ABOVE: Cytogeneticist Nettie Stevens drew these images of beetle chromosomes in 1906, labeling one chromosome pair “l” and “s” in figures 102 and 107. These chromosomes would come to be known as the sex chromosomes, X and Y.
BIODIVERSITY HERITAGE LIBRARY, N.M. STEVENS, STUDIES IN SPERMATOGENESIS PART II
Why are the human sex chromosomes called “X” and “Y,” while the other 22 chromosomes are identified only by numbers?
The answer begins in the late 1800s, when insect gonad cells, whose large chromosomes are easy to view through a microscope, were the specimen of choice for investigating the cellular basis of heredity. In 1891, German biologist Hermann Henking counted 11 chromosomes in firebug (Pyrrhocoris apterus) sperm nuclei. Some nuclei, he discovered, contained an additional large chromatin element, which he identified in his drawings with “x.”
A few years later at the University of Kansas, Clarence McClung observed that half of grasshopper sperm ...
