Chromosome X

As addressed in the intro of this article by the New York Times, the coining of the name for the X chromosome is fitting for such a mysterious biological component. While inherited like the rest of the chromosomes (one from mom, one from dad), its products set it aside from the others, and research is showing that it's a lot more complex than we might have thought.

X (and Y) chromosomes are the bundles of DNA responsible for determining one's biological sex: in humans, XX usually means female, and XY usually means male, with other combos of chromosomes and genes to change that up from time to time. The two chromosomes, unlike other pairs, don't contain the same genes, and as such, XX individuals have to deal with having two times the genes that an XY individual would have. This is accomplished by X inactivation, in which a cell (seemingly arbitrarily) picks one of the two Xs to condense into an unusable state. When a cell divides, the daughter cells retain the same pattern of condensation, producing what is called mosaicism.

Picture from Michael Bodega on Wikimedia Commons by public domain release

As you can see in this tortoiseshell, some patches of cells in an XX individual turn off the X chromosome inherited from the mother, and other patches turn off the one inherited from the father. Despite one being turned off, the presence of both increases the genetic diversity of an XX individual, as an XY individual has only the X from the mother, and this occurs on varying levels. Tissues can be an even mix of mother X and father X, or they can be skewed toward one side or the other; research with mice showed some animals with eyes containing almost all cells expressing one of the Xs.

The mechanism behind this differentiation is still in testing, but scientists are working with a series of molecules that perform the condensation, which is led by the molecule Xist. Xist molecules latch onto the chromosome being condensed and signal for the rest of the molecules to come over. While the swarm of molecules might sound a little scary, cells that didn't have Xist were more likely to develop cancer due to the additional proteins being made by the second X chromosome.