Question

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?

Hint:

The letter "X" in the X chromosome originally stood for "unknown," much like an unknown variable $x$ in mathematics. Henking called it the "X-body" simply because its function was a complete mystery under his microscope in 1891.

Answer 1

Step 1: Understanding the Question:
The question asks for a description of Hermann Henking's microscopic observations during insect spermatogenesis (1891). It also asks how this observation helped establish our modern understanding of chromosomal sex determination.

Step 2: Key Formula or Approach:
Henking identified a distinct nuclear structure that was present in exactly half of the male gametes, laying the groundwork for identifying the $XX/XO$ genetic system of sex determination.

Step 3: Detailed Explanation:
Henking's Primary Observation: In 1891, Hermann Henking observed a specific nuclear structure while tracing the steps of spermatogenesis (sperm formation) in certain insects. He noted that this structure remained unpaired and did not separate like other chromosomes during meiosis. Crucially, he found that this structure was distributed unevenly among the resulting gametes: exactly $50\%$ of the sperm cells received this structure, while the remaining $50\%$ did not receive it. Because its function was unknown at the time, he named it the "X-body".
Contribution to Sex Determination: Henking did not establish its role in sex determination himself. However, later researchers recognized that Henking's "X-body" was an entire chromosome, which they renamed the X chromosome. This led directly to understanding the $XO$ sex-determination system seen in insects like grasshoppers. Scientists discovered that eggs fertilized by a sperm carrying the X chromosome developed into females ($XX$), whereas eggs fertilized by a sperm lacking the X chromosome developed into males ($XO$). This proved for the first time that sex is determined by the specific chromosome combination passed down through the gametes.

Step 4: Final Answer:
Henking observed that exactly $50\%$ of insect sperm cells received a specific nuclear structure called the "X-body," while the other $50\%$ did not. This finding contributed to sex determination by revealing that the presence or absence of this specific sex chromosome in the fertilizing sperm decides the sex of the offspring.