The genetics service of a large veterinary hospital gets referrals for three or four male tortoiseshell cats every year. These cats are invariably sterile and have underdeveloped testes. How are these tortoiseshell male cats produced?

The proportion of male tortoiseshell cats is very low. This happens because the colors are linked to sex because they are exclusively on the X chromosome, to have two colors (black and orange) must have two X chromosomes from the father and mother.

For a male cat to be tortoiseshell, a genetic anomaly must occur: the presence of two X chromosomes and one Y (XXY).