morgan drosophila experiment

“Sex Limited Inheritance in Drosophila” (1910), by Thomas Hunt Morgan

In 1910, Thomas Hunt Morgan performed an experiment at Columbia University, in New York City, New York, that helped identify the role chromosomes play in heredity. That year, Morgan was breeding Drosophila , or fruit flies. After observing thousands of fruit fly offspring with red eyes, he obtained one that had white eyes. Morgan began breeding the white-eyed mutant fly and found that in one generation of flies, the trait was only present in males. Through more breeding analysis, Morgan found that the genetic factor controlling eye color in the flies was on the same chromosome that determined sex. That result indicated that eye color and sex were both tied to chromosomes and helped Morgan and colleagues establish that chromosomes carry the genes that allow offspring to inherit traits from their parents.

Prior to Morgan’s fly experiments, other researchers were studying heredity. In 1865, scientist Gregor Mendel in eastern Europe published an article describing heredity experiments he had performed using pea plants. By mating pea plants, Mendel observed that the resulting offspring inherited characteristics, such as seed color and seed shape, in predictable patterns. Mendel hypothesized that there were heritable factors, later called genes, controlling the development of those characteristics.

By the early 1900s, other scientists aiming to explain heredity began to reapply Mendel’s theory. In the late nineteenth century, researchers discovered structures inside the nuclei of cells. Researchers called those structures chromosomes because of the way staining materials colored them. Staining chromosomes enabled researchers to observe chromosomes throughout development. In 1902, Walter Sutton, a researcher at Columbia University, and Theodor Boveri, a researcher at the University of Würzburg in Würzburg, Germany, each observed that chromosomes behaved in a manner that was consistent with Mendel’s theories. Boveri and Sutton hypothesized that chromosomes carried heritable factors, or genetic material. Researchers called Boveri and Suttons’ theory the Boveri-Sutton chromosome theory.

By 1904, Morgan had begun to study the processes that affect heredity and development at Columbia University. However, Morgan, like other scientists at that time, was reluctant to accept the Boveri-Sutton chromosome theory. Morgan argued that scientists had a bias towards associating phenomena, like the inheritance of traits, with known structures, like the chromosome. Similarly, he argued that if one gene didn’t explain a phenomenon, scientists could argue that any number of genes might. In 1910, Morgan published an article explaining why he was reluctant to accept the Bover-Sutton chromosome theory.

Later that year, Morgan made an observation that eventually provided evidence in support of the chromosome theory. In 1910, Morgan was studying Drosophila at Columbia University to find what he called mutants, or individual flies that had atypical, heritable characteristics, such as white eyes instead of the normal red eyes. In May of 1910, after breeding thousands of flies, he observed a single male fly with white eyes, which he called a white mutant. Typically, both male and female flies have red eyes. To explain the white eye mutation, Morgan bred the mutant fly and observed how the mutation was inherited throughout successive generations.

In 1910, Morgan published details of his research in an article titled “Sex Limited Inheritance in Drosophila." First, Morgan took the white mutant and bred it with pure red-eyed female flies. All of the females that resulted from that breeding had red eyes. Morgan then took those red-eyed females and mated them with the original white-eyed mutant male to determine whether or not the inheritance of eye color followed Mendel’s inheritance patterns. If Mendel’s patterns applied to Morgan’s flies, there would be one white-eyed fly to every three red-eyed flies in the resulting generation of flies, regardless of sex. Although Morgan did observe one white-eyed fly to every three red flies, that inheritance pattern was not shared equally across males and females. Most of the white-eyed flies were male. That result indicated that the flies did not follow Mendel’s ratio in a traditional sense.

After observing the white-eye inheritance pattern, Morgan hypothesized that a factor, or gene, controlling eye color was located on the X chromosome. Female flies have two X chromosomes, and males have one X chromosome and one Y chromosome. If a trait, like eye color, correlated with a specific factor on the X chromosome, then the trait was called X-linked. Because males only have one X chromosome, they display all X-linked traits. Females, on the other hand, often need an X-linked trait to exist on both X chromosomes to display that trait. Morgan hypothesized that, in his breeding experiment, the first generation of flies contained males only with white eyes because the gene controlling eye color was on the X chromosome. Males displayed the white eye trait because the trait was present on their only X chromosome. Females did not display the white eye trait because the trait was only present on one of their X chromosomes.

