Although it’s not about horse color, polydactyls are fascinating in terms of genetics. Here is a good blog post about them, complete with a radiograph that shows where the extra digit originated.

Polydactyl People and Ponies – A Gallery of Extra Digits (and Hooves)

My youngest son is an avid video game player, so he was highly amused to learn that one of the genes involved in limb development – and therefor in polydactylism – is called Sonic Hedgehog (Shh). Here is a good article on Hemingway’s polydactyl cats that explains the hedgehog genes.

For Whom the Cell Mutates