Darwin was a cat fancier. He attended the first cat show in 1871 and was particularly interested in polydactyl (extra toes) cats.
Tortoiseshell color is specific to cats and was the first X-chromosome-linked gene identified.
Male tortoiseshell cats are rare. Most have an extra X chromosome and are infertile. A few are chimeras (i.e., they have two sets of DNA because two embryos fused together in utero) of XX/XY. Only a few of these cats are fertile.
A unique anatomical finding in cats is that their auditory bullae (a bony capsule in the middle ear) is double chambered.
Cats with color points, like Siamese, may be cross-eyed or show nystagmus (abnormal eye movement) due to a defect in the optic chiasma of the brain.