World of Scientific Discovery on Thomas Hunt Morgan
Thomas Hunt Morgan was born on September 25, 1866, in Lexington, Kentucky. As a child growing up in rural Kentucky, he was surrounded by nature and wildlife. Perhaps that environment contributed to his intense interest in biology, for Morgan later majored in zoology at State College of Kentucky. After his graduation in 1886, he investigated chemistry and morphology (the study of organism development to better understand evolutionary relationships) at Johns Hopkins University, completing his doctorate in 1890. From his graduate days on, Morgan believed that heredity was in some way central to understanding all biological phenomena--especially development and evolution. His persistence in trying to prove and develop heredity theories led to his winning the Nobel Prize for physiology or medicine in 1933.
In 1903, there were several attempts to explain variations in plants and animal species. One was Charles Darwin's theory of natural selection, a process by which organisms best adapted to local environments leave more offspring that survive to spread their favorable traits throughout a population. But Morgan wondered how complex organisms such as humans could have evolved from such a process. To him, the theory seemed incomplete. Morgan viewed natural selection as a process that sorted out variations in an organism, not as one that created the variations. So what was it that determined whether a baby would be a boy or a girl, or whether it would have blue eyes or green eyes? The three widely known heredity theories of the time offered competing explanations: the Mendelian (or gene) theory, the chromosome theory, and the mutation theory.
Gregor Mendel, by cross-breeding pea plants, had first determined some of the rules of inheritable traits--those of sex determination, gene linkage (inheritance of characteristics together), and mimicry. Advocates of the chromosome theory maintained that genes located on chromosomes were responsible for specific inherited traits. Morgan was skeptical of the Mendelian and chromosome theories because the conclusions were speculative, based on nothing more than observation, inference, and analogy. Morgan wanted to be able to draw firm, rigorous, testable conclusions based on quantitative and analytical data. His strong belief in experimental analysis attracted Morgan to Hugo de Vries's mutation theory. De Vries, a Dutch botanist, had physical evidence that large-scale variations in one generation could produce offspring that were of a different species than their parent plants. Morgan set out to test de Vries's theory in animals and also to disprove the other heredity theories.
His first experiments using the fruit fly (Drosophila melanogaster) were unsuccessful; Morgan was not able to duplicate the magnitude of mutations that de Vries had claimed for plants. Then in 1910, Morgan noticed a natural mutation in one of the male fruit flies: it had white eyes instead of red. He began breeding the white-eyed male to its red-eyed sisters and found that all of the offspring had red eyes. When Morgan bred those offspring, he found that they produced a second generation of both red- and white-eyed fruit flies. Morgan was fascinated to find that all of the white-eyed flies were male. He traced the unusual finding to a difference between male and female chromosomes. The white-eye gene of the fruit fly was located on the male sex chromosome. By studying future generations of fruit flies, Morgan found that genes were linearly arranged on chromosomes. His work with the fruit fly strongly backed Mendel's gene concept and, moreover, established that chromosomes definitely carried genetic traits. For the first time, the association of one or more hereditary characteristics with specific chromosomes was clear, thereby unifying Mendelian "trait" theory and chromosome theory.
That was only the first of Morgan's discoveries. Working with students Hermann Muller, Alfred H. Sturtevant, and Calvin Bridges, Morgan went on to develop and perfect his concepts of linkage by explaining why, for instance, he occasionally found a white-eyed female in his studies. Morgan concluded that traits found on the same chromosome were not always inherited together. This genetic "mistake" was called crossing over, because one chromosome actually exchanged material with (or crossed over to) another chromosome. This process was an important source of genetic diversity. In 1915, Morgan, along with his students, published the culmination of his work, The Mechanism of Mendelian Heredity. These results provided the key to all further work in the area of genetics and laid the groundwork for all genetically-based research.
In 1904, Morgan had married Lilian Vaughan Sampson, who assisted in his research. They had one son and three daughters. Morgan died in 1945.
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