1996 - 2023 Sarah Hartwell

When this article was originally written (1996), a breeder was investigating her fertile tortoiseshell and white stud cat. This tortie tomcat only passed on the gene for red and never for black. Tissue samples found that her stud cat was a red-and-white bicolor "somatic mosaic". The black areas of his coat were due to local mutations of skin cells - much like moles or birthmarks - and he was genetically a red-and-white bicolour. Since then there have been many more studies into tortie tomcats, many of whome were fertile, and this has turned the accepted theory of tortie males being due to chromosomal abnormalities and therefore infertile on its head. In fact there are some pedigrees that have two or three generations of fertile tortoiseshell males.


Over the years and from region to region, the figures given have ranged from 1-in-1000 to 1-in-many-thousands. Some of these would have been mis-dentified poorly defined classic tabbies, especially where white patches obscure the tabby pattern. According to Roy Robinson's 'Genetics for Cat Breeders' out of a total of 125 kittens produced by black female x orange male there was 1 tortoiseshell male. Some breeds may be more prone to genetic gender anomalies than others (this has been noted in dogs, but there is currently no comparable data for cats).

Are tortoiseshell or calico tomcats valuable? You may find tortoiseshell tomcats being advertised for large sums of money. One website even said that if you found a tortoiseshell tomcat, you could sell it for a fortune and retire! Despite their rarity, tortoiseshell tomcats are not valuable in money terms. Those that are fertile don't pass on the tortoiseshell colour. As pets they are just like any other cat. As pedigree cats, there may be no colour class for them to enter so however fine they look, so they can only compete in the "Any Other Colour" class. The abnormalities that create some tortie males can also lead to auto-immune disease or testicular tumours. Next time you see someone advertising a "rare tortie male" on Ebay or Craigslist, you can wonder who is more gullible - the person hoping to make a fortune or the person who pays over-the-odds because they think the cat is going to make them a fortune! Another myth about tortoiseshell male cats is that if an owner takes the cat to be spayed (quite reasonably believing it to be female) and it turns out to be a male which the vet then castrates, the owner is entitled to get the money back from the vet for unnecessarily neutering a cat which is already sterile. Even though a male tortie or calico may be sterile, he probably still has the male hormones which make him spray. Many owners of tortoiseshell males like to mate the cat once to see if he is fertile. With so many unwanted kittens in the world, please don't do this unless you are involved in a genuine research programme.

To understand what creates tortoiseshell and calico males, we first have to look at the genes for red and for tortoiseshell. The white areas of calico cats are caused by different genes and can be ignored. The tabby pattern is also caused by other gene. In this article, "ginger" also includes "ginger-and-white" (red bicolour) and "red tabby" (with or without white). "Tortoiseshell" also includes "tortoiseshell-and-white" (calico) and "tortoiseshell tabby" (torbie) (with or without white). Cream is a dilute version of red. Blue (grey) is a dilute version of black.


Tortoiseshell (Tortie): a coat with a mix of two colours e.g. red & black or blue & cream; these can be patched or brindled
Calico: American term for a tortoiseshell-and-white cat
Torbie: a tabby-tortie where the patches are tabby-pattern instead of solid colour
Torbico: American term for tabby-tortie-and white cats

Allele: there are different variants of most genes; these variants are properly known as alleles, but for simplicity I use "gene" throughout this article.
: any chromosome other than the X or Y "sex" chromosomes.
Genotype: the genetic make-up of an organism.
Phenotype: the outward appearance of an organism. Several different genotypes can result in the same phenotype.
Dominant: most genes are inherited in pairs; the dominant gene of a pair is the one which is physically expressed.
Recessive: the recessive gene is only physically expressed if both genes in a pair are the same recessive gene, otherwise it is "hidden".

Hermaphrodite: having the sex organs of both male and female.
Intersex: Having undifferentiated sex organs, which are neither fully male nor fully female, but intermediate between the two.

Mosaic (1) having a multi-colour coat e.g. brindle, tortoiseshell or calico; (2) an individual whose cells have different genetic composition e.g. XY/XXY; (3) somatic mosaicism.
Somatic mosaicism the organism contains two (or more) genetically distinct populations of somatic (body) cells due to DNA mutations/damage or chromosome abnormalities.
Germ-line mosaicism if the mutation occured early in embryo development, some of the egg or sperm cells will carry the mutated gene which can be passed on to offspring. The mutation will show up when that cat has offspring.
Chimaera/Chimera an individual that developed from two genetically distinct germ-lines e.g. where two embryos fused in the womb. Scientists have created a "geep" chimera by mixing sheep and goat embryos; it had goatlike legs and a sheeplike torso.

