Tonkinese: Cat Breed FAQ: Genetics

by Linda Martino

Please pass along suggestions and corrections to: linda at rhythm dot com

The definitions build on information presented over the length of the article, but here are the locations of the definitions for quick reference:

I. Coat Color

Within the Cat Fanciers Association (CFA), Tonkinese, Siamese, and Burmese have the same basic four colors although they are called by different names. Other cat associations allow a wider variety of colors, but this article will not address those colors.

When I began breeding Tonkinese I knew the four color names, but I became very confused about the genetics behind them. After I read (and reread a number of times) the Book of the Cat, it became clear. Why the confusion? Because many breeders used inaccurate terms when discussing the inheritance of color.

The following discussion applies to Siamese and Burmese as well as Tonkinese. Only the color names change - not the genetics.

Every cat has many thousands of genes. Except for the sex-linked genes, cats have two copies of each gene. They inherit one copy from each parent. The basic coat colors are created by two genes, each of which has two alleles. In this case, a gene is a set of instructions (found at a specific location on a chromosome) that tells the cat's body how to make the coat color. Each of the two genes governing coat color provides instructions for a different step in the production of color. Both steps must be done correctly for coat color to be produced in a normal fashion. An allele is the instructions for what the gene actually says. Thus, because the two color genes each have two alleles, there are two versions of instructions that tell the cat's body how to make the color at one step and two versions affecting what to do at the other step in color production. There are: 2 genes x 2 alleles = 4 possible combinations of instructions that can be followed. The combinations of alleles (instructions) result in different effects on the coat colors. Gene 1 alleles result in either Black (B) or brown (b) pigment, depending on whether B allele or b allele instructions are followed, and Gene 2 is Dense (D) or dilute (d). In other words, Gene 1 basically effects the hue of the pigment and Gene 2 effects the density or lightness/darkness of the hue of Gene 1. The four combinations and their respective color names by breed are:

Color Names by Breed

Genetic name Tonkinese name Siamese name Burmese name
Black Dense Natural Seal Sable
Black dilute Blue Blue Blue
brown Dense Champagne Chocolate Champagne
brown dilute Platinum Lilac Platinum

Although the Burmese call every color except Sable a dilute (as all three are lighter than Sable), genetically a Champagne does not have the dilute gene. Champagnes and Platinums are warmer colors than Naturals and Blues. I find it useful to think of the colors in pairs depending on the orientation you are thinking of.

The uppercase letter is used to denote the dominant allele and the lowercase the recessive allele. The effect of the dominant allele will be visible in the color of the cat's coat. The effect of the recessive allele will not be visible unless the cat inherits two recessive alleles. This is because a dominant allele so overshadows a recessive one that you do not see the effects of the recessive allele. It is there but not visible. Every Tonkinese has four color genes: one Gene 1 and one Gene 2 from each parent for a total of two Gene 1s and two Gene 2s. Neither color nor coat pattern genes are sex-linked; therefore it is irrelevant which parent contributes the gene. Black and Dense are dominant and brown and dilute are recessive. Again, a dominant allele will define the color you see regardless of the second gene in the pair. For a recessive allele to be visible, both of the pair must be the same recessive allele. Therefore, only if both genes of a Gene 1 pair are brown will brown be the visible color. If either gene is Black, then black will be the only color visible. The Dense/dilute Gene 2 pair follow the same logic. Only if both genes of a Gene 2 pair are dilute will the effect of dilute be visible.

The following table shows the different combinations that an individual cat can have of their two genes and two alleles. Again, it does not matter which parent contributed which gene.

Genetic Gene Combinations by Color

Genetic name Tonkinese name Specific Combinations
Black Dense Natural BBDD, BbDD, BBDd, BbDd
Black dilute Blue BBdd, Bbdd
brown Dense Champagne bbDD, bbDd
brown dilute Platinum bbdd

If a cat shows the effects of either Black or Dense or both, then the cat has unknown, hidden or passive genes that are not visible when examining the cat. This is because it takes only one copy of the Black or Dense allele to determine the color. The second allele of the gene pair is the hidden or passive gene, because it does not have any visible effect on the color. It could be the same dominant allele or it could be the recessive allele. We say a cat carries the passive or hidden gene. It sort of goes along for the ride but is not the driver nor even a back seat driver. It is strictly a passenger that remains quiet (I know I'm not a hidden gene). All four Natural's genetic combinations in the above table would look alike, but three carry one or two hidden/passive genes.

