Thomas Hunt Morgan’s experiments on Drosophila led to the discovery of the principles of the gene-chromosome theory of hereditary transmission.

In 1904, Thomas Hunt Morgan, a young professor of biology, was invited by E. B. Wilson to join him at Columbia University as professor of experimental zoology. The new position would allow Morgan more time for laboratory research. That same year, through a friend, Jacques Loeb, Morgan met Hugo de Vries, Vries, Hugo de the Dutch biologist who had been one of the trio of scientists who in 1900 had rediscovered the work of Gregor Mendel. Mendel, Gregor (Mendel’s work had been largely ignored when he first propounded it in 1866, and it was soon thereafter forgotten.) De Vries had a theory that new species originate through mutation. The rediscovery of Mendel and the contact with de Vries influenced Morgan to initiate experiments to try to discover mutations and to test the Mendelian laws. He began to experiment with Drosophila melanogaster, the fruit fly, an enormously propitious choice for a laboratory animal. It bred rapidly and ate little, and Morgan and his students found that thousands of these minuscule experimental animals could be contained in a small collection of milk bottles that they “borrowed” from the Columbia cafeteria. Gene-chromosome theory[Gene chromosome theory]
Chromosomal theory of inheritance
Genetics;chromosomes
Chromosomes
[kw]Morgan Develops the Gene-Chromosome Theory (1908-1915)
[kw]Gene-Chromosome Theory, Morgan Develops the (1908-1915)[Gene Chromosome Theory, Morgan Develops the (1908 1915)]
[kw]Chromosome Theory, Morgan Develops the Gene- (1908-1915)
Gene-chromosome theory[Gene chromosome theory]
Chromosomal theory of inheritance
Genetics;chromosomes
Chromosomes
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[c]Science and technology;1908-1915: Morgan Develops the Gene-Chromosome Theory[02040]
[c]Genetics;1908-1915: Morgan Develops the Gene-Chromosome Theory[02040]
[c]Biology;1908-1915: Morgan Develops the Gene-Chromosome Theory[02040]
Morgan, Thomas Hunt
Bridges, Calvin Blackman
Sturtevant, Alfred H.
Muller, Hermann Joseph

Thomas Hunt Morgan.

(The Nobel Foundation)

In 1908, Morgan had one of his graduate students, Fernandus Payne, Payne, Fernandus perform an experiment in which he bred generations of Drosophila in the dark, in an attempt to produce flies whose eyes would atrophy and become dysfunctional. On Morgan’s advice, Payne secured his experimental animals by capturing some of the flies that were attracted to bananas he had left on the ledge of a window of the laboratory. Nothing came of the experiment, however; after sixty-nine generations of fruit flies, the last could see as well as the first.

Morgan’s second experiment on Drosophila was an attempt to induce mutations by subjecting flies to X rays and other environmental stimuli, such as wide ranges of temperature, salt, sugars, acids, and alkalies. Again, the results were negative. (Why these results were negative is unexplained, as Hermann Joseph Muller later demonstrated that X rays do produce mutations.)

Morgan’s experiments did not reveal mutations, nor did they bear out the Mendelian laws; rather, they turned up enough exceptions that Morgan began to doubt the validity of the laws. One day in 1910, however, he found a single male fly with white eyes instead of the standard red. Morgan bred the white-eyed male with a red-eyed female, and the first-generation offspring were all red-eyed, suggesting that being white-eyed was a Mendelian recessive factor. He then bred the first-generation flies among themselves, and those in the second generation were red-eyed and white-eyed in a three-to-one ratio, appearing to confirm that white eyes were Mendelian recessive. Morgan noted that all the white-eyed flies were males. He had discovered sex-limited heredity. Sex-limited heredity[Sex limited heredity] The Mendelian factor, or gene, that determined white eyes was located on the same chromosome as the gene that determined male sex, or, as it turned out, on the male chromosome.

