TORTOISESHELL AND TRI-COLOUR CATS
Copyright 2002 - 2013, Sarah Hartwell

Because of the way tortoiseshell is inherited, almost all tortoiseshell and calico cats are female. The very few male tortoiseshell cats are caused by genetic aberration or development abnormalities in the foetus. These are discussed in detail in "Mosaicism, Tortie Tomcats, Genetically Impossible Kittens and Gender Anomalies". This article only considers normal tortoiseshell cats.

Jean Bungartz referred to tortoiseshell-and-white cats as "Spanish cats" in his 1896 book "Die Hauskatze, ihre Rassen und Varietäten" (Housecats, Their Races and Varieties) in " Illustriertes Katzenbuch" (An Illustrated Book of Cats) though he said it was hard to explain why they got the name as they were not restricted to the Pyrenean peninsula.

HOW TORTOISESHELL PATTERNS OCCUR

The ginger colour of cats (known as "yellow", "orange" or "red" to cat breeders) is caused by the "O" gene. The O gene changes black pigment into a reddish pigment. The O gene is carried on the X chromosome. A normal male cat has XY genetic makeup; he only needs to inherit one O gene for him to be a ginger cat. A normal female is XX genetic makeup. She must inherit two O genes to be a ginger cat. If she inherits only one O gene, she will be tortoiseshell. The O gene is called a sex-linked gene because it is carried on a sex chromosome. Tortoiseshell cats are therefore heterozygous (not true-breeding) for red colour.

The formation of red and black patches in a female with only one O gene is through a process known as X-chromosome inactivation. Some cells randomly activate the O gene while others activate the gene in the equivalent place on the other X chromosome. This only shows up visibly in skin cells as these produce pigment. This occurs early on in the embryo and as skin cells multiply, they form patches. The skin is a mosaic of cells where some have the O gene active (making ginger pigment) and some do not (making black pigment). This can only happen in cats with two X chromosomes. Male cats only inherit one X chromosome so this is active in all skin cells as there is nothing equivalent on the Y chromosome which could "switch off" the O gene.

There are two main theories regarding brindled torties and patched torties. One (the "early/late deactivation theory") suggests that the time at which X chromosome deactivation occurs during foetal growth determines whether the cat has well defined patches or is brindled with intermixed black and orange hairs. Skin cells multiply during growth and spread out across the skin; as the embryo grows the skin cells multiply. If deactivation occurs early on each pigment cell has room to multiply into, a "red" cell will multiply into more red cells while a "black" cell will give rise to patches of black. If it occurs later, the patches are smaller as the cells have less room to multiply into; some "patches" will be no larger than a single hair! All "red" cats are red tabbies - where there are large red patches, the tabby pattern will usually be discernible.

All Red Cats Are Red Tabbies? Strange as it may seem, all red cats are actually red tabby because the non-agouti gene (the gene that turns a tabby into a self/solid colour) does not affect the way red pigment is deposited. A variety of other genes, called polygenes or modifiers, control the intensity of colour and contrast between markings and background colour. Selective breeding has reduced the tabby markings to produce a cat that looks solid red by breeding from those cats with the least red markings (cats with "low contrast" between markings and background colour). Because the non-agouti gene does not work on the red pigment, red tabby ghost markings can never be completely eliminated and may be seen on the tail, legs and forehead and as a darker region along the spine. Even though red cats are registered as "red self" they are still red tabbies, albeit red tabbies with very reduced markings. Because they lack the polygenes for high contrast between markings and background colour, their offspring also appear to be red self. More information can be found in Robinson's "Genetics for Cat Breeders".

The other theory regarding brindling and patching (the "migration theory") is that brindled torties occur when there are more pigment producing cells produced from the neural crest (which becomes the back and spine area). The cells are assumed to have undergone X chromosome deactivation before migration. The migrating cells carry either O (red) or o (black) and they migrate at the same rate into their final positions. Where there are many pigment producing cells, there is more competition (imagine cells jockeying for position on the skin) and they become intermingled. Where there are fewer pigment producing cells, each cell has room to grow into patches of colour (imagine plants forming clumps in a flower border).

As an interesting side note, human women can also have X-inactivation. Although there are no tortoiseshell women, there is an uncommon condition called anhidrotic ectodermal dysplasia caused by a faulty gene on the X chromosome. The faulty gene reduces the number of sweat glands per square centimetre on the skin. In women who inherit one normal gene and one faulty gene; their skin may be divided into hundreds of small patches which only become visible when they sweat. The normal patches of skin sweat normally, but the patches derived from cells with a faulty gene don't.