In a mosaic, the percentage of cells having each genotype varies depending on how early during embryo development the mosaicism originated. Mosaics and chimeras both have more than one genetically-distinct population of cells and they may look the same, but there is a clear distinction between them. In mosaics, the genetically different cell types came from a single embryo due to a mutation. In a chimera, the genetically different cell types come from two embryos that have fused together. A single individual can also have more than one form of mosaicism.

I have tried to keep the article simple so the reader doesn't need previous genetic knowledge. Readers may also be interested in the book "Cats are Not Peas" which is about George, a male calico.


Physical gender is largely due to the X and Y chromosomes. These are the only chromosomes that don't make a matching pair. Females can only pass on X chromosomes. Males can pass on both X and Y chromosomes and it's a gene on the Y chromosome that tells an embyo to develop as a male. An egg that inherits two X chromosomes normally becomes female. An egg that gets an X and a Y chromosome normally becomes male.

The ginger colour of cats (known as "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. If she inherits no O genes, she will be black.

One explanation is that some skin cells activate the O gene while others activate the gene in the equivalent place on the other X chromosome (more precisely, these are alleles, or variants, of the same gene). This occurs early on in the embryo and as skin cells multiply, they form patches. Another theory is that the cells activate and migrate across the embryonic skin surface (this is explained in Tortoiseshell and Tricolour Cats). Either way, the skin is a mosaic of cells where some have the O gene active (converting black pigment into ginger pigment) and some do not (forming "normal" black pigment). This can only happen in cats with two X chromosomes.

Normal 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. More rarely there are male cats with XXY genetic make-up (Klinefelter's syndrome). These are physically male because they have a Y chromosome. However, just like XX females, they undergo X-inactivation and are genetically mosaics. If only one of those X chromosomes carries the O gene, this can result in a tortoiseshell tomcat. The O gene is a "sex-linked gene" because it is carried on a sex chromosome. More details and the different outcomes of mating red cats, black cats and tortoiseshell cats are given in tables in Tortoiseshell and Tricolour Cats. The diagram below gives a simple summary of the possible outcomes.

Mating together a red male and a red female should not produce tortoiseshell kittens because the cats can only pass on the "O" genes to their kittens. However a tortoiseshell female called Royal Mainly's Otylia turned up in such a mating. Otylia has a small black patch on her ear, probably the result of a somatic mutation. Otylia was a "genetically impossible kitten" because neither parent had the gene for black. Tortie-looking kittens like Otylia often go unnoticed, especially in random-bred cats.

A misconception that I've seen on a newsgroup said "Male calicos are quite rare, because the gene that produces calico colouring is usually lethal in utero to male foetuses (the technical explanation's a bit complicated, but you could look it up in any good book on cats). This doesn't mean a male calico is highly valuable. For one thing, a true calico male is usually sterile, so it can't be used to breed a line of 'pure' calico cats." An explanation from 1992 was similarly mistaken when it said "The calico male is probably triploid i.e. XXY which allowed the calico recessive to show." Tortie and calico are due to an interaction of two genes; it is not a single recessive gene and it isn't lethal to male embryos. Because most tortie males are chimeras (2 fused embryos), and not XXY, they are actually fertile although they only pass on either black or orange, but not both.


Almost all female mammals are mosaics i.e. they have a patchwork mixture of two genetically different types of cell although this may not have a visible effect. Females inherit 2 X chromosomes while men have an X and a Y chromosome. While other chromosomes must be inherited in matched pairs, males get along just fine with only one X chromosome. This suggested that having two X chromosomes could cause some sort of genetic abnormalities (too many genes), but female mammals overcome that problem by inactivating one or other X chromosome. The cells of female mammals contain something called the "Barr body". In the 1960s, geneticist Mary Lyon suggested that females switch off one X chromosome in every body cell and the Barr body is the bundled up corpse of the switched off X chromosome. Very early in the development of female embryos, each cell inactivates one of its X chromosomes at random. Each of these cells eventually gives rise to a patch of cells in the adult female that has the same inactivated X.