You can sometimes use logic and knowledge of the color of the parents to determine what hidden alleles are carried by their offspring. For example, if a Natural Tonkinese has one parent that is a Champagne, the Natural must carry a hidden brown (b) allele. The Champagne parent had to pass one copy of Gene 1 to each offspring - and the Champagne could only donate one of two brown alleles. However, knowing that one parent is a Champagne does not tell the breeder anything about the dilute allele, because dilute can be hidden in Champagnes. A Platinum parent is more helpful because Platinums carry no hidden genes and must always donate one b and one d allele to each kitten.

It should now be obvious there is no Blue color gene just as there is no Champagne, Platinum, or Natural color gene. All four colors are a combination of two genes. I frequently hear breeders say a cat carries Blue referring to the hidden gene. The hidden gene is not blue as blue is a combination of two colors. This is inaccurate and confusing for others. A Tonkinese can carry both brown or dilute not blue.

Because hidden genes affect the color of the offspring they are of great interest to a breeder. A Natural with the BBDD combination will create 100% Naturals regardless of to whom it is bred. Whereas Naturals with a hidden recessive allele can create other colors. Left to nature, Naturals occur most frequently in a Tonkinese population because they carry the dominant alleles. Platinums are the most infrequent due to their doubly recessive gene requirement.

If you have wanted a Natural or Blue and found them in short supply, it is because breeders have overridden nature and controlled what colors are produced by carefully chosing the breeding pair. Why? Platinums followed by Champagnes are the most popular with judges and many breeders want winning show cats. Of course some breeders simply prefer those colors themselves. Some would say Platinums are flashy. Do blondes have more fun? Blues are subtle and as discussed in the coat pattern discussion below, they naturally have the least contrast. I find prospective pet owners either love or dislike blues with no middle ground. Naturals are very popular with pet owners but less so with judges.

Probability table for color per mating pairs

The Probability Table gives the probability of producing a specific color when mating two cats. Like the roll of the dice, the actual results are left to chance. The chart can tell which colors can't occur, but it won't tell what will get created in a specific litter. The probabilities are based on how frequently an event usually occurs in a large population.

The table creation method is:

  1. Determine all the 2-gene combinations for each parent. This is determined by making all the different combinations of one Black/brown Gene 1 and one Dense/dilute Gene 2 that can occur from the 4-genes of the parent. The table below shows the combinations for each unique type.
  2. Create a matrix using all parent combinations. Children cells = # of Parent A combinations x # of Parent B combinations Put Parent A 2-gene combinations in the header cells horizontily on top and Parent B 2-gene combinations in the header cells vertically on the left.
  3. Add the parent combinations together for a 4-gene combination and put them in each cross inner cell to create the resulting possible children combinations.
  4. Put the Gene 1s together and the two Gene 2s together.
  5. Figure out the color of each child combination. Remember Bb is the same as bB and Dd is the same as dD. Order is not important in this case, although usually I put the dominant (uppercase) allele first.
  6. Calculate the percentage of each color.

Possible 2-gene combinations from each type of parent:

4-Genes Notation Color Type 2-gene Combinations
BBDD Nat. Natural not carrying brown or dilute BD
BbDD Nat(b) Natural carrying brown BD, bD
BbDd Nat(d) Natural carrying dilute BD, Bd
BbDd Nat(bd) Natural carrying brown & dilute BD, bD, Bd, bd
BBdd Blue Blue not carrying brown Bd
Bbdd Blue(b) Blue carrying brown Bd, bd
bbDD Champ. Champagne not carrying dilute bD
bbDd Champ(d) Champagne carrying dilute bD, bd
bbdd Plat. Platinum bd

Example:

This example calculates the probabilities when mating a Natural carrying both brown and dilute alleles Nat(bd) to a Blue carrying a brown allele Blue(b).