To test his hypothesis that the white-eyed trait was on the X chromosome, Morgan mated other specific groups of flies together and observed the offspring. Prior to doing so, Morgan predicted what the sex and eye color ratios of the offspring would be if his hypothesis were true. By comparing the observed results with the predicted results, Morgan determined that his hypothesis was supported. In one mating, Morgan took a red-eyed male and mated it with a white-eyed female. He predicted and observed that half of the flies would be red-eyed females and the other half would be white-eyed males. That mating showed that the occurrence of the white-eyed trait is limited to the X chromosome, as only male offspring were capable of displaying the white-eyed trait with a single copy of the trait. Morgan showed that inheritance of a trait could differ between sexes.

In the following years, Morgan and a group of scientists at Columbia University established the chromosome theory of inheritance, which described the role that chromosomes play in heredity. In 1911, Morgan published more details of his experiments with the white-eyed mutant, an account in which Morgan explicitly stated that chromosomes carry heritable factors, or genes. In 1915, Morgan, and his colleagues, Alfred Henry Sturtevant, Calvin Bridges, and Herman Joseph Muller published the book Mechanism of Mendelian Heredity . That book contained contemporary scientific information about heredity and included the results of Morgan’s white-eyed mutant experiments.

In 1933, Morgan won the Nobel Prize in Physiology or Medicine for his work establishing the chromosome’s involvement in heredity.

• Boveri, Theodor. “Über mehrpolige Mitosen als Mittel zur Analyse des Zellkerns (On multipolar mitosis as a means to analyze the cell nucleus).” Verhandlungen der physicalisch-medizinischen Gesselschaft zu Würzburg ( Proceedings of the physical-medical company at Wurzburg ) 35 (1902): 67–90. http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/15991 (Accessed April 2, 2017).
• Kandel, Eric R. “Thomas Hunt Morgan at Columbia University.” Columbia University Living Legacies. http://www.columbia.edu/cu/alumni/Magazine/Legacies/Morgan/ (Accessed March 25, 2017).
• Mendel, Gregor Johann. “Versuche über Pflanzen-Hybriden (Experiments Concerning Plant Hybrids)” [1866]. In Verhandlungen des naturforschenden Vereines in Brünn ( Proceedings of the Natural History Society of Brünn ) IV (1865): 3–47. Reprinted in Fundamenta Genetica , ed. Jaroslav Krízenecký, 15–56. Prague: Czech Academy of Sciences, 1966. http://www.mendelweb.org/Mendel.html (Accessed March 25, 2017).
• Morgan, Thomas H. "Chromosomes and heredity." The American Naturalist 44 (1910): 449–96. http://www.jstor.org/stable/pdf/2455783.pdf (Accessed March 25, 2017).
• Morgan, Thomas H. "Sex Limited Inheritance in Drosophila." Science (1910): 120–2. http://www.jstor.org/stable/pdf/1635471.pdf (Accessed March 25, 2017).
• Morgan, Thomas H. “Random Segregation Versus Coupling in Mendelian Inheritance.” Science (1911): 384. http://science.sciencemag.org/content/34/873/384 (Accessed April 2, 2017).
• Morgan, Thomas H., Alfred H. Sturtevant, Hermann J. Muller, and Calvin B. Bridges. The Mechanism of Mendelian Heredity . New York: Henry Holt and Company, 1915. http://www.biodiversitylibrary.org/bibliography/22551#/summary (Accessed March 25, 2017).
• Nobel Prizes and Laureates. “The Nobel Prize in Physiology or Medicine 1933.” The Official Web Site of the Nobel Prize. https://www.nobelprize.org/nobel_prizes/medicine/laureates/1933/ (Accessed April 2, 2017).
• Sutton, Walter S. "The chromosomes in heredity." The Biological Bulletin 4 (1903): 231–50. http://www.biolbull.org/content/4/5/231.full.pdf (Accessed March 25, 2017).