Random inactivation means that a female mammal is a mixture of two different cell types. Some regions of her body use the X she inherited from her mother, the rest use the X inherited from her father. She is a mosaic of two cell populations. Since the X chromosome carries around 5% of her genetic material, those different patches can be genetically very different! In cats this is visible as tortoiseshell females - some X chromosomes give rise to red fur while others give rise to black fur. A similar effect is seen in human females with Anhidrotic Ectodermal Dysplasia (reduces the number of sweat glands in the skin). Where only one X chromosome carries the faulty gene, the skin becomes a mosaic of small areas of sweating and non-sweating skin which only becomes visible in certain conditions.

Although the effect of mosaicism may not be visible (or is only visible in tortoiseshell cats), the whole body is a mosaic of genetically different cells. This was demonstrated when CC the cloned kitten was born. Cloned from a tortoiseshell-tabby female, CC could have turned out either red-tabby or brown-tabby (the white patches are caused by a different gene), but not tortoiseshell tabby. Nature's equivalent of clones are identical (monozygotic - coming from a single egg) twins. X chromosome inactivation means that female identical twins may turn out to be very different and at the genetic level.

It is possible (though statistically unusual) for an embryo to use one X chromosome almost to the exclusion of the other. In female cats this can lead to genetically tortoiseshell females appearing to be wholly black or wholly ginger, but having unexpected black, ginger or tortie kittens. In apparently solid ginger or solid black females there be other coloured fur present, but it may be one or two hairs only - the equivalent of a needle in a haystack. If the cat also inherits the white spotting gene, those other coloured hairs may be obscured by white patches. Such cats are sometimes called "cryptic torties" (meaning "hidden torties")

Although X chromosome inactivation is only visible on the skin, it affects all the cells of the body. In 2006, I was contacted by Lisa Lorea whose tortie female displayed erratic behaviour and died of a neurological problem aged 9 years. Lisa had been told that torties were more prone to such problems. My own tortie girl, Motley, was put to sleep due to a brain tumour in 2006 (she had previously had a mammary tumour removed). It is possible, though I have not found any solid data or research to confirm it, that the X-chromosome inactivation that causes the tortie pattern is also linked to a higher frequency of such problems and that this might be linked to their reputation as "naughty torties".


Originally, all tortoiseshell tomcats were believed to be XXY individuals which accounted for them being infertile and having "female" colouration. Some also behaved more like females which was attributed to the supposed XXY genetic make-up. With genetics testing then in its infancy and expensive, the other conditions now known to cause tortoiseshell males were not recognised or understood.

In earlier feline genetics texts, mosaic was defined as a genetically red/red-and-white (or dilute of these) cat which has patches of black hairs so that it appears to be tortoiseshell/calico. The black patches (or hairs) are not genetically inherited; they are caused by localised mutation of skin cells in the embryo (somatic mutation). The mutant skin cells produce black pigment. This form of mosaicism was once thought to account for all fertile 'tortoiseshell' males since test matings showed them to be genetic reds/red-and-whites. Somatic mutation also occurs in females, but is mistaken for normal tortoiseshell.

As genetics knowledge advanced, the terms "mosaic" and "mosaicism" have been (and often still are) used to mean different things. Some texts use "mosaic" to mean the outward appearance, regardless of the cause while other texts use mosaic to mean a genetic effect which may or may not be visible. To avoid confusion, I have tried to keep to a consistent set of definitions e.g. somatic mosaic, XY/XXY mosaic.

The discussion of cats with unusual chromosome complements (XXY, XYY etc) also results in Gender Anomalies

. Many of the listed anomalies have not been documented in cats, but their existence in other mammals means there is a probability that they also occur in cats. Gender/fertility problems are only usually investigated in pedigree cats in breeding programs. In the much larger random breeding feline population, such anomalies are likely to go unnoticed.

In the last decade, there have been more studies into tortie and calico males which has turned the XXY theory on its head. Many of the tortie males studied were fertile and tissue samples showed them to be normal XY males. Some of them have genetically impossible colour combinations such as black, grey and white patches or red, white and grey patches. However, they only passed on one of those colours (black or grey; red or grey) to their offspring. This warranted more detailed study and the surprising result that many (or most) tortie tomcats are due to chimerism and not to the XXY chromosomal abnormality. A few others were due to mistaken identification of their colour.


