie; Pointed/Mink/Solid.
• The gene also has a weakening effect on colour. The colours seen on the points of Himalayan breeds are less vibrant than their counterparts when the Himalayan gene is not present. ie; seal (very dark brown ) is actually the gene for black and appears as jet black if not combined with the Himalayan gene.
• The longest-established and best known of these is the Siamese breed, which was used to incorporate the Himalayan gene into other breeds that now express this pattern.
• It may seem odd to think of Tonkinese as solid colour cats, but genetically they are; they have just had a "special effect" overlaid on them.

 

So what is 'Co-Dominance' as a genetic mode of inheritance?

Co-dominance (also called partial dominance) occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. Co-dominance occurs when the contributions of both alleles are visible in the phenotype.

So how does Co-Dominance affect coat colour and pattern in the Tonkinese?

The two alleles that determine coat pattern in the Tonkinese breed are cs (the allele for the Siamese coat pattern) and cb (the allele for the Burmese coat pattern).

Both alleles are from the albino series, and cause unstable pigment colouration, due to temperature sensitivity of the melanin. This results in greater colouration on the cooler parts of the body, or the points (the extremities).

The torso has a lighter colouration because body temperature is higher there. The amount of contrast between the extremities and the torso varies from a very strong contrast in the Siamese coat pattern cscs to a very weak contrast in the Burmese coat pattern cbcb.

When we have a Tonkinese with both cs and a cb (genes come in pairs - one from each parent), we get a third variation which we call a mink cscb. Because neither is dominant over other, they are co-dominant. We achieve a coat pattern part way between the other two in terms of contrast.

The usual mode of inheritance where one gene will dominate and therefore 'hide' the effect of the recessive gene, is common to how many traits are inherited. But Co-Dominance and the resulting expression of colour does occur in other domestic animals, albeit rarely. As seen below in the Palomino horse, a colour that is due to a co-dominant mode of inheritance.

In the field of Genetics, codes have been determined to describe the genes for both colour and pattern. We describe this as the 'Genotype.'

In each case these 'codes' are most usually determined by a series of 2 letters, due to the fact that genes for any one trait are transmitted in pairs, frequently where one of the genes dominates and 'hides' the other. Thus in each case the Dominant gene is described using a capital letter.

The gene for Seal (Black) = B
The gene for Chocolate (Brown) = b

therefore Seal is dominant over Chocolate, which is thus recessive to Seal

Thus a Seal-coloured Tonkinese that is Homozygous for Seal, ie; both genes are for the colour Seal is coded; BB
A Seal-coloured Tonkinese that is Heterozygous', ie; carrying Chocolate is coded; Bb
A Chocolate-coloured cat that is homozygous; for Chocolate is coded; bb

In the diagram below we have visualised the genotypes, and the legend shows which 'carried' or hidden genes are presen t... the genotype 'code' is below the colour name in each colour panel.