It is one of the most complicated genetic interactions I've ever read about
involving several genes and not only dominant/recessive, but how gene
expression modifies the expression of other genes.
It depends on the entire lineage, which parents carry which genes, which are
passed onto the young, and which ones express in the whole mix.

There are enire books written on the subject, most intended for either a
scientific audience or for cat breeders. I don't pretend to really understand
it in its entirety, but I grasp the (very) basics from doing some research on
tricolor cats.

Recent studies of a cloned cat are proving that patterns may also be
influenced by conditions in the womb as well, since the cloned cat did not
have the same markings as her genetic "mother".
So, even scientists don't fully understand it.

IOW, yeah, God's paint brush.  :)

Dunno, but tortoiseshells are always female for some reason.

They can hold recessive genes (like people) and pop out kittens
completely different than themselves. Siamese is a recessive coloring.
Of our stray gray and white tuxedo cat's 6 kittens, 3 were siamese. I
would not have guessed that in a million years.

What I think is cool too, is Siamese points are heat-activated. Their
coats are darkest at the extremities were there is less blood-flow. As a
Siamese cat gets older, many times their coats get darker all over
because of less blood flow (they are colder.)

Rhonda