I often receive photos of supposed calico tomcats that are really classic tabby-and-white cats. Where there are only small patches of tabby, the thick black markings on a brown background can be confused for tortie patches. The brown, rather than red, background is the giveaway.

A condition that can sometimes mimic tortoiseshell occurs in Norwegian Forest Cats due to the "amber" gene and another is a late colour change from silver to golden due to the inhibitor gene. Amber causes previous black fur to change to a cinnamon colour as the cat matures. The visual effect depends on the original colour - solid, black-and-white or tabby. In this photo a black-silver tabby is shown during the change from silver to golden; starting with the back. The areas already affected appear as red tabby. This Norwegian Forest Cat was bred by Yve Hamilton Bruce and shows the start of the colour change. It this stage it could be mistaken for a tortie male. Eventually the whole coat will become a golden colour.



All female mammals are mosaics due to X chromosome inactivation. There are other forms of mosaicism where the different genotypes are due to a mutation or abnormality occurring in a cell of the embryo. Mosaics occur when a mistake during cell division in the early embryo stops the correct number of chromosomes segregating to each cell, or creates a mutation in a single gene. As the cells multiply, some parts of the embryo are built from the normal cell and other parts are built from the mutant cells. Where this happens in one of the first few cell divisions after fertilisation, a large proportion of cells will inherit the mutation or chromosomal anomaly. Although all parts of the embryo come from a single fertilized egg, that egg has given rise to 2 slightly different populations of cells. If it affects the segregation of chromosomes, the mosaic will have patches of cells that have an extra chromosome (called trisomy) resulting in trisomic patches of tissue. An XY male may have some XXY tissues (another potential cause of tortoiseshell male cats).

Mosaicism and X chromosome inactivation means that female identical twins never carry exactly the same genes. At a genetic level they are not identical. When an XX embryo splits in two, the two embryos follow their own developmental paths. The random nature of X inactivation in the two embryos can create startlingly different individuals. One female twin is colour blind or haemophiliac and the other isn't (these traits are carried on the X chromosome). The occurence of such genetically different twins leads researchers to suggest that the X chromosome inactivation process can trigger some cases of twins. X chromosome inactivation occurs early on and researchers suggest that an early split ensures that one embryo inherits the lion's share of good cells (having activated the more healthy X chromosome) while the other may not survive at all (comprising cells which activated the damaged X chromosome). Possibly an area of genetically less healthy cells is ejected (but continues to develop) to ensure that the other cells develop into a viable individual.

The usually invisible mosaic nature of females is suggested as the cause of auto-immune diseases; diseases where the body turns upon itself. Autoimmune disease occurs when the immune system treats part of the body as foreign tissue and attacks it. Since some auto-immune diseases are more common in females, it could be that immune cells commanded by one particular activated X chromosome are attacking body cells where the other X chromosome is activated. XXY (Klinefelter) males also seem prone to autoimmune disease.

Leaving aside mistaken identity, there are three possibilities that can cause tortoiseshell tomcats: somatic mutation (sometimes simply termed mosaicism), Klinefelter syndrome and chimerism (a mosaic made up of of 2 different embryos).

cat with somatic mutation

Somatic mutation causes ginger cats to have small black spots, much like moles or birthmarks in humans. Occasionally, these black blemishes may be large enough to give the appearance of a tortoiseshell cat, albeit one with a low amount of black. The size of the black patches may also depend on how early in embryo development the mutation happened. Very late and it gives a spot or speckle. Where it happens earlier, the black patches are larger as the cells multiply during embryo growth. Somatic mutation is rarely noticed in female cats because tortoiseshell is an unremarkable colour of females - it may be noticed if a black or tortoiseshell female kitten appears in a litter where black or tortoiseshell is a genetically impossible outcome of the mating. In ginger-and-white cats where there is a lot of white, the addition of even small black patches can give the appearance of a calico cat.

