The Story of the X Chromosome (How It Determines Our Lives)

Author: David B. Miller (USA)
Publisher:
Publish Date: 2006-06-01
Features: How should all this be explained? Hermann Henking was intently focused, gazing down at the microscope, trying to figure out why the purple spots on the slide were behaving so strangely. It was 1890, and scientists all over the world were studying these spots, yet they remained as mysterious as ever. Many believed they held the key to the biological temple—the mechanism that allowed offspring to inherit certain traits from their parents—but no one could prove it. While sitting in his dim and dilapidated laboratory in Leipzig, Henking was on the verge of a major discovery that could have revolutionized genetics, but instead, he set the field back by a decade. His research could have directly led to our current understanding of gender and genetics, but for the next ten years, no one recognized the significance of his work. In fact, it took another thirty years for another zoologist to tell Henking the importance of his achievements. Looking back in history, the 1890s were a decade filled with frustration. Charles Darwin had already explained in On the Origin of Species that as long as parents could pass traits to their offspring, external forces could change species over time. But no one could explain how all this inheritance actually worked. Everyone knew that children often resemble their parents—calves, foals, puppies, and kitten seemed to follow suit. In fact, if you took the time to look, almost any type of animal you observed would show familial resemblance. So, inheritance was likely a universal phenomenon, though no one could explain it. Many zoologists believed that a few sperm and eggs carried instructions that made each new individual similar to its parents. Most agreed that some kind of material mechanism must exist to transfer these instructions to a new individual, though a few still insisted that inheritance was a spiritual process rather than a material one—they believed that if inheritance were a worldly matter, it might undermine God's dominion over all creation. That was when Henking appeared, with his new discovery, which later settled the entire debate. He discovered the genetic mechanism, though he was completely unaware of it. Despite the seemingly hopeless situation, Henking's weary eyes were captivated by the chromosomes before him. These chromosomes came from the reproductive organs of an insect called Pyrrhocoris (the firebug). Twenty-five years earlier, the generation of scientists exploring the genetic mechanism had injected a tiny new dye into the chromosomes (the "colored bodies"), making them visible to the naked eye under the microscope. They found that these chromosomes exhibited elegant yet bizarre behavior. Every cell in an organism contains a set of chromosomes, but to be honest, most of the time, they didn’t seem particularly special. Chromosomes suddenly come alive only when a cell divides into two daughter cells. Miraculously, they transform from blurry, disorganized matter into distinct, neatly arranged threads, skillfully positioned at the center of the cell. Even more fascinating was that the chromosomes were then carefully sorted into identical pairs, each pair entering one of the daughter cells. Given how orderly chromosomes divide between cells, they must have been accomplishing something extraordinary—so, were chromosomes the genetic instructions scientists had been desperately seeking? At the time, Henking wasn’t studying the reproductive organs of flies, as he treasured them. Scientists might be eccentric, but not to the point of absurdity. Obviously, one of the most striking features of chromosomes could only be observed in testes and ovaries, where cells divide to produce sperm or eggs (the cells that create the next generation). Compared to the cell division that produces all other cells, the process of generating sperm or eggs seemed vastly different. It wasn’t a single-step division, but rather a two-step continuous process, with a carefully designed sequence between the two steps. During the two divisions, the chromosomes remained in a state of constant contact, actively engaged. If chromosomes had nothing to do with inheritance, why would they perform such a graceful dance during the formation of sperm and eggs? It was with this question in mind that Henking made his observations, cementing his name in the history of biological development. He noticed that while the separated pairs of chromosomes were elegantly dancing within the firebug’s reproductive organs, one chromosome remained quietly nearby, not joining the celebration. Because it was so quiet, when the dance ended, it wasn’t evenly divided between the daughter cells—finally, it found its place in only one half of the sperm. It looked just like all the other chromosomes, but Henking couldn’t understand why it was sitting on the sidelines. With suspicion, he gave it the name "wallflower chromosome," though we still don’t fully understand why he chose that name. Perhaps it was because of its mystery, or because its appearance was somewhat "special," or maybe because it was clearly an "extra-chromosome." He called it the "X chromosome." This name is still used today. So far, most other chromosomes have common and mundane names, while X remains a mystery. It is special, though Henking didn’t fully realize this at the time. P1-3