How Can a Genetic Woman Be... a Woman?

Maleness is given by that Y chromosome that leads to the XY formula while females are XX, with two X chromosomes. That's why in the early development of the female fertilized egg, one of the two X chromosomes must be silenced. When by accident this does not happen, severe genetic diseases get installed.

Both X chromosomes possess a suicide gene (XIST), which, when activated, stops other genes' activity behind a RNA barrier. The suicide gene itself can be blocked by a plug of proteins but it is not clear if the silenced X chromosome is randomly chosen.

Researchers do not understanding very clearly why one X chromosome is switched off rather than the other. And the existence of certain mechanisms that permit a "best" choice to be made between things is not proven.

Now a team led by University of Warwick physicist Dr Mario Nicodemi has explained how this randomness happens and why it is beneficial.

This could explain why some women unusually don't have a completely random X chromosomes distribution; but in fact, at least 10% of our genes could behave similarly, from those encoding our immune system to our olfactory apparatus.

When both X chromosomes are brought close together within the cell, proteins for forming the plug start gathering around both of the XIST suicide genes, triggering a race between the two build-ups of protein. One of these two nascent proteins plugs narrowly wins, reaching an energy level in which it can amass both plugs into a single one.

The sole plug shuts off one of the XIST genes, its host X chromosome turning into the active one, while the other chromosome is shut down by its XIST gene. Harvard investigations confirmed this hypothesis.

Nicodemi thinks the randomness actually confers an evolutionary advantage: similar numbers of both the maternal and paternal X chromosome are kept in the gene pool, ensuring a greater genetic variety, a biological advantage.