To be completely accurate, O = orange and o = "non-orange" meaning that o allows whatever colour is carried on other chromosomes to be displayed (black, blue, chocolate etc). The O gene converts normal black/chocolate/cinnamon etc pigment into red pigment. For the sake of simplicity, this article refers to o as "black" since that is the non-orange colour most commonly seen on tortie and calico cats.

TORTOISESHELL-AND-WHITE (CALICO) CATS

The white patches in tortie-and-white (tricolour, calico) cats is caused by the piebald spotting gene discussed in Beautiful Bicolours. This is a semi-dominant gene with very variable expression ranging from nearly all white to nearly all coloured with only a few white hairs. The gene affects the embryo cells which will become pigment-producing skin cells. These cells are initially formed along the "neural crest" - the embryo's backbone area - and migrate to all over the body during formation of the skin. Where these pigment producing cells fail to get in position before the skin is fully formed, there will be areas of skin which lack pigment producing cells i.e. white areas. White areas are usually the areas furthest from the cat's backbone - paws, belly, chest and chin - these areas take longest to reach.

One effect of white spotting in tortoiseshell cats is to change the pattern from brindled to patched. Tortie cats with little or no white tend to have brindled coats. However, the more white there is, the more the black and white will also be separated out into patches instead of being intermingled. The phenomenon of tortie-and-white cats having better defined patches of colour is familiar to most cat lovers.

In the developing embryo, the pigment forming cells migrate from the neural crest. If the "migration theory" is right, cells which activate O (red) and those which activate o (black) appear to migrate at the same speed, leading to a brindled pattern. If the embryos also inherit the gene for white spotting, the fur develops as patches of colour. The bigger the white areas, the bigger the and better defined the separate patches of black and red. The presence of the white spotting means fewer pigment producing cells and less competition between them as they migrate into position. One or two cells reach an area and these multiply in situ to form a patch of colour (a clonal patch).

Red Tabby Markings in Tortie and Tortie-Tabby Cats. Where a tortie (or calico) cat has the non-agouti gene it will appear to have solid patches of colour. Due to the quirk of the red pigment being unaffected by the non-agouti gene, there may be tabby markings visible in the red areas. How much red tabby pattern is visible is due to other genes called polygenes that also affect the depth and contrast of the red colour (the illusion of solid red is due to low contrast between background and markings). Where a tortie (or calico) has the agouti gene instead, the tabby markings will be visible in all of the coloured patches. In a tortie cat, the non-red colour will be a solid colour. In a torbie cat, the non-red colour will also have tabby markings. More information can be found in Robinson's "Genetics for Cat Breeders".

TORTIE ABYSSINIANS AND TORTIE TICKED TABBIES

There is another gene which gives rise to the red colour. This is the non-sex-linked red ("sorrel") found in Abyssinian cats and other ticked breeds and it does not produce tortoiseshells. Genetically, the Abyssinian pattern is not a solid colour, but is a form of tabby, called ticked tabby, and is described in Striped and Spotted Cats. The ticked pattern changes the visual appearance of the cat's colour. Although some registries call this colour "red" ("non-sex-linked red" in Australia), many registries prefer to call it "sorrel" to avoid confusion with the sex-linked red gene described earlier. Genetically, Abyssinian "non-sex-linked red" is the equivalent of cinnamon in non-ticked cats. It appears as a reddish-brown due to the ticked pattern of the hair. Chocolate Abyssinians appear a deeper red which can be differentiated from sorrel by the chocolate's darker tail tip.

Strictly speaking, because Abyssinian is a tabby pattern, the tortie versions should be termed ticked torbies (tabby-torties) rather than ticked torties. This photo shows Harley (Harlequin) and Spot (provided by owner Lizzi) who were resuced as feral kittens. Harlequin is a ticked torbies, while littermates Spot is ticked tabby with white, and Blaze and Mr Grey are mackerel tabbies with white. The two mackerel tabbies are quite lightly marked on a ticked background and show some rufousing (red tints).

