MOSAICS, CHIMERAS & TORTIE TOMCATS
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.
HOW COMMON ARE TORTIE TOMCATS?
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.
Autosome: 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.
THE GENE FOR GINGER
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.
X CHROMOSOME INACTIVATION
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".
EARLY THEORIES EXPLAINING TORTIE TOMCATS
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.
MISTAKEN IDENTITY TORTIES
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. This 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 to amber; 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 amber colour change. It this stage it could be mistaken for a tortie male. Eventually the whole coat will become amber.
MORE ABOUT MOSAICS
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 identiy, there are three possibilities that can cause tortoiseshell tomcats: somatic mutation (sometimes termed mosaicism), Klinefelter syndrome and chimerism.
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.
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.
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.
RECENT TORTIE AND CALICO MALE CATS
In December 2013, Joy Fatooh adopted a tortoiseshell kitten from an animal shelter who, according to her friends, looked as if it had been designed by committee or assembled from random cat parts: a tuxedo tortie with orange tabby hind legs. The adoption papers and the vet said "female" but when Joy took 13 week old Monster for vaccinations, the vet admitted she hadn't taken a good look under Monster's tail before. Monster had a distinct set of male attributes under HIS tail. Monster also has large, low-set ears and wavy whiskers, similar to a Devon Rex. His mother is grey and his brother is a normal black tuxedo cat. The photos show Monster at 9 and 13 weeks, showing his odd coloration from several angles, including his male attributes! When Monster is neutered, blood samples will be sent to Leslie Lyons for testing, but if these are inconclusive, tissue samples would be required from the different coloured areas.
Fernando of Canpaza is a cream-coloured German Maine Coon (born 2002) with two small blue spots on the top of his head which may be due to a localised mutation of skin tissue. Several breeders told Fernando's breeder, Elvi Weidemann, to neuter this unusual tomcat and sell him as a pet. However, a veterinary hospital in Hanover (Germany) was running a study project on tortoiseshell tomcats and Fernando was included in that project. A blood test proved he was a normal XY male. Breeder and researchers were keen to see if Fernando could father kittens - and what colour he passes on to his kittens. In December 2003, the results of Fernando's matings were announced. He was mated with Roxanna a black-silver tortie resulting in 8 kittens weighing between 70 and 95 grams (smallish, not unusual for a large litter): 1 cream-smoke male (Bucky), 1 black-silver tortie female, 1 red tabby female, 3 red males, 1 black tortie female, 1 cream male and a black-silver tortie male. Sadly the latter 5 kittens died of a bacterial infection; the black-silver tortie male being an especial loss. Fernando had one further test mating before being neutered. It was particularly interesting that a tortie male produced a tortie son, but there are now several reports of two, or even three, generations of tortie males in a pedigree.
Are there a disproportionate number of tortie males in the Maine Coon breed? Pretty Boy Floid is a famous fertile red, grey and white Maine Coon tortie male (he is an XY/XY chimera). Another Maine Coon tortie male, Koonikki Feirfiz Geezabird (Geezer), had intersex traits - testicular tissue in the position of an ovary plus one normally descended testicle. Geezer reportedly had both male and female behaviour and the vet believed him to be XXY. Like Skipper (described in Tortie Tomcats: Chimeras), he solicited other males to mate him, but he also attempted to mate females. His father has since sired another tortie male (tortie tabby from a brown tabby female) so possible the father is producing aberrant XY sperm. A third tortie Maine Coon, Stormwatch Maxwell, was reported in 1997. Sired by a black smoke, Maxwell was mostly bright red, but had a black patch on one side of the face. Perhaps all the brouhaha over Pretty Boy Floid has simply made people more likely to report these unusual males.
Marie Hope, Coldstream Cattery, has another tortie Maine Coon, this one a tortie-tabby whose red is mainly along his lower back and the base of his tail. One side of his face is black/brown tabby while the other is red tabby. This is more than the rufousing that is sometimes seen on the muzzles of tabby cats.
Miky (Kapamba's Home Miracle) is a Dutch-bred tortie-tabby-and-white male Norwegian Forest Cat. He was probably sterile as he has now been castrated (to eliminate other male behaviour such as spraying) and there are no reports of him siring offspring prior to castration. His chromosome make-up hasn't been investigated, but the well defined patched pattern suggests either XXY (Klinefelter) or a chimera. The cattery appears to specialise in tortoiseshell-and-white Norwegian Forest Cats.
The cat above is a seal-tortie point Birman, Grand International Champion Dar es Salaam's Varsha. Varsha was a stud cat that sired a number of very good kittens and he breeds as though he were a red-point. The distribution of black on his coat suggests he is a chimera. It is interesting that he has achieved Grand championship status as many registries cannot accommodate tortie males in competition, except as "Any Other Colour".
It is interesting to me that so many of the recently reported tortie males belong to longhaired breeds such as Maine Coons, Siberians, Norwegian Forest Cats, Birmans and Persians.
In 2005, breeder Peta Cohen (Kamikai Kattery, Victoria, Australia) came across two Scottish Shorthair (straight-eared Scottish Fold variants) cats with unusual coat patterns. One was a red-cream-and-white which also had two dark red patches on the coat. The other was a black-grey-and-white patchwork. These cats both had a mix of dilute and non-dilute colours, indicating chimerism. They came from a Scottish Shorthair/Fold breeder who had also bred a chocolate-lilac-cream female cat, and several more red-cream-and-white patched cats and black-grey-and-white cats. In 2008, a male torbie-and-white Scottish Fold was reported, but there was no indication it was related to the Scottish Shorthairs reported previously.
Very occasionally, the tortoiseshell "male" is not male at all. It is genetically a female cat which, due to hormonal problems during embryo development, has developed the external characteristics of a male cat. This is the normal condition in female hyenas where the clitoris is elongated to form a pseudo-phallus and there is also a false scrotum. It is occasionally been reported in other animals which appear to be hermaphrodite, intersex or are masculinised females (genetic females which look like males). Depending on the cause, the false scrotum may contain fatty tissue or ovarian tissue. This may explain those cases of tortoiseshell "males" which act like females - they are genetically female with an external male appearance. These gender anomalies are discussed in more detail in a later section.
A mosaic that made online headlines in 2012 was not a male tortie, but a strikingly marked female known as "Venus the Chimera Cat". Venus has a brindled body and a face that is a perfect half-and-half of black and ginger. Intriguingly, her eye on the ginger side is blue, while that on the black side is green. This doesn't necessarily make Venus a chimera (fused embryos) cat - that would require DNA testing of the black and ginger sides to determine. The solid black area could also be a result of cell mutation (resulting in an effect like a birthmark). The colour of the fur is linked to eye colour, so a mutation in that area would affect both skin pigment and eye pigment. The blue eye on the ginger side is unusual, but not unknown and may be related to her having the white-spotting gene.
Next Page: Early Reports of About Tortie Tomcats