1. Obtain the 2-gene parent combinations from the above table.

2. Create a matrix of all parent combinations. There will be 8 children cells.

Parent A BD Parent A bD Parent A Bd Parent A bd
Parent B Bd
Parent B bd

3. Add the parent combinations together for a 4-gene combination and put them in each cross inner cell to create the resulting possible children combinations. Put the Gene 1s together and the two Gene 2s together.

Parent A BD Parent A bD Parent A Bd Parent A bd
Parent B Bd BBDd bBDd BBdd bBdd
Parent B bd BbDd bbDD Bbdd bbdd

4. Figure out the color of each child combination.

Parent A BD Parent A bD Parent A Bd Parent A bd
Parent B Bd BBDd Natural bBDd Natural BBdd Blue bBdd Blue
Parent B bd BbDd Natural bbDD Champagne Bbdd Blue bbdd Platinum

Remember that BdDd is the same as bBDd. I usually write the dominant (uppercase) allele first regardless of whether it came from the horizontal or vertical cells. In this example, I always put Parent A's allele first, so I could illustrate that the order of the two alleles (withinin each set of Gene 1s and Gene2s) is not important. BdDd and bBDd are the same. Putting the dominant allele first just makes it easier to recognize the colors.

5. Since there are 8 possible results, each results represents a probability of 1/8. Adding up the number of occurrences you get the following probabilities:

This process was used to create the following table.

Table 1: Mating Color Probabilities for Kittens

Nat. Nat(b) Nat(d) Nat(bd) Blue Blue(b) Champ. Champ(d) Plat.
Nat. Nat. Nat. Nat. Nat. Nat. Nat. Nat. Nat. Nat.
Nat(b) Nat. 3/4
Champ. 1/4
Nat. Nat. 3/4
Champ. 1/4
Nat. Nat. 3/4
Champ. 1/4
Nat. 1/2
Champ. 1/2
Nat. 1/2
Champ. 1/2
Nat. 1/2
Champ. 1/2
Nat(d) Nat. 3/4
Blue 1/4
Nat. 3/4
Blue 1/4
Nat. 1/2
Blue 1/2
Nat. 1/2
Blue 1/2
Nat. Nat. 3/4
Blue 1/4
Nat. 1/2
Blue 1/2
Nat(bd) Nat. 9/16
Blue 3/16
Champ. 3/16
Plat. 1/16
Nat. 1/2
Blue 1/2
Nat. 3/8
Blue 3/8
Champ. 1/8
Plat. 1/8
Nat. 1/2
Champ. 1/2
Nat. 3/8
Blue 1/8
Champ. 3/8
Plat. 1/8
Nat. 1/4
Blue 1/4
Champ. 1/4
Plat. 1/4
Blue Blue Blue Nat. Nat. 1/2
Blue 1/2
Blue
Blue(b) Blue 3/4
Plat. 1/4
Nat. 1/2
Champ. 1/2
Nat. 1/4
Blue 1/4
Champ. 1/4
Plat. 1/4
Blue 1/2
Plat. 1/2
Champ. Champ. Champ. Champ.
Champ(d) Champ. 3/4
Plat. 1/4
Champ. 1/2
Plat. 1/2
Plat. Plat.

For a Table which includes the hidden gene results see Table 2: Mating Color Probabilities for Kittens - Hidden Genes. Instead of summing up the Naturals as 3/8 in the Example above, it lists 1/8 as Natural carrying dilute and 1/4 as Natural carrying dilute and brown.

Question:
If I mate a Natural with a Platinum and all the kittens are Naturals, does this mean the Natural is not carrying brown or dilute?
Answer:
The tables give you odds and in some cases tell you what you can't get. A single test breeding will not eliminate the possibility of a cat having hidden genes. Probabilities are based on what the results will be - on average - if an event (kitten birth) occurs many, many times, forming a large population of many thousands of kittens. A large litter may seem huge to a Queen but in the mathematical world a litter is a small population. In small populations, there can often be a considerable difference between what the odds predict and what actually occurs. On the positive side, some things are impossible even with small sample populations. For example, it is not possible for two Champagne parents to produce a Natural or blue kitten.

