The naming of X and Y

The Scientist this month has a nice short article by Joseph Keierleber that recounts some of the early history of scientific investigation of the sex chromosomes: “How Chromosomes X and Y Got Their Names, 1891”.

It begins with the German biologist Hermann Henking, who studied firebug sperm and found a large lump of chromatin that he labeled “x”. It was this “x” that eventually won out in the naming of the sex-determining chromosomes, but not before some additional complexity:

In the early 1900s, Nettie Stevens at Bryn Mawr College and Edmund Beecher Wilson at Columbia University tackled the puzzle of how chromosomes relate to sex differences in insects. Although they worked independently, they followed each other’s research. In 1905, Wilson, the more established scientist, described a pair of unequally sized chromosomes, which segregated in a 50:50 ratio among insect sperm. A month later, Stevens reported a similar discovery in beetle gonads. Half of beetle sperm carried a small chromosome, which Stevens labeled “s,” and half carried its larger companion, “l.” Female somatic cells contained two copies of the large chromosome, while male cells contained one small and one large. “This seems to be a clear case of sex determination,” Stevens wrote, concluding that sperm carrying the small chromosome, not McClung’s large accessory chromosome, determined male sex.

As the story goes on to relate, it was Wilson who went on to name “X” and “Y” by following Henking’s example.