Tortoiseshell Cats
I’m sure a lot of you already know this but it’s new to me as I never really thought to look in to it. Today I found out why you can only get female tortoiseshell (aka calico) cats. For those of you who don’t know, it’s through a process called X-inactivation.
Basically, as female mammals have two X chromosomes (XX) and males only have one (XY), cells in the female body ‘silence’ one X chromosome while the organism is still an embryo, as having two can give rise to a potentially toxic dose of proteins. This inactivated chromosome becomes a ‘Barr body’. This happens in us too. Coat colour in these cats is X-linked, so males are either orange or black depending on whether the X chromosome they inherit codes for orange or black. In female tortoiseshell cats, each cell randomly switches off one of the two X-chromosomes, meaning if the female possesses one X chromosome with the gene for orange, and the other for black, each cell shuts one of the two colours down. This results in some areas of skin producing orange pigment and others black.
This is a really simplified version of what’s going on in these cat’s genomes. In real life other genes contribute to the final coat denoting further spotting, additions of white areas and dulling of colours.
Very rarely a male toirtoiseshell is born. This is usually due to an extra chromosome (XXY) meaning the animal is infertile.
I find this picture sums it up perfectly.