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• Linkage and Recombination

Linkage and recombination are phenomena that describe the inheritance of genes. A linkage is a phenomenon where two or more linked genes are always inherited together in the same combination for more than two generations. The recombination frequency of the test cross progeny is always lower than 50%. Therefore, if any two genes are completely linked, their recombination frequency is almost 0%. The phenomenon of linkage was studied by the scientist T.H. Morgan using the common fruit fly or Drosophila melanogaster.

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Morgan’s Experiment

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Morgan picked Drosophila melanogaster  as his subject for the following reasons:

• He noticed a white-eyed male drosophila instead of the regular red eyes.
• It was small in size
• They have a short lifespan and so many generations can be studied in a short time frame.
• They have a high rate of reproduction

He crossed a purebred white eyed male with purebred red-eyed female. As expected following Mendel’s laws, the F1 progeny were born with red eyes. When F1 generation was crossed among each other, the ratio of red-eyed to white eyed progeny were 3:1. However, he noticed that there was no white- eyed female in the  F2 generation.

To understand further, he performed a cross between a heterozygous red-eyed female with a white-eyed male. This gave a ratio of 1:1:1:1 in the progeny(1 white eyed female, 1 red eyed female, 1 white eyed male and 1 red eyed male). This made Morgan think about the linkage between the traits and sex chromosomes. He performed many more crosses and determined that the gene responsible for the eye color was situated on the X chromosome.

Types of Linkage

Linkages are primarily of two types: Complete and incomplete

• Complete Linkage: When the combination of characters appears together in more than two generations in a regular manner, it is called as a complete linkage. Due to this complete linkage, only two types of gametes are formed. Example: Drosophila melanogaster
• Incomplete Linkage: When there is an incomplete linkage, new gene combinations are formed in the progeny or offsprings. This occurs due to the formation of a chiasma or crossing over between the linked genes.

Linkage Significance

• Due to the linkage between genes, desired characters cannot be brought together by breeders. This would be possible only if the genes would sort independently.
• The characters that are linked remain so as there is no chance of recombination of the linked genes.

Sex- chromosome Linked Diseases in Humans

Diseases like haemophilia, color blindness, male pattern of baldness are sex-linked diseases. Where color blindness and haemophilia are X- linked diseases, male pattern of baldness is a Y-linked one. This indicates that the X-linked diseases will express themselves in a male whereas the female is always a carrier until both the genes are recessive in the female. Male pattern of baldness being a Y-linked trait expresses itself only in the males while females are never affected by it.

Crossing Over

This is a phenomenon where genetic material is exchanged between non-sister chromatids of homologous chromosomes which results in a new gene combination. The process of crossing over occurs in a sequence of following steps:

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• Duplication of chromosomes
• Crossing over
• Chiasmata formation
• Terminalization

Solved Example for You

Q1: If the genes are completely or fully linked, what are the chances of recombination?

Sol. The correct answer is the option ”c”. If the genes are completely or fully linked, then the chances of recombination are 0%. 

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Principles of Inheritance and Variation

• Laws of Inheritance
• Introduction to Genetics
• Mutation and Chromosomal Disorder
• Sex Determination

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• Introduction

Experiments in embryology

The work on drosophila.

• How is genetic testing done?

Thomas Hunt Morgan

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• Public Broadcasting Service - Biography of Thomas Hunt Morgan
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• Table Of Contents

Thomas Hunt Morgan (born Sept. 25, 1866, Lexington , Ky., U.S.—died Dec. 4, 1945, Pasadena , Calif.) was an American zoologist and geneticist, famous for his experimental research with the fruit fly ( Drosophila ) by which he established the chromosome theory of heredity . He showed that genes are linked in a series on chromosomes and are responsible for identifiable, hereditary traits. Morgan’s work played a key role in establishing the field of genetics . He received the Nobel Prize for Physiology or Medicine in 1933.

