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CC’s genetic donor is a calico (the American
term for tortoiseshell and white) domestic
shorthair. CC is a tabby-and-white domestic
shorthair. Why isn't CC a calico like her genetic
mother? The answer is due to ‘X-linked
Inactivation'.
Tortoiseshell and calico cats are almost always
female, which means they have two X chromosomes.
One of those X chromosomes contains the gene for
orange coat color. The other X chromosome contains
the gene for black coat color. As the embryo
develops, a process called ‘X-linked
inactivation’ occurs in its tissues. One or the
other X-chromosome in every cell in a
tortoiseshell or calico cat embryo is randomly
inactivated.
This shows up in hair and skin cells as patches
of different colors. If the X-chromosome
containing the gene for orange coat color is
inactivated, that cell goes on to produce black
coat color. If the X-chromosome containing the
gene for black coat color is inactivated, the cell
produces the orange coat color. If the
deactivation happens early on, it produces
relatively large and well-defined patches of color.
If it happens later during development, it
produces a brindled effect.
The white patches and the tabby pattern are
both caused by different genes entirely.
Regardless of which cell was used to produce
CC, because that cell is already an adult cells,
one or other of its X chromosomes would have been
inactivated while the donor cat was an
embryo. CC had an equal chance of being
orange-and-white (or orange-tabby-and-white) or
black-and-white (or tabby-and-white), but would
never be calico. Unless a way can be found to undo
X-linked inactivation at the embryo stage, owners
wishing to clone a tortoiseshell cat will have to
settle for a cat of a different color entirely. If
the X-linked inactivation can be reset, the
inactivation appears to be a random process so the
clone will have the right colors, but not in the
same places as the donor cat.
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