First off, what is a calico cat? A calico cat is not a breed of cat, it is a color pattern. To be called "calico", three colors must be present: black, white and orange. Variations of these colors include gray, cream and ginger. A "true" calico cat has large blocks of these three colors, a "tortoise shell" or "tortie" cat has a mix of these three colors (blended/swirled together more than distinct blocks of color).
Now that a calico cat has been defined as a cat with three colors, the question is: why are they nearly always female? The answer is in genetics. Coat color in cats is a sex-linked trait, a physical characteristic (coat color) related to gender. Female animals have two X chromosomes (XX), males have one X chromosome and one Y chromosome (XY). The genetic coding for displaying black or orange color is found on the X chromosome. The coding for white is a completely separate gene.
Since females have two X chromosomes, they are able to "display" two colors (orange and black, or variations thereof) and white; creating the 3-color calico mix. Since males have only one X chromosome, they can only be orange OR black. It is more complicated than simply having the color genes -- it is a complex process of dominant and non-dominate genes interacting on the X chromosomes, but that is the basis for coat color in calico cats.
Can a calico cat ever be male? Yes, in rare instances. In this situation, the cat has two X chromosomes and one Y chromosome (XXY). Cats with this chromosomal configuration are usually sterile (not able to breed). This is similar to a condition in humans called Klinefelter's syndrome http://en.wikipedia.org/wiki/Klinefelter_syndrome, or XXY Syndrome.
Interesting calico cat fact: on October 1, 2001, the calico cat became the official cat of the state of Maryland in the United States.
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.[5] 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.
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