Loving torti cats & genetics: Chimera Cat more likely to be X-inactivation?

Posted on 24/08/2012


venus chimera cat x-inactivation

Venus – internet cat sensation of the week, channeling Phantom of the Opera

Okay, that’s it! Someone else shared ‘I F***ing Love Science‘s chimera cat story in my Facebook feed and said they wanted a chimera pet too. Could someone please show me the evidence that it’s a chimera, because I can’t find it.

It’s a cat with fantastic markings that’s doing the rounds on the interwebs! And you’re welcome to call her a chimera because it sounds catchy or you’re into Greek mythology. I just object to a Facebook page about how much people *expletive* love science, purporting to give a detailed scientific explanation without any evidence to back it up. And then people see a supposed smartscientifical explanation and copy the wrongness around the internet.
According to I F***ing Love Science:

Meet chimera cat. She recently went viral on the internet – for obvious reasons. Chimera cat is one individual organism, but genetically its own fraternal twin. A chimera is typically formed from four parent cells (either two fertilized eggs, or two early embryos that have fused together). When the organism forms, the cells that had already begun to develop in the separate embryos keep their original phenotypes and appearances. This means that the resulting animal is a mixture of tissues and can look like this gorgeous (but bizarre) kitty.
She also has complete heterochromia, a condition when the eyes are different colours.

If the cat hasn’t been genetically tested then you can’t know it’s a chimera. And, given that it’s a female tortoiseshell, there’s another likely explanation for her colour - X-inactivation.

I ♥ tortoiseshells and genetics

chimera cat

Genetics perfectly illustrated.

The orange and black patchy markings on tortoiseshell cats are the classic example used in genetics lectures to illustrate X-inactivation (if you’ve studied genetics or are a torti-lover, tune out for the next few lines). Tortoiseshell cats are (almost) always female. Why?

Females have two X-chromosomes, XX, whereas males are XY. To avoid receiving double the dose of all the genes on the X chromosome, female animals switch off one of the two Xs: X-inactivation.

In  cats, the gene for orange vs black cat coat colour is X-linked, and a female cat inherits one X from each parent. You get a tortoiseshell when a female cat inherits different X alleles,  e.g. mum could be a black cat and dad could be an orange cat. The patches of orange or black fur on a tortoiseshell arise depending on which X chromosome was inactivated in that patch of fur.

tortoiseshell genetics

The best tortoiseshell cat in the world (*note the author may be slightly biased), looking very undignified.

More torti pics off the interwebs (all those people who say it must be photoshopped clearly haven’t met many tortoiseshells, or studied genetics):










Cats can also have different coloured eyes, called heterochromia.

File:June odd-eyed-cat.jpg

There are some other very rare genetic conditions that can produce a tortoiseshell (e.g. a male XXY or a chimera). This isn’t to say Venus can’t be a chimera, but given it’s a female cat, Ockham’s razor would suggest that X-inactivation is the most probable answer. I could be proved wrong if someone does a genetic test and shows she’s a chimera.

Venus the chimera cat has her own Facebook page, and the owners say her genetics hasn’t been tested – and fair enough, why on earth should they know or care? For us cat-science-lovers it’s a spectacular cat with spectacular X genetics.

tortoiseshell cat chimera heterochromia

Awwww … but, of course, still not as cute as my torti is right now asleep in front of the heater ;)

Posted in: biology