Domestic cats may have not been bred to such aesthetic extremes as all the different breeds of dogs out there, but the centuries of selective breeding have still left their mark on our feline friends. Consider this genetic weirdness: most ginger cats are boys, while calico-patterned cats can only be female.
Fur colours in cats work quite differently to human genes for say, hair colour (which is why we don’t see a huge gender split in ginger-haired people, for example).
For cats, the colour gene is always carried on the X chromosome. Chromosomes are the same in cats as they are in humans, meaning males have an X and a Y, while females have two X chromosomes. So when looking at ginger cats, female kittens need to inherit two copies of the ginger gene to get that colouration, while males only need one.
While this means not all ginger cats are always going to be male, it’s far more common for them. There are about three male ginger cats for every one ginger female.
However this quirk of colouration also leads to a phenomenon almost solely found in females: calico colouring.
Calico refers to the colouration where a cat is predominantly white with splotches of two other colours, usually orange and black. These two other colours range in intensity from washed out pale pigmentation to bright orange and pure black.
This is what happens when a female cat inherits, say, an orange colouration gene and a black colouration gene. It also makes it impossible to find this colouration in male cats, as they can only inherit one colour-carrying chromosome. Very rarely calico male cats can be found, but these animals usually display a chromosomal mutation such as an XXY, and are most often sterile.
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