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Calico cats are domestic cats with a spotted or parti-colored coat that is predominantly white, with patches of two other colors (often, the two other colors are orange tabby and black). Outside of North America, the pattern is more usually called tortoiseshell-and-white. In the province of Quebec, they are sometimes called chatte d'Espagne (French for '(female) cat of Spain'). Other names include tricolor cat, mi-ke (Japanese for 'triple fur'), and lapjeskat (Dutch for 'patches cat'); calicoes with diluted coloration have been called calimanco or clouded tiger. Occasionally, the tri-color calico coloration is combined with a tabby patterning. This calico patched tabby is called a caliby.
"Calico" refers only to a color pattern on the fur, not to a breed. It is absent from lists of breeds. Among the breeds whose standards allow calico coloration are the Manx, American Shorthair, British Shorthair, Persian, Japanese Bobtail, Exotic Shorthair and Turkish Van.
Because genetic determination of some coat colors in cats is linked to the X chromosome, calicoes are nearly always female. Because of the genetics involved, calico males are rare, and generally have impaired vitality and are almost always sterile.
The coat pattern of calico cats does not define any breed, but occurs incidentally in cats that express a range of color patterns; accordingly the effect has no definitive historical background. However, the existence of patches in calico cats was traced to a certain degree by Neil Todd in a study determining the migration of domesticated cats along trade routes in Europe and Northern Africa. The proportion of cats having the orange mutant gene found in calicoes was traced to the port cities along the Mediterranean in Greece, France, Spain and Italy, originating from Egypt.
In genetic terms calico cats are tortoiseshells in every way, except that in addition they express a white spotting gene. There is however one anomaly: as a rule of thumb the larger the areas of white, the fewer and larger the patches of ginger and dark-or-tabby coat. In contrast a non-white-spotted tortoiseshell usually has small patches of color or even something like a salt-and-pepper sprinkling. This reflects the genetic effects on relative speeds of migration of melanocytes and X-inactivation in the embryo.
Serious study of calico cats seems to have begun about 1948 when Murray Barr and his graduate student E.G. Bertram noticed dark, drumstick-shaped masses inside the nuclei of nerve cells of female cats, but not in male cats. These dark masses became known as Barr bodies. In 1959, Japanese cell biologist Susumu Ohno determined the Barr bodies were X chromosomes. In 1961, Mary Lyon proposed the concept of X-inactivation: one of the two X chromosomes inside a female mammal shuts off. She observed this in the coat color patterns in mice.
Calico cats are almost always female because the locus of the gene for the Orange/non-orange coloring is on the X chromosome. In the absence of other influences, such as color inhibition that causes white fur, the alleles present in those orange loci determine whether the fur is orange or not. Female cats — like all female placental mammals — have two X chromosomes. In contrast, male placental mammals, including chromosomally stable male cats, have one X and one Y chromosome. Since the Y chromosome does not have any locus for the orange gene, there is no chance that such a male could have both orange and non-orange genes together, which is what it takes to create tortoiseshell or calico coloring.
One exception is that in rare cases faulty cell division may leave an extra X chromosome in one of the gametes that produced the male cat. That extra X then is reproduced in each of his cells, a condition referred to as XXY (see Klinefelter's syndrome). Such a combination of chromosomes could produce tortoiseshell or calico markings in the male, in the same way as XX chromosomes produce them in the female.
All but about one in three thousand of the rare calico or tortoiseshell male cats are sterile because of the chromosome abnormality, and breeders reject any exceptions for stud purposes because they generally are of poor physical quality and fertility. In any event, an (unusual) fertile XXY male could not normally pass on any of those X chromosomes to any male offspring, so his male kittens would practically always be normally coloured, either ginger or non-ginger, but not tortoiseshell or calico.
As Sue Hubble stated in her book Shrinking the Cat: Genetic Engineering before We Knew about Genes,
"The mutation that gives male cats a ginger-colored coat and females ginger, tortoiseshell, or calico coats produced a particularly telling map. The orange mutant gene is found only on the X, or female, chromosome. As with humans, female cats have paired sex chromosomes, XX, and male cats have XY sex chromosomes. The female cat, therefore, can have the orange mutant gene on one X chromosome and the gene for a black coat on the other. The piebald gene is on a different chromosome. If expressed, this gene codes for white, or no color, and is dominant over the alleles that code for a certain color (i.e. orange or black), making the white spots on calico cats. If that is the case, those several genes will be expressed in a blotchy coat of the tortoiseshell or calico kind. But the male, with his single X chromosome, has only one of that particular coat-color gene: he can be not-ginger or he can be ginger (although some modifier genes can add a bit of white here and there), but unless he has a chromosomal abnormality he cannot be a calico cat."
It is currently impossible to reproduce the fur patterns of calico cats by cloning. "This is due to an effect called x-linked inactivation which involves the random inactivation of one of the X chromosomes. Since all female mammals have two X chromosomes, one might wonder if this phenomenon could have a more widespread impact on cloning in the future."
Calico cats may have already provided findings relating to physiological differences between male and female mammals. This insight may be one day broadened to the fields of psychology, psychiatry, sociology, biology and medicine as more information becomes available regarding the complete effect of random X-inactivation in female mammals.
Cats of this coloration are believed to bring good luck in the folklore of many cultures. In the United States, these are sometimes referred to as money cats. The Japanese Maneki Neko figurine is almost always a calico cat.
A cat of the calico coloration is also the state cat of Maryland in the United States.
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