Buzz
The reason behind the colorful coats of cats, such as tortoiseshells and calicos, is rooted in genetics, and it’s more common in females.
Females, across all species, possess two X chromosomes, whereas males have one X and one Y chromosome. To manage the genetic load from two X chromosomes, which each carry over 1,000 genes, females undergo X-chromosome inactivation. This process effectively silences one of the X chromosomes in each cell.
Kat McGowan details this in Nautilus:
The X chromosome in cats contains a gene that determines skin and fur color, with two alleles—one for orange fur and the other for black. When a female cat inherits both an orange allele and a black allele from her X chromosomes, each cell carries both versions, but X-inactivation causes some cells to express orange and others black. This inactivation happens early in development when the cat is just a cluster of cells. The skin cells that carry the active orange allele form distinct orange patches, while those with the black allele form black patches.
The donor cat (left) and the cloned kitten (right, with surrogate mother). Image Credit: Shin et al., Nature (2002)
No two calico cats are identical in appearance. The pattern of their coat colors is determined by chance, so even siblings will have different markings. Even identical twins will have different fur patterns because it’s random whether a cell will code for orange or black fur. This randomness is also true for clones. In 2002, scientists cloned a calico cat named Rainbow, but the clone had a completely different coat color, despite having the exact same genetic makeup.
X-inactivation only occurs when there are multiple X chromosomes in a cell, which is why these color patterns are usually found in female cats. Rare genetic mutations can lead to a male cat with extra chromosomes (XXY), resulting in a male calico or tortoiseshell, but this is a rare occurrence. For the most part, calicos are a female-exclusive phenomenon.
