The genetics of tuxedo cats:The gene for white spotting affects the embryo cells (melanoblasts) which will become pigment-producing skin cells (melanocytes) which make the pigment for hair. One theory is that the melanoblasts arise from the "neural crest" - the area along the back of the embryo - and they migrate to all over the body during formation of the skin. Where these cells fail to reach their allotted positions before the skin is fully formed, those areas of skin will lack pigment producing cells i.e. be a white spot.
This explains why the white is most often found on the paws, belly and chest - these areas are the most remote from the neural crest and take longest to reach. The slower the migration of cells, the less color there will be. This also explains why the back and the tail may be colored in an otherwise all white cat - those areas are closest to the neural crest and the pigment cells didn't have to migrate very far at all!.
The alternative hypothesis is that the patterning of black and white is the result of the skin surface "cracking" during early embryo development. The skin of such bicolor cats would be basically black, but the dominant S (white spotting) gene, causes cracks to appear, breaking up the black surface into black domains. These black regions drift apart over the embryonic surface as the embryo grows. The white areas form in the regions between the black domains. It may help to think of it as similar to the movement of the earth's continents with areas of sea inbetween. The black domains can end up pushed together to form a single tuxedo style black area. The white belly area might be a ventral seam crack.
Many cats have patches which look as though they could be fitted together like jigsaw pieces; for example a colored spot on a leg might correspond to an inlet on a colored patch on the flank, exactly as though a piece of colored area has broken off and migrated elsewhere. Spots could end up almost anywhere depending on the timing of cracking, the size of the colored domains, and the trajectories they take over the embryonic surface as it grows. In some cats it is possible to fit all the colored areas of cat coat together. The seams can then be projected onto the surface of a sphere (the embryo). As the sphere expands, the colored area cracks apart. Think of a balloon covered in solidified chocolate: the balloon can expand, but the chocolate can't. The cracked solid surfaces then gave patterns just like those found on bicolor cats.
And if you have an all white cat it really isn't white, but some other color. It's just covered completely in white spots.
It reminded me of that time that I decided it would be a good idea to make twelve individual three-layer cakes for a dinner party (almond cake with rasberry coulis filling). So I made eight sheet cakes and cut thirty-six cake tiers with biscuit cutters. The plan was to cover them with ganache, because it would be so easy to frost them by simply pouring the melted ganache over the tops and letting it cascade down the sides. But it was June. June in Arizona. And chocolate can't set up when it's 102 degrees in the kitchen. There was more ganache on the floor than on the cakes. And on the wall, too, because I threw a couple spatulas.
