Intro to sex chromosomes: why my calico cat is a girl

Internet, meet Emmy, the most precious cat in the world. Emmy likes to sit on laps, purr loudly, chew on flowers, and occasionally get so happy that she drools a bit. Most pertinent to this science blog though is that she is a calico. Look at those orange and black splotches.

Emmy the calico

Almost all calico and tortoiseshell cats are lady cats, like Miss Emmy, and if you find a boy calico, he is almost certainly sterile. Why does that happen?

By the way, this is a discussion of biological sex only. Gender is complicated and not the same as sex!

In mammals, females have two X chromosomes whereas males have one X and one Y chromosomes. Those two chromosomes of course determine if an individual develops as a male or females. It also creates a bit of a quandary though.

You see, the 1000 or so genes on the X chromosome could be made twice as much in females as in males because there’s two copies. Making the correct amount of a gene is pretty important though. Females just don’t make twice as much of all those genes as males. Somehow we mammals had to evolve a way for females and males to make the same amount of those X genes.

And that is called dosage compensation, meaning how females make up for having so much more DNA. Sadly, more is not always better.

To make dosage compensation happen, females turn off one copy of the X chromosome in each and every cell. Bam, that chromosome is no longer open for business. Those genes do not get used. The chromosome gets packaged up, almost like putting shoved in a box in deep storage, so it can’t be accessed.

So how does this all relate to my adorable calico cat?

The X chromosome contains a gene for coat color. That gene, depending on its precise sequence, can result in either orange or black fur. One of Emmy’s X chromosomes has the gene type for orange while her other X has the gene type for black. Let’s call them the orange and black chromosomes for short.

As you can tell by looking at Emmy, different X chromosomes get silenced in different cells. In some places, the orange chromosome gets boxed up, leaving the black one active. The opposite occurs at her orange patches. Because she is both orange and black, you can tell she definitely has two X chromosomes.

Sex chromosomes are pretty amazing.  Their evolution, how they get silenced, the regions that aren’t silenced, what happens when someone gets an atypical number of sex chromosomes…many fun subjects! In the future, expect to read about more recent research that’s been done. Find out if the Y chromosome will ever just disappear, how long pieces of RNA are used to turn off a chromosome, and more!


One thought on “Intro to sex chromosomes: why my calico cat is a girl

  1. If you can find the video of David Page on The Colbert Report, it is totally worth checking out for the defense of the Y (David’s group showed that it is here to stay pretty definitively about a year ago).

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