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C2007-23

Dear Christopher Cat

Most of our barn cats, though unrelated, have one thing in common: white bellies. Is that a coincidence?

Christopher Responds

No, it’s a common pattern.

I am a handsome, long-haired silver and black striped tabby, and I have a white belly. Moreover, most of the other cats in my family, even the calicos with their colorful coats, have white undersides.

This pattern develops very early in life, when we cats are just ball-shaped embryos.

At the “top” of the spherical embryo, along what will become the cat’s spine, are pigment cells that will determine the color of the cat’s skin and hair coat.

As the embryo’s development continues, these pigment cells migrate down the outside of the sphere, along what will become the cat’s skin.

If they reach all the way to the “bottom” of the sphere, the cat’s tummy will have color. My sister Claire is an example: her entire hair coat is solid gray.

Conversely, if the pigment cells don’t migrate the full distance, the cat’s tummy will remain white, like mine.

Take a look at other animal species – dogs and horses, for example – and you’ll see a similar pattern.

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