Question:
If I breed a Natural with a Platinum and I get a Platinum does that mean the Natural is carrying brown and dilute alleles?
Answer:
Yes. Since Platinum can have only brown and dilute alleles, then each parent has to have one of each of those to contribute. Therefore the Natural is BbDd.

Question:
Is there any way of knowing if a Natural has hidden genes before breeding the cat?
Answer:
Yes. In some cases, logic and the colors of the parents can identify the hidden genes. If a parent was a Platinum, the Natural must carry both brown and dilute. If a parent was a Champagne, the Natural carries brown with dilute possible but unknown. With a Blue parent the Natural carries dilute with brown possible but unknown. With both Champagne and Blue parents, the Natural would carry both brown and dilute.

II. Coat Pattern

Our Tonkinese are color coordinated with three coat patterns. The color varies with intensity but is essentially the same color. The coat pattern gene is independent and different from the two color genes. We get twelve color/coat pattern combinations, three coat patterns for each of the four colors.

There is a major difference between the gene that determines the coat pattern and the two that determine color. Each of the two color genes has one allele that is dominant and one that is recessive. In the Tonkinese breed, the two alleles of the coat pattern gene, neither are dominant over the other, but are codominant to each other. Codominance means that the effects of both alleles will be visible in cats that inherit the dissimilar pair of alleles together. Also, the combination of those effects will cause the cat to look quite different from cats that inherit two identical copies of the same coat pattern allele.

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, a group of many different alleles that all cause unstable pigment coloration. The instability is temperature sensitive. This results in greater coloration on the cool part of the feline body, or the points (the extremities). The torso has a lighter coloration 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 (remember 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 codominate. We achieve a coat pattern part way between the other two in terms of contrast. Neither side wins.

Genetically, the allele that produces a truly solid colored coat is given the symbol C and it is dominant over cs and cb. The C allele results in stable pigmentation and the color does not vary based on temperature. This gene is not present within the Tonkinese breed. cs and cb allele are recessive to the dominant C, while they are codominant to each other.

Many people get confused when trying to determine the color/coat pattern of a Tonkinese. They will describe the cat as dark. Dark in what way? The key is to look first at the extremities for color and then second at the body for amount of contrast to those extremities to determine the coat pattern. Again it is intensity that is changing - not color.

Unfortunately, a further source of confusion is created by the names of the three patterns: Point for the Siamese Coat Pattern, Solid for the Burmese Coat Pattern, and Mink for the in-between Coat Pattern. All of the Coat Patterns have points which are darker than the body and none of them is solid. The amount of contrast varies for each color, with Champagne having the most contrast and blue the least. A Blue Mink may have about the same contrast as a Champagne Solid. A Natural or Blue Solid adult may appear solid even though it contains no C allele. When looking at a young Natural kitten or a Champagne Solid adult, it is apparent that they are not solid - meaning that the color is not distributed evenly over the body surface and throughout the fur. However this contrast is small in comparision to a point.

Color/Coat Pattern Names

Natural Point Natural Mink Natural Solid
Blue Point Blue Mink Blue Solid
Champagne Point Champagne Mink Champagne Solid
Platinum Point Platinum Mink Platinum Solid
Tonkinese darken with age. They are born without any point color; the latter starts to develop within a few days. It is fun to see the mask (dark area on the face) start small around the eyes and grow to cover the whole face. The entire cat then darkens very slowly over the years. The paws pads start such that they look like they have walked in ink and it increases to cover the whole paw. The eye color is blue at birth and starts to change at around six weeks. It can continue to change for many months. Because coat color is lightened by high body temperatures and darkened by lower temperatures, the color can be altered by a bandage covering the fur, illness, cold weather, or the reproductive cycle of a female.