cat with somatic mutation

A somatic mosaic lion called Ranger was born at Glasgow Zoo, Scotland in about 1975, the offspring of some lions acquired from Manchester's Belle Vue Zoo. At birth, Ranger had a black patch that stretched from his right paw, all the way up the inside of his leg and across his chest. It was believed to be the first time melanism, even partial melanism, had been recorded in the African lion (apart from anecdotal cases). Ranger (he was sponsored by Glasgow Rangers Football Club!), frequently mated with proven fertile female lions at the zoo, but failed to produce any offspring. Zoo staff believed he had a chromosome abnormality causing him to be sterile. Due to age and illness, Ranger was put to sleep in 1997 and his body was sent for post mortem at Glasgow Vet School. It was hoped that blood samples could be tested for the suspected chromosome abnormality, but it was not possible to get testable blood samples from the body. Sadly it seemed no-one thought to analyse tissue samples from the black area and golden area, nor a sample of testicular tissue. The pathologist believed that the melanistic patch was similar to that sometimes seen in domestic cats and which also results in sterility. However melanistic patches (somatic mutation) rarely cause sterility in domestic cats. Ranger was unlikely to have been XXY (which does cause sterility) because black is not seen in lions. The black patch on his fore-quarters should have had no effect beyond the affected cells though possibly Ranger's mutation also affected internal tissues, including the testes.

Non-Disjunction Normally, when body cells divide, each chromosome unzips into 2 strands (called chromatids) and each daughter cell gets one of the strands. In non-disjunction, both strands go into one daughter cell and no strands go into the other. In partial non-disjunction, a fragment of the chromatid - and therefore the genes on that fragment - is missing from the daughter cell. The effect is visually the same as a somatic mutation.

Mosaic Albinos. Another different type of mosaicism was reported in a shaded cameo Devon Rex kitten. The cat had golden eyes but was photosensitive, he remained scrawny and suffered chronic eye and respiratory infections. Investigation showed that his immune system was very poor. After months of tests to find out what was wrong, the diagnosis of mosaic albino was made when it was noted that the cat's fur was growing progressively lighter in colour and he was becoming more light sensitive. He ended up almost pure white with a tiny amount of red on the edges of his ears. The eventual diagnostic symptom of his condition was that his scrotum was pure white. Any whole male with colour should have darker fur on the scrotum. The poor immune response was linked to albinism. The mosaic albino condition is found in horses but is uncommon in cats.

Germ Line Mosaicism. Early in the development of an embryo, the cells that will give rise to sperm or egg cells in adulthood become separated from the rest of the developing embryo. These are known as germ cells and the cells set aside are called the germ line. Mutations in germ line cells rarely affect other body cells (somatic cells), but the mutation can be passed on to offspring though eggs of sperm which arise from those mutant germ cells. Thos mutations will become part of the offspring's DNA and can be passed on to future generations. Where some germ cells are affected and others are not, this is called germ line mosaicism or gonadal mosaicism. For example, a mutation might occur in the germ cells which go on to produce sperm and the stud cat sires kittens which are 'genetically impossible' according to his own coloration or type. An example of a germ-line mutation is Treker, Bonnie Arnold's normal-sized Persian stud who sired a proportion of miniature Persians due to a mutation in his sperm-producing cells. Treker's sperm-producing cells are a mosaic of cells some of which have the gene for normal size and others with the dominant gene for miniature size. Offspring which inherit the gene for miniature size grow into miniature cats; because the gene is part of their DNA they can pass it on to their offspring.

Chimerism. Early in the development of an embryo it bumps into, and merges with, another embryo. Where these embryos have are different colours, the resulting single embryo develops into a cat that is a composite of these colours. As well as producing bleck/red tortoiseshells (both male and female), this can produce some very unusual tortoiseshell-like patterns in impossible colour combinations.

mosaic karyotypes


There was an interesting Thesis presented at University Claude Bernard Lyon 1 (Medicine – Pharmacy) by Morgane Canu in 2022, “The Contribution of Molecular Genetic Tools in the Identification of Genetic Anomalies in Canine and Feline Mosaics, Chimeras And Monozygotic Twins.” This contained a section about a number of mosaic cats.

A somatic single gene point mutation, if transmitted by mitotic division to a new cell line, can give rise to single gene mosaicism in an individual. The individual is made up of two cell populations: a population of original cells possessing the original allele and a cell line carrying the mutated allele. This situation would explain ginger cats with an isolated black patch. A single gene point somatic mutation may be a reverse mutation e.g. if the patch is of wild colour (e.g. black) on a coat of mutated colour (blue).

An epigenetic mosaicism refers to an alteration in the expression of a gene/group of genes, without any associated change in the nucleotide sequence. This is the case for X-linked mosaicism (X inactivation) in female cats, but it can also occur in autosomes.

