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.