Thread by @ZiziFothSi

One of my many charming traits is that every time a piece of media calls a tortoiseshell or calico cat “he” I pause it to give the long and detailed lecture on cat genetics that nobody asked for

All tortoishell (black/orange) and calico (black/orange/white) cats are female because they need to have two X chromosomes to express this colouring. Rare tortie/calico males have XXY chromosomes instead of the regular XY and are usually infertile.

Also, dilute tortoiseshell or calico colouring exists (same XX chromosomal link), where the black is grey and the orange is a pale peachy colour, and it’s super pretty

And as for ginger cats, that’s a mite more complex. Most (approx 80%) ginger cats are male. Again, this is an X chromosome thing!

Male cats have one X chromosome, and if it has the ginger gene, they’re ginger, end of. However, female cats have TWO X chromosomes!

If a female cat has two ginger X chromosomes, she’s ginger. If she has one ginger and one NON ginger, she’ll be tortie or calico (some ginger in coat). If she has two X chromosomes without the ginger gene, she won’t be ginger at all!

Here’s a nifty diagram I ganked from bio.miami.edu/dana/dox/calic…

Extra credit info: XX (female) cats, as they develop, will “turn off” one of their X chromosomes in each cell at random, to prevent duplicate data. So female cats end up with some cells expressing the X from their dad, and some with the X from their mum.

If the cat has two ginger X chromosomes, one from mum and one from dad, it doesn’t matter which one expresses itself in each cell - they’re both ginger, so they both cause the same colour coat to grow! Same with a female cat with two non-ginger X chromosomes.

BUT if a female cat has ONE ginger X and one non-ginger X, they end up with patches of different colour - in skin cells where the ginger X gets switched off, the fur will be black or white; in skin cells where the NON-ginger X gets switched off, the fur will be ginger!

This process is called Lyonisation and takes place very early in development. From then on, every time a cell copies itself, the new copy will have the same X inactivated as before.

**CORRECTION: above I implied that the white colouring on cats is X-chromosome linked. TURNS OUT that partially white cats have a mutation on a DIFFERENT gene that prevents pigment from migrating all the way across the body!

Pigment starts in the neural crest in the foetus, which is sort of where the spine will be, and then migrates down towards the belly and feet. With the part-white mutation, it doesn’t quite get all the way there, which is why partially white cats often have white bellies & socks!

Image of pigment migration direction from theconversation.com/how-the-cat-go…

Update to the update: earlier on I said that pigment migrates, but it’s actually melanoblasts. These will go on to become melanocytes, which are melanin-producing cells that create and push pigment into nearby skin cells! They have tentacles so they can touch all their friends

Anyway, I’m not an expert and I taught myself like three quarters of this while I was tweeting it so please correct me if I’ve got anything wrong!

Anyway, as a reward for reading all this way, my terrible sons whomst live in the trash