The secret history of X and Z – how sex chromosomes from humans and chickens found common ground

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 ...