Genetics 101

Any cat lovers out there? Well you have made an awesome choice in liking this animal. If not, you’ll hopefully be interested by the end of this article. And you’ll learn a good way to recognise a boy cat from a girl cat!

Cats are extraordinary – but not just because they’re useful in funny cute cat videos, or because they can jump six times their height. They are also extraordinary animals for geneticists.

Now, I’m sure you want to know the trick for telling whether a cat is female or male at a glance! (Or maybe you don’t really care what sex they are, just that they are cute. But I promise this article will make you look out.) And no I don’t mean look under them. Just check: is it both orange aaand black? If yes, it’s a girl! – but don’t come complaining if it’s a male because then you’ve just spotted one of the many exceptions in nature, well done.

But why do we know this? Okay let’s delve.

You know about DNA, heredity, genes… chromosomes! Yes, those little Xs of DNA you see during “mitosis” (cell division (for precise biologists “nuclear” division)) and in karyotype (see photo 1). Those are what are causing female cats to be tortoiseshell (orange and black).

There are two types of chromosomes: autosomal, and sex chromosomes. Sex chromosomes are the “XX” (for female) and “XY” (for male) which determine sex – you’ve probably heard of them already. Autosomal are all the rest – we won’t really care about them for now.

The X chromosomes are big and contain over 1000 genes (“hereditary units” – they code for various proteins), while the Y chromosome are puny little specks in comparison (look at the picture), containing less than 100 genes. The gene for orange or black coat colour in cats is located (it’s “locus” is) in the X chromosome. Female cats, therefore, have a duplicated set of X genes, and thus a duplicate of the colour gene. Let’s say a particular female cat has the “orange fur” gene (called “allele”) on one X, and the “black fur” allele on the other.

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Photo 1: Human male karyotype (would be pretty much the same for a cat, except less numbers of chromosomes) – notice the tiny Y chromosome in comparison to the X.

At a certain moment during early embryo development (around 200 to 300 cells), one of the X chromosomes in females becomes inactivated – like any two people in a team, work has to be shared between them, and the cell boss is always too lazy to think about attributing tasks. This happens at random. Some of the female embryo’s cells inactivate the X chromosome with the orange fur allele, others with the black. So now she has a dappled mixture, a “mosaic,” of colours in her cells. After one inactivation, the cells continue to divide; the “black” cells divide into more “black” cells, making a patch of black in the kitten as she develops, and same with the “oranges.” When she’s born, she’ll have patches of orange, and patches of black, making her tortoiseshell! And it makes sense that as the cat continues to grow, she will keep her particular colour pattern, since as she grows, those particular cells divide into more cells of the same colour in the same area.

So why can’t males be tortoiseshell? The answer’s simple once you know all this! Males only have one X chromosome (remember, they have XY sex chromosomes). So they can only have either the black, or the orange gene. Voila! That simple.

Exceptions? Yeah, sometimes nature makes random differences (that’s where evolution comes from!), and males are sometimes born with an Xtra X chromosome (pun intended). Then a male can also be tortoiseshell, but this only happens in 1 in 3000 cats, plus he has to have different alleles in his two X chromosomes… (if both Xs have the gene for the same colour, only one colour will show, even with inactivation). So probability is low.

Small detail: two colours doesn’t always mean female, though. The white colour in a cat’s coat is determined by a gene in an autosomal (other than sex) chromosome, present in both males and females.

Let me also tell you about an interesting event: the first cat to be cloned. Her name was Rainbow – you can see her in the picture (photo 2), with her clone, CC (for Carbon Copy – yes weird name). “But aren’t clones supposed to look exactly the same?” Yes.

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Photo 2: On the left, CC the clone, and on the right, Rainbow

Mais bien sûr! Rainbow is a “her”! She is a tortoiseshell! She has both the allele for “black” and for “orange” fur on her X chromosomes! Remember I told you that X chromosome inactivation in the embryo is random? Well, now you have a beautiful proof of that randomness. (If you just look at the coloured area versus the white area, you start seeing their similarities.)

On this note, I will close off, hoping to have sparked at least a small interest in the study of Biology.

Lilia E.