This week’s Nature has a picture of the X chromosome on the cover. The issue has two articles on the topic. The first contains an analysis of the recently completed sequence. As I wrote before on the Y chromosome, women have two copies of the X. Previously, it had been thought that one was suppressed and the other is expressed. In the second paper, it is shown that about 15% of genes are still expressed on the inactive chromosome. The same genes on the active chromosome are also expressed so this could mean that more of the protein is produced. This could provide a basis for the differences between individual women and for this month’s hot button topic – the differences between women and men.
Ironically, the X chromosome may actually be more important for men than for women. Since men only have one X, any advantageous mutation for men will we be selected for strongly even if it is somewhat deleterious for women. Conversely, a deleterious mutation will be strongly selected against. It has been proposed by Horst Hameisterat of the University of Ulm in Germany that females may be responsible for intelligence in humans. His group proposes that early females had a preference for intelligence and if the genes for superior intelligence and the preference for intelligence are found near each other then they would be selected together and this could lead to an instability because both lead to enhanced survival. It had been known for some time that many of the genes related to mental retardation are found on the X and now it is confirmed that there are many crucial genes on the X for neural development. Hence, the X could be what makes humans human.