Ever since the evolution of sexual differentiation in mammals, which is thought to have started when one of a pair of chromosomes picked up a gene that gave males a selective advantage, the sex chromosomes have been at war.

As the sex chromosomes diverged over time, the Y chromosome lost the ability to recombine - its genes that are not essential have accumulated mutations and either been shut down or lost.

Now, the Y chromosome is reduced to a stump, clinging onto the genes that offer males a competitive advantage, and expressing only some of the genes it does have.

In some cases, genes on the Y chromosome may be detrimental to females. By residing on the Y, however, they can survive - as they never occur in females, they can escape being discarded by the selective forces acting in the interests of females. And by improving the fitness of males that carry them, they wage their own battle in the ongoing conflict between the sexes.

The DNA map of the Y chromosome shows that it includes large repeated chunks, termed 'amplicons', which are so similar that they are almost impossible to tell apart. Many of the male-specific genes lie in these regions, such as those involved in testes development and sperm production, and it is possible that this duplication is an attempt to ensure that the genes aren’t lost in the battle with the X (safety in numbers!).

Another surprise is a large 3.5 Mb block, which is in one orientation in some males, and in the opposite orientation in others. It is feasible, though speculative, that this large inversion, and other variations in the numbers and orientation of the amplicons, may underlie variations in male fertility and virility.