Morgan’s father, Charlton Hunt Morgan, was a U.S. consul, and his uncle, John Hunt Morgan , had been a Confederate army general.

Early in life, Morgan showed an interest in natural history. In 1886 he received the B.S. degree from the State College of Kentucky (later the University of Kentucky) in zoology and then entered Johns Hopkins University for graduate work in biology . At Hopkins, Morgan studied under the morphologist and embryologist William Keith Brooks . After being awarded the Ph.D. in 1890, Morgan remained there a year before accepting a teaching post at Bryn Mawr College .

During the period 1893–1910, Morgan applied experimental techniques to fundamental problems of embryology . In order to identify causally related events during development, he analyzed such problems as the formation of embryos from separated blastomeres (early embryonic cells) and fertilization in nucleated and nonnucleated egg fragments. As examples of the effects of physical factors, he analyzed the way in which the spatial orientation of eggs affects their future development and the action of salt concentration on the development of fertilized and unfertilized eggs. In 1904 he married one of his graduate students at Bryn Mawr, Lillian V. Sampson, a cytologist and embryologist of considerable skill. The same year, he accepted an invitation to assume the professorship of experimental zoology at Columbia University , where, during the next 24 years, he conducted most of his important research in heredity.

Like most embryologists and many biologists at the turn of the century, Morgan found the Darwinian theory of evolution lacking in plausibility. It was difficult to conceive of the development of complex adaptations simply by an accumulation of slight chance variations. Moreover, Darwin had provided no mechanism of heredity to account for the origin or transmission of variations, except his early and hypothetical theory of pangenesis. Although Morgan believed that evolution itself was a fact, the mechanism of natural selection proposed by Darwin seemed incomplete because it could not be put to an experimental test.

Morgan had quite different objections to the Mendelian and chromosome theories. Both theories attempted to explain biological phenomena by postulating units or material entities in the cell that somehow control developmental events. To Morgan this was too reminiscent of the preformation theory—the idea that the fully formed adult is present in the egg or sperm—that had dominated embryology in the 18th and early 19th centuries. Although Morgan admitted that the chromosomes might have something to do with heredity, he argued in 1909 and 1910 that no single chromosome could carry specific hereditary traits. He also claimed that Mendelian theory was purely hypothetical: although it could account for and even predict breeding results, it could not describe the true processes of heredity. That each pair of chromosomes separates, with the individual chromosomes then going into different sperm or egg cells in exactly the same manner as Mendelian factors, did not seem to be sufficient proof to Morgan for claiming that the two processes had anything to do with each other.

Morgan apparently began breeding Drosophila in 1908. In 1909 he observed a small but discrete variation known as white-eye in a single male fly in one of his culture bottles. Aroused by curiosity, he bred the fly with normal (red-eyed) females. All of the offspring (F 1 ) were red-eyed. Brother–sister matings among the F 1 generation produced a second generation (F 2 ) with some white-eyed flies, all of which were males. To explain this curious phenomenon, Morgan developed the hypothesis of sex-limited—today called sex-linked—characters, which he postulated were part of the X-chromosome of females. Other genetic variations arose in Morgan’s stock, many of which were also found to be sex-linked. Because all the sex-linked characters were usually inherited together, Morgan became convinced that the X-chromosome carried a number of discrete hereditary units, or factors. He adopted the term gene , which was introduced by the Danish botanist Wilhelm Johannsen in 1909, and concluded that genes were possibly arranged in a linear fashion on chromosomes. Much to his credit, Morgan rejected his skepticism about both the Mendelian and chromosome theories when he saw from two independent lines of evidence—breeding experiments and cytology—that one could be treated in terms of the other.

In collaboration with A.H. Sturtevant, C.B. Bridges, and H.J. Muller, who were graduates at Columbia, Morgan quickly developed the Drosophila work into a large-scale theory of heredity. Particularly important in this work was the demonstration that each Mendelian gene could be assigned a specific position along a linear chromosome “map.” Further cytological work showed that these map positions could be identified with precise chromosome regions, thus providing definitive proof that Mendel’s factors had a physical basis in chromosome structure. A summary and presentation of the early phases of this work was published by Morgan, Sturtevant, Bridges, and Muller in 1915 as the influential book The Mechanism of Mendelian Heredity. To varying degrees Morgan also accepted the Darwinian theory by 1916.

