Monthly Archives: March 2013

Why torties are…well…torties.

Hi there,

Tortoiseshell cats are quite unique and there is an epigenetic reason for the characteristic speckling and mottling that mark the breed. Tortoiseshell cats also happen to be exclusively female due to the nature of the epigenetic process at work.

The coat colours of cats are determined by genes localised to the X-chromosome, and just like humans, female cats have two X-chromosomes and male cats have one. The neat trick is here – femal torties express different copies of the coat-colour gene in different cells, randomly – a hallmark of a process called X-inactivation.


X-inactivation is actually extremely important in order to prevent an overexpression of genes in female animals – in order to maintain steady levels of gene expression similar to that seen in males you need to express only one copy of the X chromosome, despite having two. The process of X-chromosome inactivation is how cells achieve this.

The process itself involves the production of long non-coding RNAs from a gene cluster on X chromosomes that we call the X-inactivation centre, the most famous of which is Xist. Xist spreads along the X chromosome that inactivates it and then recruits other factors that lock in epigenetic modifications such as DNA methylation to silence expression of that particular X chromosome, permanently. X inactivation does not happen in males though because to X-inactivation centers have to be points where X chromosomes “kiss” to trigger the cascade of changes that shuts all but one of them down. Here’s another cool thing – If you read Xist from the opposite side it encodes a long non-coding RNA called Tsix (Clever name – see if you can figure it out) – and if it is expressed, Xist isn’t. You can induce inactivation in other chromosomes by putting in copies of a chromosome that have been spiked with the X-inactivation centre.

Here’s a cool video explaining the process…

It is all very very intriguing because the X-inactivating centre is composed entirely of non-coding RNAs as far as we know it.  There’s more to biology than proteins (prejudice declared – I don’t like protein work much). You can read a lot of the backstory and study details at Scitable here.