Currently, only Minks can be shown for Championship status in CFA. Other associations allow all three coat Patterns to be shown. One thing that cannot be stressed too much is that these are all Tonks and, regardless of the coat pattern, share the same standard for body type and the same personality. The only other characteristic described in the Tonkinese standard that is affected by coat pattern is eye color. Normally Points have blue eyes, Minks have aqua eyes, and Solids have green/gold eyes. This is due to the differing effects of body temperature on pigment in the eye; the effects of temperature on eye pigmentation are different depending on which pattern alleles are inherited.

We will always have three coat patterns and can not 'breed out' the two patterns of Points and Solids. It requires a cs and cb to make a Mink: when you mate a mink to a mink you will always get a mix of coat patterns in the resulting litter.

Probability tables

What happens when you mate Tonkinese cats that differ in their coat patterns? The following shows the probability table for each combination.

Mating Coat Pattern Probabilities for Kittens
Point cscs Mink cscb Solid cbcb
Point cscs Point Point 1/2
Mink 1/2
Mink
Mink cscb Point 1/2
Mink 1/2
Point 1/4
Mink 1/2
Solid 1/4
Solid 1/2
Mink 1/2
Solid cbcb Mink Solid 1/2
Mink 1/2
Solid
The same caveats apply to these probability tables as to the previous ones. They tell you in some cases what you can't get, but how many of each type you will get in a specific breeding are left to the fates.

Normally you want to pick cats to mate that have been proven in the show ring to be good specimens of their breed. The irony of Tonkinese genetics is that because a Point to a Solid breeding creates all minks, you get a higher percentage of kittens that are eligible to be shown. Many of our Distinguished Merit (DM) cats were never shown themselves.

We have much to learn as breeders about what is the ideal standard for our points and solids to create the best minks.

Question:
In other breeds which have Solids and Points, two Solids can produce a Point. The table above implies that is not true for Tonkinese. Why not?
Answer:
Remember that the Tonkinese 'Solid' is not created by the dominant C allele but by having two identical copies of the cb from the albino series. The C dominant allele can hide a cs allele. A true solid cat could be Ccs or (CC). Two Ccs cats could both pass a hidden cs to create a pointed child with cscs. A Tonk Solid is cbcb so genetically it only has a cb alleles to donate to its offspring. There are many things a cat can hide genetically, but in a Tonkinese coat pattern is not one of them. There are no hidden genes in the Tonkinese coat pattern.
Our Solids are compared to Burmese more than our Points are compared to Siamese. This may have occurred because some breeders and judges have moved away from the standard towards the Burmese conformation. The Cat Fancy does not see as many different breeds with the Burmese coat pattern as it does with the Siamese coat pattern. I wonder if this also leads to more comparison between Tonkinese Solids and Burmese.

III. Conformation and Personality

In the above discussion we started with color with either/or options. Things are black or white (dominant/recessive) - or in the case of coat pattern there is also a gray (codominance). These types of genes are called major genes. Major genes are genes with very strong visible characteristics such as color. Their effects are very obvious. When we come to body type and personality we enter a whole new realm genetically. The genes affecting these attributes are called polygenes. Polygenes are a group of genes working together to create an attribute. Instead of just one or two individual genes with very large obvious effects, polygenes are a whole group of genes with each individual gene contributing a small amount. Also, many of the different mixes/combinations of these genes can produce similar physical results. This concept is especially relevant to understanding the Tonkinese and circumnavigating the confusion generated by the breed's hybrid origin.

Like most breeds, the Tonkinese breed was developed from older, established breeds. Tonkinese were created by crossing Siamese and Burmese. The breed was a product of the desire to create a moderate breed between the two. The first Tonkinese were half-siblings and first cousins to Siamese and Burmese. What is critical to understand though is there is no such thing as a genetic map for any specific breed beyond the inclusion or exclusion of certain main genes for color or bobbed tail or folded ears. In other words, it is not possible to list every one of the many thousands of genes cats carry and identify which alleles of those genes belong to which breeds and which do not. You will not see probability tables in this section. In particular, the contributions of the polygenes to complex feline traits are mostly small and subtle. In fact, most of the polygenes have never been identified and researched by geneticists. Thus, they are not tracked and are not associated with a specific breed. There are no specific genes labeled Siamese head or Burmese head.