In 1928 Morgan was invited to organize the division of biology of the California Institute of Technology . He was also instrumental in establishing the Marine Laboratory on Corona del Mar as an integral part of Caltech’s biology training program. In subsequent years, Morgan and his coworkers, including a number of postdoctoral and graduate students, continued to elaborate on the many features of the chromosome theory of heredity. Toward the end of his stay at Columbia and more so after moving to California , Morgan himself slipped away from the technical Drosophila work and began to return to his earlier interest in experimental embryology. Although aware of the theoretical links between genetics and development, he found it difficult at that time to draw the connection explicitly and to support it with experimental evidence.

In 1924 Morgan received the Darwin Medal; in 1933 he was awarded the Nobel Prize for his discovery of “hereditary transmission mechanisms in Drosophila ”; and in 1939 he was awarded the Copley Medal by the Royal Society of London, of which he was a foreign member. In 1927–31 he served as president of the National Academy of Sciences; in 1930 of the American Association for the Advancement of Science; and in 1932 of the Sixth International Congress of Genetics. He remained on the faculty at Caltech until his death.

Among Morgan’s most important books are those dealing with (1) evolution: Evolution and Adaptation (1903), in which he strongly criticizes Darwinian theory; and A Critique of the Theory of Evolution, (1916), a more favourable view of the selection process; (2) heredity: Heredity and Sex (1913), his first major exposition of the Mendelian system in relation to Drosophila; and with A.H. Sturtevant, H.J. Muller, and C.B. Bridges, The Mechanism of Mendelian Heredity (1915; rev. ed., 1922); and The Theory of the Gene (1926; enlarged and revised ed., 1928); the latter two works firmly established the Mendelian theory as it applied to heredity in all multicellular (and many unicellular) organisms; and (3) embryology: The Development of the Frog’s Egg: An Introduction to Experimental Embryology (1897), a detailed outline of the developmental stages of frogs’ eggs; Experimental Embryology (1927), Morgan’s statement on the value of experimentation in embryology; and Embryology and Genetics (1934), an attempt to relate the theory of the gene to the problem of embryological differentiation and development.

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Anecdotal, Historical and Critical Commentaries on Genetics

Thomas hunt morgan at the marine biological laboratory: naturalist and experimentalist.

IN the early 1910s, researchers at the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts, might have wondered why a colleague, Thomas Hunt Morgan ( Figure 1 ), began shipping fruit flies from his Columbia University lab to the MBL each summer. After all, the Woods Hole currents supplied the MBL with a rich variety of marine organisms and Morgan, an avid practitioner of experimental embryology, made good use of them.

T. H. Morgan in 1920. This portrait of Morgan was taken by A. F. Huettner. Courtesy of MBL Archives.

Yet those who knew Morgan well would not have been surprised by his insect stocks. A keen naturalist, Morgan studied a veritable menagerie of experimental animals—many of them collected in Woods Hole—as a student and later researcher at the MBL from 1890 to 1942. Moreover, Morgan always had a diversity of investigations going on simultaneously. “This was the way Morgan worked: he wasn't happy unless he had a lot of different irons in the fire at the same time,” wrote A. H. Sturtevant, Morgan's long-term collaborator ( S turtevant 2001 , pp. 4–5). In Morgan's first 3 decades at the MBL, for instance, he studied at least 15 different species, including the now-famous fruit fly, while investigating a variety of problems related to his central interests in development and heredity ( M organ 1888–1937 ; M arine B iological L aboratory 1909 ).

