[F]emales have two X chromosomes whereas males have one, and females compensate by ‘switching off’ one of them while still an embryo. But for each cell, the X that gets switched off is random – so an adult female is a melting pot for two different families of cells. This is the subject of a recent book by geneticist Sharon Moalem, called ‘The Better Half: On the Genetic Superiority of Women’. He explained to Phil Sansom that not only does this mosaicism help females in loads of ways, but that it’s more complicated: some of the genes from the ‘switched off’ X chromosome somehow survive the switching off… and go on to help out even further.
[Moalem:] What we’re discovering right now is not only that females are made up of two populations of cells that are cooperating, but actually within the individual cells, females have access to about 25% of the so-called silenced X. So what that actually means is another 250 genes. It’s much more genetic horsepower, so to speak, in every female cell compared to male cells. And many of those genes are involved in the prevention of cancer, in immunological function, in the building and maintaining of a human brain. So although having an extra 1000 genes overall to use might not sound that significant, because we have about 20,000 genes, these genes are crucial.