Despite the fact that X is much larger than the tiny Y, it seems that both evolved from a pair of conventional chromosomes in early mammals sometime in the past 300 million years – an idea first proposed in 1967.

Previously, our main clue that X and Y had a common ancestry was that they swap a few small sections during one kind of cell division, just as pairs of ordinary chromosomes swap much larger chunks.

After X and Y had taken up their role in sex determination, their paths diverged. We already know that the Y shrank and lost almost all of its genes (��è����Ƶ, 24 August 2002). Non-sex chromosomes have also changed greatly, acquiring or losing huge chunks.

Now sequence comparisons with rats, mice and dogs show that the X chromosome seems to have changed little since the evolution of placental mammals, supporting the idea that once genes are transferred to X, they stay there.

This is thought to be a result of X inactivation, the process whereby most of the genes on one X chromosome are switched off to prevent an “overdose” of X genes.