The true sex-linked red Abyssinian is bright orange with a red tail tip. It is important for breeders to know whether they have sex-linked red or non-sex-linked red as this will affect the breeding program. Where there is sex-linked red, there can also be tortoishells. Tortie Abyssinians occur, but since the breed does not permit white, these are always brindled. The combination of brindling and ticking can make it almost impossible to determine whether a female is tortie or not just from a visual inspection. Sometimes a female Abyssinian is only known to be a genetic tortie when she produces a mix of red and black kittens!

Ticked tabby also occurs in non-pedigree cats. In randombred cats with the ticked tabby pattern, white markings may be present as well, resulting in "ticked tortie tabby and white" (ticked calico). The photo shows part pedigree tortoiseshell ticked tabby kittens with white markings.

DIFFERENT TORTOISESHELL COLOURS

The basic tortoiseshell combination is black and red. Because red does not occur as a true solid colour, the "red" patches are actually red tabby and this will be more apparent on some cats than on others. The addition of a dilution gene gives rise to the other tortoiseshells combinations. In Burmese cats, where there is a different dilution gene at work, diluting black into brown.

 

The action of caramel is noted as theoretical, in reality the colours may be indistinguishable from other colours unless the presence of caramel is known from the cat's pedigree; the entries are based on data from solid colour cats. The colour intensifier is also theoretical, based on the possible "indigo" gene which turns blue into a deeper richer colour. This could equally be due to polygenes (multiple genes having a cumulative effect). I have included it after seeing one cat which was dark-blue/hot-cream tortoiseshell i.e. where both colours were intensified.

NON-DILUTE COLOUR

DILUTE VERSION

CARAMELISED VERSION OF THE DILUTE VERSION (DOUBLE DILUTION)

INTENSIFIED VERSION OF THE DILUTE VERSION
("INDIGO" GENE)

Tortoiseshell
Black/Orange Tortoiseshell
(Brown Tortoiseshell in
Burmese cats)

Blue-Cream Tortoiseshell

Caramel/Apricot

Indigo/Hot Cream (dark-blue and hot cream) (theoretical)

Cinnamon Tortoiseshell
Milk chocolate and cream

Fawn Tortoiseshell:
Fawn and cream

Caramel/Apricot

??

Chocolate (Chestnut) Tortoiseshell
Warm milk chocolate, red, and cream

Lilac (Lavender) Tortoiseshell:
Frosty lilac-grey and cream

Taupe/Apricot (theoretical)

??

Above are black (smoke), solid chocolate and chocolate tortie kittens. The black is shown for colour comparison. Chocolate and choc-tortie are more common in pedigree cats than in randombreds, but these kittens were born to non-pedigree parents. Photo courtesy of Leva Cygnet.

In Siamese cats, only the points (legs, tail and face) show as tortoiseshell and because the gene causing the Siamese pattern turns black into dark brown, the tortoiseshell pattern will contain dark brown rather than jet black.

TORTOISESHELL TABBY AND TORTOISESHELL TIPPED

Tortoiseshell combines with the tabby pattern to create tortoiseshell-tabbies, also known as patched tabbies or (in the US) torbies. The tabby pattern may be classic (blotched), mackerel or spotted.

 

In a usual tortoiseshell, the black patches become brown tabby (i.e.. black markings on brown background), the red patches become more obviously red tabby (red patches on paler red background). Patched tabbies occur in other tortie colours e.g. blue-cream patched tabby.

In combination with silver, this would produce tortoiseshell markings on a silvery background e.g. black-silver (black markings on pale grey) and red-silver patches; though such combinations are not common.

The tortoiseshell markings can also be combined with the genes for tipped, smoked or shaded in which case the tortoiseshell colours are restricted to the hair-tips rather than going all the way to the root. This gives the effect of a shimmering tortie pattern on a pale undercoat.

BREEDING TORTOISESHELLS

In order to produce tortoiseshell kittens, at least one of the parents must carry the O gene. Red male and red female only produce red kittens. The combinations which produce tortoiseshell kittens are red male and non-red female, red female and non-red male. The following can also produce tortoiseshell kittens: tortie female and non-red male, tortie female and red male. It also depends on some kittens being female.

Parents

Kittens

Female

Male

Males

Females

Red

Red

Red

Red

Black

Red

Black

Tortie

Tortie

Red

Red

Red

 

 

Black

Tortie

Tortie

Black

Red

Tortie

 

 

Black

Black

Red

Black

Red

Tortie

Black

Black

Black

Black

 

This is what is happening when a tortoiseshell female is mated to either a red or a black male. Because a tortie female is heterozygous, she can pass on either the gene for red or the gene for non-red (black). The colour of the kittens depends on whether they are male or female and what gene, if any, they inherit from the male.

