Tortoiseshell is a coloring found in
cats caused by a
combination of specific genetic traits. It is a mix of
phaeomelanin based colors (red) and eumelanin
based color (black, chocolate and cinnamon). The pattern results in a cat with
patches of red and patches of black, chocolate or cinnamon. The size of the
patches can vary from a fine speckled pattern to large areas of colour. Dilution
genes may modify the coloring, lightening the fur to a mix of cream and blue,
lilac or fawn. Occasionally silver or ginger
patterns are also seen (sometimes called "torbies"). Tortoiseshell also can be
expressed in the
points pattern. The description "tortoiseshell" (also called 'calimanco' or
'clouded tiger' in North America) is correctly reserved for cats with coats that
have no white markings, while those that are largely white with orange and black
patches are described as calico (in the US) or tortoiseshell-and-white
(in the UK). Tortoiseshells and calicos are not specific
cat. The tortoiseshell markings appear in many different breeds.
Coat coloration in cats is complex. The genes involved include the Orange
gene, O, which in its dominant form, XO, produces orange fur, and
in its recessive form, Xo, produces black fur.
For a cat to be calico, it must simultaneously express both of the two genes,
O and o, which are located at the same location on the
X chromosome. Males normally cannot do this: they can have only one allele, as
they have only one X chromosome. Over 90% of tortoiseshell cats are females.
Occasionally a male calico is born. These may have Klinefelter's syndrome,
carrying an extra X chromosome, and will almost always be sterile or they may be
a chimera resulting from the fusion of 2 differently coloured embryos.
Spotting gene causes white patches to cover the colored fur. Although there
is no genetic difference, the amount of white is artificially divided into
mitted, bicolour, harlequin and van,
going from almost no white to almost completely white.
In normal female tortoiseshell cats and in Klinefelter males, the position of
the patches depends on which X-chromosome is active in each cell and which is
inactivated to become a Barr body.