Morgan was also a vocal proponent of experimentalism, and at the MBL he (quite successfully) joined with Jacques Loeb in arguing for a quantitative, predictive foundation for biological studies ( A llen 1969 ). Morgan was interested only in scientific problems that could be experimentally tested. Deeply wary of ungrounded hypotheses, he sought not overarching theories, but experimental methods that would allow him to identify proximate causes. This stance would triumph in Morgan's work with the fruit fly, Drosophila melanogaster . Morgan initially began breeding this animal in his search for an experimental approach to evolution: he was testing an alternative to the theory of natural selection, which he felt was insufficient to explain the origin of new species. But when a sex-linked mutation appeared in his Columbia University stocks in 1910, Morgan's attention was diverted to analyzing the material basis of sex determination and inheritance. By 1912, he and his colleagues were mapping the location of genes on chromosomes. These epoch-making studies launched the field of experimental genetics.

Morgan's penchant for maintaining multiple, diverse lines of investigation paid off in important ways, as this review of his work at the MBL up through the mid-1920s shows. First, Morgan was able to synthesize his research on many different organisms in his book Regeneration ( M organ 1901 ), which today provides a useful and insightful perspective on regenerative medicine. Second, evidence from originally distinct studies conceptually converged for Morgan. An example is his post-1910 work at the MBL on the insects phylloxeran and aphid, which confirmed his early Drosophila results on the relationship of the chromosomes to sex determination and inheritance.

Morgan's dual characteristics as a “naturalist and experimentalist” ( Figure 2 ) place him historically in an era when biology was transitioning from a descriptive and often speculative field to an experimental one ( A llen 1969 ). Yet they may indicate also why Morgan was a successful scientist, one who received the first Nobel Prize ever awarded in genetics in 1933 and became the first in a now-long list of Nobel Laureates affiliated with the MBL. Morgan's appreciation of natural diversity and his wide-ranging investigations, coupled with his skepticism toward a priori theories, could have left him flailing in a biological wilderness. What anchored him was his strict experimentalism, his insistence on choosing problems that could be analytically tested.

Naturalist, experimentalist, and trustee. This plaque in the lobby of Lillie Laboratory at the MBL commemorates T. H. Morgan's long-term and wide-ranging activities at the laboratory. Courtesy of Matthew Person.

EARLY YEARS: MARINE ORGANISMS, MORPHOLOGY, AND EXPERIMENTAL EMBRYOLOGY

In 1886, when T. H. Morgan was 20 years old and about to start graduate studies in zoology at Johns Hopkins, he attended the summer marine laboratory in Annisquam, Massachusetts, where he first learned how to collect and handle marine organisms for basic biological research. “Altogether, I am delighted with myself for being here and without doubt the work will be of the greatest assistance to me next winter,” he wrote to a friend ( A llen 1978 , p. 25). As it turned out, the Annisquam laboratory closed down after that summer, and its benefactors moved its glassware, apparatus, boats, furniture, and fixtures to Woods Hole, where they established the MBL in 1888 ( L illie 1944 ). When Morgan died in 1945, he was “the last surviving personal link” between the MBL and its predecessor at Annisquam, wrote Edwin G. Conklin of Princeton University, Morgan's close friend and 45-year colleague at the MBL ( C onklin 1947 , p. 14).

At Johns Hopkins, Morgan trained with embryologist W. K. Brooks, who promoted the use of marine organisms for studies of early development, as was then practiced at the Naples Zoological Station and other European marine laboratories. Through Brooks' arrangement, Morgan spent the summer of 1889 at the U.S. Fish Commission Laboratory in Woods Hole, and the following summer Morgan was one of 20 investigators at the nascent MBL, which had opened in 1888. During these two summers, Morgan collected and studied sea spiders for his doctoral research. Morgan, like Brooks, was then working within the paradigm of descriptive morphology; in his thesis, he sought to trace the phylogenetic relations of sea spiders with other arthropods by studying their embryological development. In 1891, after defending his thesis and accepting an assistant professorship at Bryn Mawr College, Morgan returned to the MBL and did so again for the next two summers.

