by Taylor Tibbs
Calico cats have a secret hidden under all those cute black, white, and orange spots. While the pattern and shade of these classic Neapolitan kitties can vary widely, there is something they all share in common: all calico cats are female!
Calico is not a breed of cat, but rather a name for a particular tri-color coat pattern. All calicos have at least three colors of fur. Tortoiseshell cats are similar to calicos, but can have two (orange and black) or three colors (orange, black, and white) in their coat (see above illustration). Tortoiseshells often also have smaller speckles of color rather than the large quilt-like patches on calicos.
While all calico cats are tortoiseshell, not all tortoiseshell cats are calicos. Similar to how all squares are rectangles (have four sides), but not all rectangles are squares (all sides of the same length). Calicos and torties (as tortoiseshells are often called) can also come in a variety of shades. My cat, Zoey, is considered a “dilute tortoiseshell” because the black, white, and orange in her fur are less vibrant, giving a coat that is more gray overall.
The reason why all calico cats are female is the same for tortoiseshell cats. The answer lies in their genetics or DNA. DNA is the blueprint for all living things, including you, me, and our furry whiskered companions. DNA determines why we are born with black hair or blonde hair, or with brown eyes or green eyes. Genetics also determine our biological or assigned-at-birth sex, specifically our X and Y chromosomes (in mammals). Chromosomes are thread-like structures of DNA located within the cells that make up our bodies. Males are born with one X chromosome and one Y chromosome (XY), while females are born with two X chromosomes (XX). In cats, the gene (portion of DNA) that determines coat color comes in two forms (called alleles): orange or black. This gene for coat color is located on the X chromosome. Since all females are XX and males are XY, that leaves females with twice the number of coat color genes compared to males.
My cat Zoey is actually a good example of how the genetics of calicos/torties work. Zoey’s mom was solid gray and her dad was an orange tabby. She received one X chromosome from each parent – one X from gray mom and one X from orange dad. When her mom gave birth, she had one solid gray boy, one solid black boy, and two tortoiseshell girls (see illustrated diagram of their family tree).
The boys inherited one X chromosome from mom and one Y chromosome from dad; therefore, they were only given one gene for coat color, which was black. Zoey and her sister on the other hand, inherited both a black X from mom and an orange X from dad.
You may be asking, if mom only had black color genes on her Xs, why is she and one of her male offspring gray? Shouldn’t they all be black? While the genes for orange and black coat color lie on the X chromosome, there are “influencer” genes on other chromosomes that can modify the shade of coat color. For example, the gene that controls white coloring, or rather the lack of color, is not located on the X chromosome. This gene causes a condition called “piebalding” which is similar to albinism. It causes the fur to lose its pigment or color. Since this gene is not located on the sex chromosomes, all sexes can have white fur!
In the case of mama kitty and her gray son, they inherited a black X and a shade-influencing gene on another chromosome. You can think about how these other genes influence coat color much like mixing paints. If you start with black paint (black gene) and mix in a little white (such as the white piebald gene), you will get gray. For simplicity in the illustrated diagram below, mom has both a black X and a gray X. The gray X represents a black gene that is being influenced by another gene. If one of her kittens gets a black X, then they will have a black base coat. If the kitten gets a gray X, they will have a gray base coat.
So, what about the orange? Why is the coat color of the girl kittens patchy and not some equal mixture of black, orange, and white? It turns out that only one X chromosome can be turned “on” at a time within a cell. Because the girls have two Xs, their cells can turn on either the X they got from mom or the X they got from dad.This allows them to express the black color gene from mom’s X chromosome in some cells and the orange color gene on dad’s X chromosome in other cells, a phenomenon called “x-inactivation.”
Since each cell can only activate one X at a time, this is what produces a random pattern of oranges and blacks across the cat’s fur. Because the boys only have one X, they can only activate the black color gene they got from mom.
X-inactivation is the reason why even though Zoey and her sister are both tortoiseshells, they look completely different. The way in which each of the color genes are activated on each chromosome is completely random, therefore every calico/tortoiseshell cat has their own unique pattern. Zoey is also unique for having 26 toes (just 2 shy of the world record, by the way)! This is due to a harmless genetic condition called polydactyly, which she also inherited from her dad.
Despite all the talk about calicos and torties only being female, it is possible to have a male calico cat, but they are extremely rare. The only way a male calico cat can be born is if they have an extra X chromosome (XXY instead of XY); however, these cats are sterile and often have health problems.
Now you know that every calico cat you see is female (with very few exceptions). Next time you want to try and impress your friends, you could show off your near “telepathic” ability to predict a cat’s biological sex just by looking at their fur!
