Cats
torties occur when there are more pigment producing cells produced from the neural crest
(which becomes the back and spine area). The cells are assumed to have undergone X
chromosome deactivation before migration. The migrating cells carry either O (red) or o
(black) and they migrate at the same rate into their final positions. Where there are many
pigment producing cells, there is more competition (imagine cells jockeying for position on
the skin) and they become intermingled. Where there are fewer pigment producing cells, each
cell has room to grow into patches of colour (imagine plants forming clumps in a flower
border).
As an interesting side note, human women can also have X-inactivation. Although there are
no tortoiseshell women, there is an uncommon condition called anhidrotic ectodermal
dysplasia caused by a faulty gene on the X chromosome. The faulty gene reduces the number
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