The marvels of female DNA

8th March 2020 – Colette Christiansen

For all the cat lovers out there, have you ever wondered why there are female calico cats with beautiful tortoiseshell coats, but no male calico cats? You may be surprised to find that human females also have a mosaic pattern – we just can’t see it.

We have 23 pairs of chromosomes which contain instructions – genes – for making what our body needs to develop and maintain itself. One chromosome of each pair is inherited from our mother and one from our father. One of these pairs is what determines whether we are male or female. This sex chromosome pair has two X chromosomes for females and one X and one Y chromosome for males.

If females have two copies of an X chromosome, this means that they have twice the number of copies of genes that are on the X chromosome than males do.  For all other chromosome pairs having two copies of each gene is normal and necessary and both males and females have these.  However, when it comes to the X chromosome this is a special exception.  As can be seen from males, who are XY rather than XX, two copies of X are not needed.

Does having extra instructions matter though? We know from conditions like Down’s Syndrome where people have three copies of chromosome 21 rather than two that extra chromosomes do affect how we develop. Three copies of a larger chromosome with more genes leads to Patau’s Syndrome which is generally fatal in the first weeks of life.  Women might not be able to survive with two fully working X chromosomes.

As ever, biology has an answer. When females are just a small collection of cells in the womb, one of the X chromosomes is inactivated. This is a random process, so the result is that around half the cells inactivate one X and the other half inactivate the other. As these original cells go on to divide into many more and create you, the result for females is a pattern which varies throughout.

The instructions for fur colour in calico cats are contained in the X chromosome, which is how some parts end up different colours from others, depending on which X chromosome is active in which patch.  If instructions for human skin colour was also on the X chromosome, human females could also have patterned skin.

So the next time you admire the beautiful coat of a calico cat, make sure you admire also the wonder that is you or the women in your life.

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