The term calico refers to the similarity of the cat's coat and the patchwork patterns typical of textiles exported from Calicut, India .
The gene for coloration in cats is found on the X-chromosome. The dominant allele for this gene (O) expresses itself in the melanocytotic production of phaeomelanin, a pigment responsible for red and red-derived colors like orange and gold. The recessive allele (o) expresses itself in the production of eumelanin, a pigment responsible for black- and brown-colored patches . A separate, autosomal gene is responsible for the white patches uniquely expressed by calico cats .
The calico pattern is only present in females (XX) with the heterozygous genotype (Oo) of the coloration gene. The patchwork pattern arises from the random silencing of one of
the two copies of the X-chromosome in each cell. X-silencing occurs early in embryonic development and the randomly selected inactive X-chromosome remains inactivated for its entire lifespan and in all progeny of that cell. Depending on which of the two copies of the X-chromosome is silenced, melanocytes will express either phaeomelanin or eumelanin, resulting in the distinct calico patchwork pattern.
The X-chromosome is inactivated by the Xist (X-inactivate specific transcript) RNA gene. This non-protein coding region produces an RNA sequence that binds and tightly coils the X-chromosome to inactivate it. This inactivated X-chromosome is referred to as a Barr Body after its discoverer, Canadian physician Murray Barr .
In extremely rare cases, the calico pattern can be found in male cats with Klinefelter's Syndrome (XXY), a congenital condition in which males inherit an extra X-chromosome .
: Lyon MF (1961). "Gene action in the X-chromosome of the mouse." Nature 190 (4773): 372-3.
: http://en.wiktionary.org/wiki/calico accessed: 8/30/2014
: "Cat Colors FAQ: Cat Color Genetics" http://fanciers.com/other-faqs/color-genetics
: "Cat Coat Genetics" http://en.wikipedia.org/wiki/Cat_coat_genetics accessed: 8/30/2014
: "XIST (gene)" http://en.wikipedia.org/wiki/XIST_(gene) accessed: 8/30/2014