Chromosomes—which appear as stringlike structures within cells—had been discovered by cell researchers in the 1850’s. Scientists had theorized, without much hard evidence, that they were involved in heredity. Mendelian theory originally had nothing to do with chromosomes (Mendel did not know about chromosomes). By the first decade of the twentieth century, when Mendelian chromosome theories had been suggested, again without much hard evidence, Morgan had considered such theories and rejected them. Meanwhile, researchers had discovered an odd-shaped chromosome (all other chromosomes occurred in similarly shaped pairs) that seemed to be related to male sex (now called the Y chromosome). The discovery of sex-limited heredity revealed the association of Mendelian genes with chromosomes and the function of chromosomes in heredity.

Morgan’s experiments revealed such results as the following: A. A red-eyed female is crossed with a white-eyed male. The red-eyed progeny interbreed to produce offspring in a ratio of 3/4 to 1/4. All the white-eyed flies are male. B. A white-eyed male is crossed with its red-eyed daughter, giving red-eyed and white-eyed males and females in equal proportions.

(Electronic Illustrators Group)

Following the discovery of sex-limited heredity, Morgan saw that a concerted effort would be required to expound fully the Mendelian chromosome theory, and he therefore enlisted a group of exceptional students to share the work in his so-called fly room. Calvin Blackman Bridges, Alfred H. Sturtevant, and Hermann Joseph Muller formed the nucleus of the group. From 1910 to 1915, Morgan and his team developed and perfected the concepts of linkage, Linkage, genetic
Genetic linkage in which various genes are found to be located on the same chromosome and the appearance of their associated characteristics in the offspring occur together, and crossing-over, Crossing-over, genetics[Crossing over]
Genetics;crossing-over[crossing over] in which paired chromosomes break and rejoin during meiosis. Crossing-over produces an effect contrary to that of linkage. That is, characteristics that would be expected to appear in the offspring because they are associated with genes on the same chromosome will not appear because during meiosis (when the paired chromosomes split apart, whereupon only half of the pair is conveyed to the new cell) part of the chromosome and the genes on that part are replaced by a portion of the paired chromosome.

Using their understanding of linkage and crossing-over, the team members were also able to create chromosome maps, plotting the relative locations and distances of the genes on the chromosomes. That is, the occurrence of combinations and recombinations (the latter produced by crossing-over) of linked characteristics indicates the relative locations of the genes for those characteristics on the chromosome. The frequency of recombinations should be proportional to the distance between the genes for the recombined characteristics.

The culmination of the work of Morgan’s team was the publication in 1915 of The Mechanism of Mendelian Heredity, Mechanism of Mendelian Heredity, The (Morgan et al.) coauthored by Morgan, Sturtevant, Muller, and Bridges. For the next twelve years, the strictly genetics studies were performed mainly by Sturtevant and Bridges and other team members, while Morgan returned to his previous areas of interest of embryology and evolution, pursuing connections between those areas and the new discoveries in genetics. Morgan also was occupied in publicizing the new views of heredity and their ramifications through publications and lectures.

By the late 1920’s, Morgan was acknowledged as the world’s leading geneticist. In 1927, Robert Andrews Millikan was reorganizing the California Institute of Technology in Pasadena to make it one of the premier scientific schools in the country, and he asked Morgan to organize the Division of Biology. Morgan accepted and joined a staff that included physicist J. Robert Oppenheimer and chemist Linus Pauling. In 1933, Morgan was awarded the Nobel Prize in Physiology or Medicine (the first Nobel Prize in the field of genetics) for his work on hereditary research. Nobel Prize recipients;Thomas Hunt Morgan[Morgan]

The discovery and demonstration that genes reside on chromosomes was the key to all further work in the area of genetics. Mendel, who in 1866 discovered genes, was extremely fortunate in the design of his experiments in that each of the characteristics he investigated in his pea plants happened to reside on a separate chromosome. This facilitated discovery of the Mendelian hereditary principles but made for experimental results that were rather tidy. When a larger number of characteristics is investigated, because of linkage of genes located on the same chromosome and crossing-over of chromosomes, the results are much more complicated and do not reveal the Mendelian pattern so clearly. This is what happened in Morgan’s early experiments. It was only through careful examination—using tweezers and magnifying glass—of generation after generation of the tiny Drosophila that Morgan began to discern the mechanism of inheritance.

