Q.What did Henking observe in his experiments on insects that led to the discovery of sex chromosome ? How did his observation contribute to our understanding of sex determination ?

Henking was studying the chromosomes of male insects, specifically the firefly Pyrrhocoris. He noticed that during sperm formation, one chromosome did not pair with any other chromosome and moved to only one pole during meiosis. This odd chromosome was present in only half of the sperm cells.

He called this unpaired structure the "X body" because he did not know what it was. He could not yet connect it to sex determination — he simply recorded that it was an extra, unpaired element in the male. …

In the late nineteenth century, the mechanism of sex determination was a complete mystery. Biologists knew that offspring could be male or female, but no one understood what caused the difference. It was in this context that the German cytologist Hermann Henking, while studying the testes of the firefly Pyrrhocoris (a type of insect), made a curious observation under the microscope.

Henking was carefully tracking the behaviour of chromosomes during sperm formation — a process called meiosis. He noticed that during the first meiotic division, one particular chromosome stood out from the rest. Unlike the other chromosomes, which paired up and separated neatly, this one seemed to move to one pole of the dividing cell without a partner. It was a dark, compact structure that stained differently, and Henking simply called it the “X body” or “X element” — the letter X being a placeholder for the unknown.

What made this observation truly striking was its distribution. When Henking examined the mature sperm cells, he found that exactly half of them contained this X body, while the other half did not. In other words, the X body was present in only 50% of the sperm. This was a clear, repeatable pattern, but Henking himself did not fully grasp its significance. He thought it might be a peculiar nuclear structure, perhaps a leftover from the nucleus, and did not immediately connect it to sex determination.

The real breakthrough came a few years later, when other scientists — particularly Clarence McClung in 1902 and Nettie Stevens in 1905 — built on Henking’s work. They studied other insects and found that the X body was actually a chromosome, and that its presence or absence in sperm correlated perfectly with the sex of the offspring. In many insects, females had two X chromosomes (XX), while males had only one (XO). Later, in other species, a second sex chromosome — the Y — was discovered, giving the familiar XX/XY system. …