- Picasso, a tortoiseshell Maine Coon male (diluted: blue and cream) (fertile) - 38 XY, with no abnormalities. 3 alleles present for some markers = chromosomal mosaic or chimera.
- a tortoiseshell Persian male (neutered) – no karyotype data obtained. 3 alleles present for some markers = chromosomal mosaic or chimera.
- Peter, a European tortoiseshell type male (tabby and white) (unknown fertility) – no karyotype data obtained. 3 alleles present for some markers = chromosomal mosaic or chimera.
- Tweety, a European-type tortoiseshell male (diluted(neutered): blue and cream, and white) (unknown fertility) – no karyotype data obtained. His genetic fingerprint showed a maximum of two alleles for each marker, but 2 markers showed a different alleles present in the cheek swab when compared to the blood/hair samples.
- Norman, a Maine Coon male supposed tortoiseshell (silver, tabby and white) (neutered) – no karyotype data obtained. Only a cheek swab available, no genetic anomalies detected. No conclusion reached because further samples were not available.
- Sonata, a male European tortoiseshell type (unknown fertility) - 38 XY, with no abnormalities (cheek swab, red hair and black hair samples). Because his colours were well intermixed, a somatic mutation (either black to red, or red to black) at an early embryo stage, giving rise to 2 cell lines was most likely. Alternative hypotheses were micro-chimerism or translocation of the Orange locus on an autosome (where a gene breaks off one chromosome and attaches itself to a different one during cell division).
- a blue-cream European type male, tortoiseshell (diluted: blue and cream) (neutered) – no karyotype data obtained. Cheek swab, blue hair and cream hair all matched. Since only 2 alleles were present for each marker, XXY was more likely.
- Spritz, a tortoiseshell (and white) European type male (neutered) – no karyotype data obtained. 3 alleles present for some markers = chromosomal mosaic or chimera.
- Pio, a European tortoiseshell (and colourpoint) type male (red point with black forepaw) – no karyotype data obtained. Cheek swab, red hair and black hair samples all matched. Somatic mutation (reversion from red to black) most likely.
- Narnia, a British shorthair male with black and blue coat (and minimal spotting) (fertile) – no karyotype data obtained. Results for the black hairs differed (in 2 markers) from the cheek swab and blue hairs. Testing found Narnia to be black, carrying dilution (blue) = D/d, but his blue hairs matched his cheek swab. If there is a somatic mutation, it is not on any of the tested markers.
- a male cat (name unknown) of the European black and blue (and white) type (neutered, blue and white with black patch inside one thigh) – no karyotype data obtained. 2 alleles present for all markers, all identical. Somatic mutation (reversion of blue to black in one cell, giving rise to a clonal patch of black fur.)
- Kodak, a tortoiseshell European short haired male with a tuft of long red hair on his back (unknown fertility) – no karyotype data obtained. 2 alleles present for all markers (red hair, black hair and ticked hair), all identical. Possibly micro-chimerism where a few cells l/l (homozygous recessive longhair) and XO/Y cells had been transmitted to a L/- Xo/Y foetus. Gene sequencing of the hair length gene (L) for long and short hairs, and red and black hairs is needed.


In theory, what else could cause non-sex linked tortie? The action of the extension gene E has only recently been documented in cats thanks to the emergence of "amber" (Norwegian Forest Cat) and "russet" (Burmese), but is well-known in rabbits where it causes the black/tan harlequin pattern. The extension gene affects the banding on individual hairs by switching between black and red pigment production. "E" (full extension – normal/wild-type) is dominant and a cat that is "EE" or "Ee" switches between the production of eumelanin (black-based) and phaeomelanin (red-based) pigments as the hair grows. In a cat that is "ee" (non-extension) cells completely switch off eumelanin and switch red-pigment instead. In combination with the agouti gene, the dominant "E" gene produces the normal blackish/yellowish banding on normal agouti hairs, but where the cat is ee – homozygous for non-extension – cells produce red pigment despite the cat being genetically black (or blue) which results in Amber (Norwegian Forest Cats) and Russet (Burmese).

