The science of calico cats


Why is it that when you cross a black cat and a ginger cat, you get some black male kittens, some ginger male kittens, and female kittens with a calico pattern?

It has to do with X chromosome inactivation, something all female mammals (including you, ladies) go through as embryos. A male cat embryo has a fat X chromosome bursting with relevant genes and a puny, shrunken Y chromosome that gives him nothing but balls, to be blunt about it. Meanwhile, a female has two of those fat X chromosomes. She can’t express the genes on both in the same cell – it would create too many proteins and gum up the system.

Her DNA has an ingenious way of protecting itself from such disasters. Early in gestation, one X chromosome in each cell gets “deactivated” – bound up in proteins and effectively closed for business – through a fascinating process I’ve been learning about in an online class called Epigenetic Control of Gene Expression. The details are not for those uninitiated in molecular biology, but the gist is that female cats have different cell lines in their bodies. In some lines, the paternal X chromosome (the one from dad’s sperm) will be active. In other lines, the maternal X chromosome (the one from mom’s egg) will be active. Since the genes for coat color are on the X chromosome, female kittens will express mom’s genes in some areas and dad’s genes in others. That’s what gives calico cats their trademark patchwork look.

It’s also why you rarely see calico toms. Male cats don’t need X inactivation because they only have one X chromosome to begin with. As cat breeders know, there are exceptions – sterile male calicos with an XXY karyotype, the feline equivalent of Klinefelter syndrome.

The study of calico cats was important for the development of epigenetics, or the study of how genes get turned on and off inside cells. In 1949, Murray Barr was examining cells taken from calico cats when he noticed tangled blobs at the edge of the nuclei. These “Barr bodies” were inactivated X chromosomes, and Mary Lyon would describe them in a 1961 paper that foreshadowed decades of epigenetic discoveries to come. So if you own a calico cat, you own a piece of scientific history…kind of.

In humans, X chromosome inactivation has profound implications for diseases where the pivotal gene is on the X chromosome (like hemophilia) and diseases which result from a person having too many chromosomes (like Down syndrome.) Down syndrome is an interesting case because researchers just figured out how to silence the extra chromosome that causes it using a process that mimics X inactivation – in a test tube, at least. Let’s hear it for the scientists!