Although Morgan did not pursue medical studies, his research laid the groundwork for all genetically based medical research. As the presenter of the Nobel Prize to Morgan stated, without Morgan’s work, “modern human genetics and also human eugenics would be impractical.” It is generally accepted that Mendel’s and Morgan’s discoveries are responsible for all subsequent advancement in the investigation and understanding of hereditary diseases.

The darker side of genetics research is that it also laid the groundwork for Adolf Hitler’s iniquitous eugenics Eugenics experiments and associated fantasies of racial purity. Morgan was always extremely distrustful of any such experiments on the human species, however. In his Nobel acceptance speech, he noted that through suitable breeding, geneticists were now able to produce populations of species of animals and plants that were free from hereditary defects. He went on to say, however, that it would not be advantageous to perform genetic experiments on humans, except to attempt to correct hereditary defects to improve human health. Morgan believed that any attempt to “purify” the human race would be improper. Gene-chromosome theory[Gene chromosome theory]
Chromosomal theory of inheritance
Genetics;chromosomes
Chromosomes

• Allen, Garland E. Thomas Hunt Morgan: The Man and His Science. Princeton, N.J.: Princeton University Press, 1978. Scholarly yet readable biography of Morgan and exposition of his work. One of the most thorough works on Morgan available. Includes an extensive bibliography of primary and secondary works.
• Carlson, Elof Alex. The Gene: A Critical History. 1966. Reprint. Ames: Iowa State University Press, 1989. Rather than a standard, comprehensive review of the subject, this book is thematic in outline. The chapter titled “The Drosophila Group: An Enigmatic Appraisal” provides Muller’s perspective on Morgan, which gives less credit to Morgan and more to his assistants for the ideas of the group after Morgan’s initial discoveries.
• Cummings, Michael. Human Heredity: Principles and Issues. 6th ed. Monterey, Calif.: Brooks/Cole, 2002. This highly illustrated text aimed at nonscience students presents the complex topic of heredity clearly, without oversimplifying the concepts discussed. Also addresses the social, cultural, and ethical implications of the use of genetic technology.
• Dunn, L. C. A Short History of Genetics: The Development of Some of the Main Lines of Thought, 1864-1939. 1965. Reprint. Ames: Iowa State University Press, 1991. Summarizes the general history of theories of heredity and gives a sense of the multifaceted nature of the search for the principles of genetics. Intended more for the specialist than for the general reader. Includes a glossary and a bibliography of both primary and secondary sources.
• Morgan, Thomas Hunt, Alfred H. Sturtevant, Hermann Joseph Muller, and Calvin Blackman Bridges. The Mechanism of Mendelian Heredity. Rev. ed. New York: Henry Holt, 1922. The classic text in which Morgan and his students describe their Drosophila research.
• Shine, Ian, and Sylvia Wrobel. Thomas Hunt Morgan: Pioneer of Genetics. Lexington: University of Kentucky Press, 1976. This very readable biography and popular treatment of Morgan’s work focuses on his likable personality and contains many personal anecdotes. Covers Morgan’s interesting family history: His uncle was a Confederate Civil War hero, and his father, as U.S. consul to Italy, discarded protocol and fought at the side of Garibaldi.
• Sturtevant, Alfred H. A History of Genetics. 1965. Reprint. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 2001. A firsthand view of the development of classical genetics by a major participant. Rather technical in approach. An interesting feature for a book in the area of heredity is an appendix on the intellectual pedigrees (presenting “genealogical” charts) of the scientists and philosophers involved in the development.
• Sturtevant, Alfred H., and G. W. Beadle. An Introduction to Genetics. 1939. Reprint. New York: Dover, 1962. This textbook on genetics from 1939 is useful as a historical document. Beadle was a Nobel Prize winner in 1958.

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Sutton Proposes That Chromosomes Carry Hereditary Traits

Punnett’s Mendelism Includes Diagrams Showing Heredity

Bateson and Punnett Observe Gene Linkage

Hardy and Weinberg Present a Model of Population Genetics

Johannsen Coins the Terms “Gene,” “Genotype,” and “Phenotype”

Sturtevant Produces the First Chromosome Map