Morgan's activities in those years are not much noted in descriptions of the MBL written by his contemporaries. Yet Morgan's profile in Woods Hole rose significantly after he spent 10 months at the Naples Zoological Station in 1894–95, carrying out research with the German embryologist Hans Driesch. Through Driesch, Morgan came into direct contact with the European school of experimental embryology that had begun in the 1870s with Wilhelm His, who developed methods for sectioning embryos and argued for a cleaving of the field from phylogenetic studies. Morgan had already been attracted to these new methods and this approach and had devised experiments on teleost and echinoderm eggs at the MBL in 1893. Other MBL investigators were interested, too. A few months before Morgan left for Naples, W. M. Wheeler translated Wilhelm Roux's manifesto for an experimental and mechanistic approach to embryology, or Entwicklungsmechanik , and presented it as a Friday Night Lecture at the MBL ( R oux 1895 ; M aienschein 1991 ).

Morgan's Naples experiments, which were designed to identify causal factors controlling development of the egg cell, made a singular impression on his Woods Hole contemporaries. Edmund B. Wilson, Morgan's longtime friend and colleague at Columbia University and at the MBL, described a “beautiful experiment” Morgan conducted in Naples in which he manipulated the relative position of frog blastomeres and gave “most conclusive evidence that each of the (first) two blastomeres contains all the materials, nuclear and cytoplasmic, necessary for the formation of a whole body, and that these materials may be used to build a whole body or half-body, according to the grouping that they assume” ( M organ 1895 ; W ilson 1897 , p. 319). In another experiment, Driesch and Morgan showed with ctenophore eggs that if part of the cytoplasm is removed, the remainder gives rise to incomplete larvae showing defects corresponding to the part removed ( D riesch and M organ 1895 ). “Thus the way was prepared for theories of organ-forming germ regions in the egg and later of ‘organ-forming substances,’” wrote Frank R. Lillie in his history of the MBL. “The chapter in experimental embryology that immediately follows from this is a long one, with important contributions from Woods Hole investigators,” particularly Wilson, Conklin, Lillie, and Morgan himself ( L illie 1944 , p. 128).

REGENERATION AND ARTIFICIAL PARTHENOGENESIS: FROM EARTHWORMS TO SEA URCHINS

After his Naples stay, Morgan next returned to the MBL as an investigator in 1897. At that point, he also became deeply involved in organizational matters at the MBL and was named a trustee, a position he would hold for the rest of his life ( C onklin 1947 ). Over the next five years, Morgan's research interests at the MBL would dovetail closely with those of Jacques Loeb, whom MBL director C. O. Whitman had recruited to establish a department of physiology at the MBL in 1894. Loeb had also been influenced by Driesch and was even more adamant than Morgan in his experimentalist, mechanistic approach to biology. Together, Morgan and Loeb waged battle in Woods Hole against the descriptive, phylogenetic tradition. “Loeb has been here [in Woods Hole] … all summer and I have learned to know him so much better,” Morgan wrote to Driesch in 1899. “We agree on so many fundamental views (and differ on these points from most of the people here) that we have become very good friends and strong allies. We have done battle with nearly all the other good morphologists and still survive their united assaults” ( A llen 1978 , p. 326).

The first line of research Loeb developed at the MBL was regeneration ( L illie 1944 ). Morgan, beginning in 1897, also pursued experiments on regeneration in a variety of organisms, including earthworms, hermit crabs, and teleost fish. By 1901 he had published more than 30 articles and a book on the topic, and much of this work was carried out at the MBL ( M arine B iological L aboratory 1909 ; M aienschein 1991 ). Morgan saw regeneration as essentially similar to normal development; he understood the value of using regenerative organisms to study both. One outcome of Morgan's extensive studies on regeneration was they led him to initially reject the Darwinian theory of natural selection, particularly the idea that adaptations have arisen due to their usefulness. How could the regenerative power have been slowly acquired through selection, Morgan argued, since it is useful to the animal only if the injured part entirely regenerates in a single generation? “The building up of the complete regeneration by slowly acquired steps, that cannot be decisive in the battle for existence, is not a process that can be explained by the theory (of natural selection),” he wrote ( M organ 1901 , p. 129). These considerations would lead Morgan to explore alternative mechanisms for the origin of species, which later led to his experimental use of fruit flies.

