The ‘white’ in a Calico Cat

The X-chromosome inactivation pattern is seen in tortoiseshell cats (see picture on the left), and calico cats are tortoiseshell with white (Helix, on the right). So where does the white come from? The gene that causes white ‘spotting’ is not on the X chromosome, so I didn’t discuss it before. You see white on males or females, and on different colours of cats. The gene that is changed is involved in cell migration and/or proliferation, so the size of the ‘patches’ is larger on calico cats, making them my mascot for X inactivation. I think the mutation may impact migration as it is generally ventral white spotting – the white patches are often seen at the ‘ends’ of the cat – tips of paws, tip of tails – and belly (yes that is the end as the ‘start’ is on the back!).   However, in an interview with Jane Gitscher, Mary Lyon – the first person to propose the X-chromosome inactivation process – suggested the defect was in proliferation. See PLoS Genetics for the lovely interview: