The two sexes must be equal. This is true in us, the cells in both men and women need the same amounts of the essential proteins that run these cells. It is also true in all other animals. These proteins are made from the genes on our chromosomes and this is called gene expression. For example muscle cells are the same in men and in women – in both their job is to exert forces – and so in both there will be an optimum amount of a protein that needs to be made from a gene.
But when there are two sexes that are genetically different there is a problem. In humans and other mammals, females are XX and males are XY, i.e., I have only one X chromosome, my sister has two. What if a muscle gene is on the X chromosome? If this made at the same rate from each of my sister’s two X chromosomes as it is from my one X chromosome then one of us is in trouble. If the rate is optimal for me, then my sister will have twice as much protein as is good, while if the rate is good for my sister, my muscle cells will have only half the amount they need.
What to do? Logically, there are a few possibilities: 1) shut down one of the two X chromosomes in females; 2) halve the protein production from each of the two X chromosomes in females; 3) double the protein production from the single X chromosome in males. Oddly, all of these strategies are in use in some animals. Mammals, including us, go for strategy 1). Fruit flies go for 3). C. elegans goes for strategy 2).
As we use strategy 1), if you are a woman in the nucleus of every cell in your body is a Barr body – a silenced X chromosome. You have many trillions of these Barr bodies in total. I don’t have any of course. For a consequence of this and why the tortoiseshell cat shown above is almost certainly female see a post on the Department’s blog.
It looks like there are a number of ways of achieving equality of gene expression between the sexes, that are all roughly equally as good. So although it is necessity to have some mechanism it is just chance during evolution that we mammals have ended up with a silenced chromosome in every female cell.