Chromosomes, Mutation, and the Birth of Modern Genetics: Thomas Hunt MorganIn 1900 several scientists across Europe came to the same realization about heredity that Mendel had some 40 years before. But they arrived at the discovery from a very different direction.
Nineteenth century cell biologists discovered that animal and plant cells had a central compartment known as the nucleus. Each nucleus contained a set of rod-shaped structures, and when a typical cell divided, a new nucleus complete with a new set of rods was created. These rods were named chromosomes for the way they absorbed colored stains. But sperm and eggs contained only half the normal set of chromosomes. When a sperm fertilized an egg, the chromosomes combined to create a full complement.
Scientists realized that the chromosomes stored the information necessary for building an individual, and heredity consisted of the transfer of that information from generation to generation. Each chromosome contained information for many different traits, and scientists dubbed each chromosomal chunk that was responsible for a particular trait a "gene."
Perhaps, scientists speculated, evolution took place as genes were altered. DeVries claimed that if a gene changed if it "mutated" it would create a new species in a single jump. But no one could say for sure what mutations did until they could be studied up close. That became possible in the laboratory of a Columbia University biologist, Thomas Hunt Morgan (left).
Morgan bred fruit flies by the thousands, and his team tried to create mutant flies with x-rays, acids, and other toxic substances. Finally, in one unaltered lineage of flies, the researchers found a surprise. Every single fly in that line had been born with red eyes, until one day a fly emerged from its pupa with white eyes. Something had spontaneously changed in the white-eyed fly.
Mutation does not equal speciation
Genetics is born
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Cell images courtesy of Florida State University Molecular Expressions Microscopy Gallery; Morgan image (PA96M3 #44) courtesy of University of Kentucky Special Collections and Archive; Flies image courtesy of University of Massachusetts Amherst Biology Department
Understanding Evolution © 2015 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California