Why all the cats?

Calico cats – ‘X’plained… Why calico (and tortoiseshell) cats are the mascots of my lab

No – we don’t study cats – but a calico cat is a pretty example of X-chromosome inactivation.

Mammalian females have 2 X chromosomes, while males have 1 X chromosome and the sex-determining Y chromosome.  Long ago, these were a pair of chromosomes, and contained around 1000 genes, but as the Y chromosome became specialized for its role in sex-determination, it lost many of these genes.  This poses a ‘dosage’ problem as females now had twice the copy number as males – so the process of X-chromosome inactivation came to the rescue and silences most genes on one X in females.  We study how this inactivation occurs, and then is maintained in all cells of a female – as is seen in a calico cat.


Inactivation occurs early in female development, and either X chromosome (the one inherited from the dad – paternal; or the one inherited from mom – maternal) is chosen to be silenced.  Once that choice is made, it is passed down through cell division, resulting in females being a mosaic mix of cells with either the maternal or the paternal X active.  We can see this mosaicism in cats because there is a gene for coat colour on the X chromosome.  This gene has two forms (alleles) – either black (B) or orange (O).  Male cats, having only a single X chromosome, are either black or orange.  But female cats can be black (BB), orange (OO) or black/orange (BO), in which case cells that have silenced the O-carrying X chromosome are black, while those silencing B are orange.

See the “Calico and tortoiseshell cats” gallery for examples of the wide range of colour patterns that X-inactivation can cause.  Many of these cats were photographed at the Richmond Animal Protection Society (RAPS) shelter, in Richmond BC.  Some of these cats have been adopted but some may still be looking for homes (along with other cats, dogs, rabbits and sundry other animals than need a place to live).

For more questions, see:

A poetic explanation of the Calico Cat

But there are three colours – what about the white?

Is it true that calico cats are always female?