In cats only E and e have been documented so far, but in rabbits there are at least 4 variants of the Extension gene (alleles). In order from dominant to recessive in rabbits, these are:

1: Steel, Extension of Dark Colour (Es)
2: Full Colour Extension (E) – normal agouti banding
3: Harlequin (Japanese Brindle) Pattern (ej) – non-sex-linked tortoiseshell pattern
4: Extension of Light Colour/Non-Extension of Dark (e) – russet/amber in cats

The "ej" (Japanese brindle) allele creates Harlequin in rabbits. Very early in the development of the embryo, Harlequin causes the switching mechanism to break completely. Some cells lose the ability to make black pigments, while others are stuck permanently in ‘black mode’. The cells continue to divide and they form clonal patches of either red or black. Being recessive, the presence of dominant ‘E’ (Eej) gives solid black (it overrides the action of ej). In rabbits, the presence of recessive ‘e’ (eje) gives wholly red colour. But ejej (homozygous ej) gives a mixture of red patches and black patches. In cats, this would probably be indistinguishable from a normal tortoiseshell pattern because – regardless of the mechanism - the visual effect is a turning on of either red or black pigment. The major difference would be the mode of inheritance – ‘ej’ isn’t sex-linked and it could be transmitted and displayed by both males and females.


Calico male Skipper (Hokkaido, northern Japan) was documented by zoologist Jeremy Angel in the 1980s. Skipper originally came from Sapporo. Angel noted that perhaps one in every one or two hundred tortoiseshells is male and that they were once prized by superstitious boat owners. In 1981, 15 month-old Skipper was described as looking un-tomcat-like. His head shape and general conformation more closely resembled a female, but his genitalia were unambiguously male. He was calico with clearly defined patches of orange and black and unusually for local Japanese cats, he had a straight tail, not a bobtail. His mother was a longhaired calico, but his father was unknown. Skipper was described as exceptionally calm and friendly, was extremely fastidious in toilet habits, and did not spray. He showed no interest when his mother came into oestrus. When introduced to other cats, including full toms, in an enclosed colony, Skipper was curious, nonchalant, non-aggressive and did not spray.

Other tomcats treated him as a female and attempted to mount him. This was initially thought to be dominance behaviour towards the newcomer, but Skipper remained attractive to several of the tomcats and did not object to being mounted. He sometimes appeared to enjoy this attention and remained crouched and receptive, even after the other tomcat had dismounted. He also adopted the lordosis position and chirruped to attract his suitors. During summer, he was mounted frequently enough that his neck became callused (common in highly active oestrus females). Skipper's attitude to females was very different. Aged almost 2 years old, he still did not spray and still showed no interest in mating with females. He was sometimes aggressive towards females and frequently fought with them. He was always the instigator of any fight. Tomcats do not normally fight with females, particularly with oestrus females although females will fight among themselves. In this respect, Skipper's behaviour was female.

In 1982, Skipper showed signs of male behaviour. He stopped picking fights with females and began to show sexual interest in them. He also began to spray. Compared to a normal tomcat, his spraying and his sexual behaviour were half-hearted. Angel isolated Skipper with an oestrus female. Although not an enthusiastic suitor, Skipper mated her three times, but failed to impregnate her. Angel had intended to mate him with other females to determine whether he was fertile, but Skipper died during an outbreak of cat flu. Angel concluded that Skipper was either a slow starter or possessed an additional X chromosome which feminized his behaviour. This was the prevalent theory at that time. Because, at different times, Skipper showed both female and male behaviour, he was possibly an XX/XY chimera with external male genitalia but some internal structures (perhaps the brain) being genetically female or producing female hormones.

An extra X chromosome (XXY, Klinefelter Syndrome) does not normally result in female behaviour in phenotypical males. However feminisation used to be considered the norm and in 1997 a cat owner stated "Most calico toms are born infertile with a single testicle. We are currently in the process of having a calico tom checked to see if he is fertile. If he is he will have some sperm 'banked' before being neutered. He is white, tan/orange tabby patches, and light to dark blue (grey) spots (and a few stripes)." The only male calico encountered by this owner’s vet was a long time ago and very obviously a genetic error, since the cat was sterile and had numerous other problems.

Rachel E Gibson wrote in 1997 of a male calico which only mated with very sexually aggressive females and which aligned himself with the females more than the males. The other males treated him like a female so he may not have many male hormones. His behaviour corresponded with the theory of the time that XXY made tortie tomcats feminine in their behaviour. Boo, another calico male, was also believed to be sexually confused because of his extra chromosome. He attempted to mate with both females and males and also tried to nurse kittens or carry them around like a mother cat. Boo continued to spray after being neutered and was described as somewhat fat, not uncommon in XXY cats.


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