In this weeks’s Nature there is an article on X-chromosome silencing. This sounds very technical, and it is, but it has fun consequences, namely tortoiseshell cats (and tortoiseshell-and-white or calico cats, as seen in the picture). You may know that tortoiseshells are almost always females, but you probably don’t know why.

In mammals like cats and us, males have an X and and Y chromosome, and females have two X chromosomes. However, in the cells of mammals only 1 X chromosome is needed. Indeed 2 X chromosomes is too much, as if protein is made from both of them, the cell would end up with twice as much of that protein as it needs. This would cause problems.

So, regardless if you are a man or a woman, your cells each have one working X chromosome in them. If you are man that is it. If you are woman then in your cells one of the X chromosome is silenced, and it is the same in female cats. The other X chromosome in females is switched off, it is silenced. The Nature paper discusses this silences, it is here, but it is quite technical and you have to pay to see it unfortunately.

The fun thing, I think, is that which X chromosome is silenced is random, and is not the same in all cells. If you are a woman then it may be the cells in your left hand, say, have the X chromosome you got from your dad silenced, while the ones in your right hand have your mum’s X chromosome silenced.

As human’s don’t have furry coats with different colours coded for in genes in the X chromosome, this fact is not visible. Your hands are the same colour. However, in female cats if one X chromosome codes for white fur and other one for greyish fur then cells with different X chromosomes silenced are different colours and you a cat like the one in the picture.