GENES FROM RED MALE

GENES FROM TORTOISESHELL FEMALE

 

GENES FROM BLACK MALE

GENES FROM TORTOISESHELL FEMALE

 

O (red)

o (non-red [black])

 

 

O (red)

o (non-red [black])

O (red)
X chromosome

OO

Red female

Oo

Tortie female

 

o (non-red [black])
X chromosome

Oo

Tortie female

oo

Black female

No gene (Y chromosome)

O-

Red male

o-

Black male

 

No gene (Y chromosome)

O-

Red male

o-

Black male

 

Because other conditions can give the impression of being tortoiseshell, some matings which theoretically have a high probability of producing tortie kittens will only ever produce red or black kittens. At the other extreme, a genetically tortoiseshell cat may appear to be solid ginger or solid black because of the way the X chromosomes activated - these cats will produce unexpected tortie kittens in appropriate matings.

CLONING TORTOISESHELLS

This may seem an odd topic, but if you clone a tortoiseshell cat you will end up with a cat of one or other of the constituent colours and not a tortoiseshell clone. If you clone a red/black tortoiseshell, the clone will be either red (ginger) or black.

DNA tests on the tabby-and-white cloned kitten ("Cc") proved that she was a clone of her tortie-and-white genetic mother (the cat whose cell was used to create Cc). The pattern of pigmentation in multicoloured animals is the result of genetic factors combined with developmental factors within the womb. This means bad news for owners who want an exact replica of a tortoiseshell or calico cat. Why isn't Cc also tortie-and-white? The answer is due to ‘X-linked Inactivation'.

Tortoiseshell cats have two X chromosomes, one carrying the gene for orange coat colour and the other carrying the gene for black coat colour. As the embryo develops, a process called ‘X-linked inactivation’ occurs in its tissues. One or the other X-chromosome in every cell in a tortie cat embryo is randomly inactivated. This only shows up in pigment producing cells, producing the familiar tortie effect.

Regardless of which cell was used to produce Cc, because that cell is already an adult cells, one or other of the cell's X chromosomes would have been inactivated while the donor cat was an embryo. Cc had an equal chance of being orange-tabby-and-white or black-tabby-and-white, but would never be tortie-tabby-and-white. Unless a way can be found to undo X-linked inactivation at the embryo stage, owners wishing to clone a tortoiseshell cat will have to settle for a cat of a different colour entirely. If the X-linked inactivation can be reset, the inactivation is a random process so the clone will have the right colours, but not in the same places as the donor cat - it may have well-defined patches of colour while the donor cat was thoroughly brindled.

TRI-COLOUR BENGALS

Confusingly for cat lovers, tri-colour Bengals are not tortoiseshell cats.  In the Bengal breed, tri-colour refers to a cat with black rosettes on a golden brown background. The centre of the rosette is generally a darker version of the background colour, hence the cat has three colours: pale background, black markings, dark rosette centre.

Tortoiseshell Bengal variants have occurred during breed development, but it is not an accepted colour in a breed which aims to re-create a wild look.

ANOMALOUS TRI-COLOURS

A number of anomaous tricolours are discussed in "Mosaicism, Tortie Tomcats, Genetically Impossible Kittens and Gender Anomalies". These include a white, black and grey tricolour and a white, red and grey tricolour. A further unusual tricolour is best described as a "white brindle".

The "white brindle" was one of the most striking "tortoiseshell" cats I have seen. Imagine a brindled tortoiseshell in which all the black areas have been converted to white. The result was a cat with intermixed fur of white, cream and ginger; the fur being brindled like a tortie rather than being patched like a red-and-white bicolour. This was seen while on holiday (either Turkey or Tunisia) and unfortunately I didn't have a camera with me. I have never seen anything like it since. A likely explanation is a genetic mutation in the fertilised egg had caused a pigment production fault in the areas which should have been black. It would have been interesting to see if the trait could have been perpetuated.

One reader suggestion for the "white brindle is a red-tabby-and-white smoke shorthair. This would give a similar effect, but the colour in a smoke cat does not go down to the roots and the brindled effect would not be so thorough. I am currently inclined to think that the cat was a conventional brindled tortie which was not producing black pigment.

MESSYBEAST : COLOURS, CONFORMATION & FUR TYPES