Why Are Calico Cats Almost Always Female (and Always Look Different)?

There’s a reason that colorful cats, like tortoiseshells and calicos, tend to be female. It comes down to genetics.

Females (of all sorts, not just cats) have two X chromosomes, while males have an X and a Y. Rather than being overwhelmed by the double dose of genes provided by having two X chromosomes—each carries more than 1,000 genes —lady organisms have developed something called X-chromosome inactivation, a process that effectively hits the mute button on one of the two X-chromosomes in a cell.Â

Kat McGowan explains in Nautilus :Â

The donor cat (left) and resulting cloned kitten (right, with surrogate mother). Image Credit:Â Shin et al., Nature (2002)

No calico cat will ever look identical to another. The particular pattern of a multi-colored cat’s coat comes down to chance, meaning that even among the same family, no cat will have the same coloration. Even with the exact same genetic make-up, a calico cat’s coloration would be different than her twin’s because it’s random whether a cell codes for orange or black fur. The same goes for clones. In 2002 , when scientists cloned a calico cat named Rainbow, the clone kitten had vastly different coloring from Rainbow, even though their genes were exactly the same.Â

Because X-inactivation only happens if there are multiple X chromosomes in one cell, coloration patterns that stem from the process tend to only pop up in female cats. Rarely, a genetic mutation can result in a cat being born with an extra chromosome  (XXY), leading to a male calico or tortoiseshell cat, but for the most part, it’s purely a ladies’ club.Â

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