This is one of those things I learned about in biology class in high school that only got cooler the more I read about it- which is why we are going to talk about it now!
I'm not a big animal person- I know that's really jarring for a lot of people to hear, but I grew up in a household where we couldn't have pets (my mom is allergic to everything with hair/fur and we moved every 2-4 years, so the practicality of having a pet just wasn't there). The good news is, that makes me very equally a dog and cat person, and one of my favorite types of cats when I was growing up were calico cats. If you don't know what I'm talking about, those are the cats with different patches of fur that are a variety of colors and patterns, like these:
There was plenty about them that I liked, but my favorite thing about them was that no two looked alike. They came in a whole variety of colors and patterns- little did I know, this patterning was true for basically all two-X-chromosome-containing animals (including female humans!).
The phenomenon I'm talking about has to do with the way that our biological sexes get "assigned" to us while we are developing in utero. If you are born with two X chromosomes (one from mom, one from dad), you were considered biologically female when you are born. If you were born with one X chromosome from mom, and one Y chromosome from dad, you were considered biologically male. This is because of the "sex-determining" region that exists on the Y chromosome (basically the only thing on the Y chromosome, it's teeny tiny due to lots of years of evolution and all the important stuff residing on the X chromosomes and the other autosomal chromosomes).
From a basic biology and genetics standpoint, this should be a little alarming- genetics is a very tightly regulated thing, so the idea that some animals can walk around with two copies of all the genes on the X chromosomes and some can get away with only one sounds absolutely wrong.
But like most things in biology, nature has it all worked out for us, and the way this dosage issue gets resolved is through something that was first discovered in the late 1940s by a medical researcher named Murray Barr. He noticed some dark-staining chromatin (protein-bound DNA) around the nuclear envelopes of female somatic cells in mammals that didn't seem to exist in their male counterparts. For a while, this dark staining mass was called the "sex chromatin", and later renamed a "Barr body". This mysterious mass was later discovered to be a condensed and genetically inactive, single copy of the X chromosome. So the way around having two X chromosomes in females, and only one in males? Just shut one off in females, and the balance is restored!
When I first heard this story I really thought that our biology might be inhibiting those of us with two X chromosomes from having superpowers or something- like, what if we did have both copies of the X chromosome active?! Would that make us unstoppable? Is this just another form of female oppression?? Turns out the answer is no- having two X chromosomes active during embryo development is lethal, meaning these individuals do not survive long enough to be born (so we will never really know if they're super I guess, but I digress).
Anyway! So the balance is set to where males and females have the same number of active X genes- great! What does this have to do with cats though??
When we are developing and our X chromosome gets turned into a Barr body- it happens randomly. I say this because we have two X chromosomes- one from mom, and one from dad. Those chromosomes are going to look quite different from each other (ideally), and when it comes time for a developing embryo to pick one to switch off, the process is random in each cell. So some cells will switch off mom's copy, and some will switch off dad's copy. After this happens, each cell with a unique mom versus dad X chromosome active undergoes many more rounds of cell division over the course of embryonic development, and eventually you get this sort of mosaic-style pattern all over our bodies of different regions of cells that have either mom or dad's X chromosome active.
But how does this tie into cats?
Cats' coat colors are determined by a gene on the X chromosome with two alleles- one that results on orange fur, and the other that results in black fur. So as you might have guessed, when the X chromosomes randomly inactivate during development, a cat with one orange X chromosome and one black X chromosome will be randomly and uniquely patterned by the two colors when it is grown up!
You might be wondering- but calico cats also have white fur? What about those patches? The white fur on a calico cat is actually the result of piebalding, which actually occurs in many different mammals and is the result of no melanocytes (which produce melanin, which makes animal fur the colors that it is) being present in that patch of fur.
Alternatively, if you have a calico cat with no piebalding, you get a tortoiseshell cat, where the patches are exclusively black and orange.
An added fun fact from all of this- all calico cats are female, or XX.
So, that's the tea on calico cats and why not only are they so cool to look at, they are so cool genetically. They are an excellent example of something that was biologically mysterious for so long, and continue to be an incredible tool for teaching about Barr bodies!
