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. 

Kat McGowan explains in Nautilus: 

On the X chromosome of cats is a skin- and fur-color gene that has two variations (alleles) that dictate either orange fur or black fur. If a female cat inherits one X chromosome with the black allele and one with the orange version, each cell will have both versions, but X-inactivation means that some of her skin cells will code for orange and some for black. The inactivation happens very early in development, when the cat-in-the-making is still just a ball of cells, and the particular nature of skin tissue is that cells and their progeny stay close together. One of those primordial skin progenitor cells that happens to have an active orange allele will give rise to a cohesive blob of millions of cells in the fully developed cat, forming a big orange blotch. The same is true for those coding for black.

The donor cat (left) and resulting cloned kitten (right, with surrogate mother). Image Credit: 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. 

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. 

[h/t: Nautilus]