In humans, two chromosomes – X and Y – determine whether we are male or female. Of the two, Y tends to get more attention because of its small, degenerate size. Both X and Y probably evolved from a pair of ordinary chromosomes that have nothing to do with sex (also known as autosomes). The story goes that one of these autosomes developed a gene that immediately caused its bearer to become male, and eventually became the Y chromosome of today. The other one became X.
Throughout its history, Y has been a hotbed of genetic change, gaining, losing and remodelling its genes at breakneck pace, and shrinking by 97%. Its partner – X – has allegedly had a less eventful past, and should faithfully represent the ancestral autosome. This history of X and Y was first proposed in 1914 by Herman Muller, and ever since, his assumptions about X’s stability have gone untested. Now, it seems that Muller was wrong. Daniel Bellott from the Howard Hughes Medical Institute had uncovered the secret history of X, which turns out to be no less storied than Y’s tale.