On the other hand, it is possible to select for specific combinations of polygenes, as well as other unidentified genes, simply by trying to breed cats that match the breed standard as closely as possible. Breeders also can select for certain complex traits that are not defined in the breed standard, such as friendly personalities and intelligence.

Therefore, most of what defines a breed is establishing a standard and developing a gene pool to get combinations of the polygenes that will yield consistent results. Though it does not need to be so, the refinement is usually accomplished by using a few good cats many times over which reduces the gene pool until there is less variation. This reduction in gene pool produces considerable consistency in desired breed traits, but unfortunately can have the side effect of making the breed more vulnerable to health problems. Potentially, there can also be conscious or unconscious changes in traits not specified in the breed standard, such as personality.

Within CFA, Tonkinese kittens born after January 1, 1984 cannot have Siamese or Burmese parents (there are some special rules related to cats registered in TICA that I will not address here). Using the selection process, all three breeds have changed a great deal since then and are substantially different today. Tonkinese breeders essentially try to emphasize many of the very characteristics that Siamese and VBurmese breeders breed out. If we were to start the breed today from the current Siamese and Burmese populations, we would see a different Tonkinese. All three gene pools are very different from 1984. There would be fewer occurances of the characteristics we are breeding for within the current Siamese and Burmese gene pools. The result is that Tonkinese cats now are more like kissing cousins rather than half-siblings to cats from the two parental breeds.

It is critical to understand that although coat pattern and eye color are a hybridization, the rest of the cat is not 1/2 Burmese and 1/2 Siamese. A cat has many thousands of genes; coat pattern and eye color are just two of them. Put another way, Tonkinese are much more closely related to each other regardless of coat pattern, than they are to the parental breeds. Don't let the visible genes of coat pattern and eye color distract you from the much larger number of differences between cats from different breeds. Would you consider a Himalayan a Siamese? The cs coat pattern is found in both breeds but they are obviously not the same breed. The Persian breed provides an obvious example of how much the selection process can change a breed. If you saw a Persian of 100 years ago you would not think it is the same breed as a Persian of today, even though the single gene difference that defined the breed then (longhair) is still present in Persian cats today. If breeds were rigidly defined by the presence of alleles from only major genes, then they would not change so easily over time by the selection process.

In summary, the Tonkinese Solid is not a Burmese and a Tonkinese Point is not a Siamese. They must always share alleles from a single common gene, but that is all. The rest of the cat has evolved away from the parental breeds; Tonkinese cats have grown up, had children, and grandchildren, etc. Many generations later they have emerged in their own right as unique - unlike cats in any other breed.

I doubt the uniqueness of the breed is more obvious anywhere than in the area of personality. Although personality is not part of the standard, breeds have distinctive personalities. Siamese, Tonkinese, and Burmese are all different. Although you can see where a Tonkinese got certain aspects from each of the parent breeds (thank you Siamese and Burmese), it manifests itself very uniquely. This is why it is somewhat amusing to hear people say, isn't a Tonk Solid really a Burmese? No one who has lived with these different breeds would question the fact that a Tonk Solid is all Tonkinese and its only the coat pattern and eye color that resemble Burmese. Both breeders and pet owners have gravitated to this breed because of its unique personality. I know many people who have two or all three of these breeds because they like a variety of personalities. Sometimes we lose sight of what really makes a cat a cat. Its not just a beauty contest. Look at the whole cat and not just one feature.

One topic I'm not going to tackle is whether there are personality differences for each color. Many breeders have apocryphal stories about certain colors and their distinctive personalities as well as some cats being color bigots. Since this is not generally agreed upon and there are many exception stories, I'll leave this topic out of the article. Just as conformation is the result of many genes, so is personality. It is a complex arena and a lively topic of debate. [an error occurred while processing this directive]