Have you ever asked your pet, “Why are you so cute?!” I do not know about you, but I ask my cat this all the time. Obviously science asked this too, because they have discovered some really neat genetic anomalies that explain why some cats are so cute!

Moggy the Tortoiseshell Cat

Tortoiseshell cats, like Moggy, are almost always female and are characterised by their mottled mish mash of black, chocolate, blue and ginger fur. They are adored by mad old ladies and loving families alike. Moggy has some interesting genetics going on and it has something to do with the phenomenon of Random X-chromosome Inactivation. What on earth is “Random X-chromosome Inactivation?” I hear you say…

Well, if you are a placental mammal like a cat or say a human like you or me, then you have two sex-determining chromosomes in every cell of your body.  Sex chromosomes contain, as well as a lot of other genes, the genes necessary to direct you to be the gender that you are today! These sex chromosomes are called X and Y chromosomes.

• Girls have two X-chromosomes
• Boys have one X-chromosome and one Y-chromosome.

Gene Dosage of the X-chromosome

However, female mammals were faced with a problem; a problem of gene dosage. If males only need one X-chromosome, won’t females be over-compensated with two X-chromosomes? Well, yes they would be. However, there is a solution to this gene dosage problem in females.

During embryo development one of the two X-chromosomes is randomly deactivated so every cell only has one active X-chromosome. Which X-chromosome is deactivated is totally random, so some of her cells will have one X-chromosome deactivated while other cells will have the other X-chromosome deactivated.

Look at any girl in your vicinity right now and I guarantee you that she is the same. Now that gene dosage of the X-chromosome is the same in males and females, our problem is solved!

Cat Coat Colour and Random X-Chromosome Inactivation

“But how does this relate to my tortoiseshell cat?” you say. Well the gene that makes your cat ginger, let’s call it the “ginger gene” (X^G), is sex-linked and therefore located on the X-chromosome. Once expressed, it will mask any of Moggy’s natural coat colours and she will be ginger.

Another version, or allele, of this gene – let’s call it the “non-ginger gene” (X^g) – conversely produces black pigment.

The cells that express these pigments are called “melanocytes” and they are embedded in the skin.

Moggy, like all tortoiseshell cats, is quite special in that she is heterozygous; she has both gene varieties X^GX^g, one on each of her two X-chromosomes. Her coat colour can form patches of ginger or black depending on which X-chromosome is activated.

For example, patches of her fur will be ginger if the ginger gene (X^G) is activated during cell development. However, patches of her fur will be black if the non-ginger gene (X^g) on the alternative X-chromosome was activated. Moggy’s coat is coloured by many differently X-deactivated melanocytes and this accounts for her mottled coat.

Moggy is a pretty special cat and she demonstrates some fascinating genetics. She is a visible manifestation of the Random X-chromosome Inactivation phenomenon occurring in all placental mammals. Now you can explain to your tortoiseshell cat or whoever is around, why tortoiseshell cats look like they do! Meow meow (=^ェ^=)

Words & illustrations by Kate Tepper.