In 1899, Morgan was at the front lines of Jacques Loeb's spectacular discovery of artificial parthenogenesis at the MBL, which brought the lab much publicity ( L oeb 1899 ). Morgan reported in 1896 and in 1898 on the induction of artificial asters in sea urchin eggs by the use of hypertonic seawater and in 1899 on the effects of various salt solutions on unfertilized sea urchin eggs, finding that they caused irregular cell division ( M organ 1896 , 1898 , 1899 ). These latter experiments, carried out at the MBL, nearly “come to a complete anticipation of Jacques Loeb's famous discovery,” Lillie wrote ( L illie 1944 , p. 133). According to A. H. Sturtevant, later Morgan's long-term collaborator, there was “at the time a rather general feeling that Loeb had taken more credit than was due him for his discovery of artificial parthenogenesis,” and Morgan “clearly felt that Loeb had been secretive about his own work and had used every opportunity to find out just what Morgan was doing”; however, Morgan “was not as resentful as were some other members of the Woods Hole group on his behalf” ( S turtevant 1959 , p. 288). The artificial parthenogenesis episode seems not to have unduly troubled Morgan; he and Loeb remained friendly until Loeb's death in 1924.

SEX DETERMINATION: INSECT STUDIES

Beginning about 1906, Morgan also pursued studies of sex determination in insects at the MBL, focusing on phylloxerans and on aphids that he later reported collecting from bearberry plants at nearby Quissett, Massachusetts ( M organ 1915 ). The relationship of the chromosomes to sex determination was at that time a topic of vigorous investigation at the MBL, particularly by Thomas H. Montgomery, Edmund B. Wilson, and Nettie Stevens, a former student of Morgan's at Bryn Mawr ( M arine B iological L aboratory 1909 ). Parallel cytological studies by Wilson and Stevens in 1905 (not at the MBL) provided strong evidence that the X chromosome determined sex, but Morgan remained unconvinced, believing the cytoplasm and physiological development played a more important role.

The evolution of Morgan's thought on sex determination is apparent in his papers on the phylloxerans and aphids, both of which have a life-cycle phase in which parthenogenetic eggs produce both males and females. Morgan wanted to find out what in the egg determines what sex it will become. In 1906, Morgan reported finding no discernible difference between the chromosomes of the male-producing and the female-producing phylloxeran eggs, nor in their cytoplasm. In watching them develop, though, he noted one importance difference: “the precocious development of the relatively enormous reproductive organs of the male,” suggesting that “a pre-existing mass of cytoplasm from which the testis develops may be present in the egg.” This led him to suggest that “the immediate determination of the sex is a cytoplasmic phenomenon” ( M organ 1906 , p. 206). In 1908, he reported having discovered that somatic cells in the female phylloxeran have six chromosomes, while those in the male have only five; thus at some point in the parthenogenetic egg the ones that will become male lose a chromosome. He was still considering cytoplasmic influences: “It follows that the egg as well as the sperm has the power of determining sex by regulating the number of its chromosomes,” he wrote ( M organ 1908 , p. 57). In a 1909 study of both phylloxerans and aphids, Morgan concluded that the sex of the egg is determined to be male or female before any change in chromosome number. “Clearly, the egg as well as the sperm contains factors that determine sex,” he wrote ( M organ 1909 , p. 235).

Then, an event in Morgan's Drosophila research, which he had initiated in about 1908, catalyzed a distinct change of course in his thought. In May 1910, Morgan discovered a male fly with a white-eyed mutation in his Drosophila stocks at Columbia University. By June, he had done enough crosses to realize he had in his white-eyed fly “a splendid case of sex-limited inheritance,” as he wrote to a friend from Woods Hole ( S chwartz 2008 , p. 179). Morgan submitted his classic Science paper describing his new Drosophila results from Woods Hole, and it was published in July ( M organ 1910 ). Although Morgan described the expression of the white-eyed mutation in males only, it is noteworthy that he did not mention chromosomes in this paper. Yet Morgan soon found additional sex-linked traits in Drosophila, which he first publicly reported in a lecture at the MBL in July 1910. These findings would lead him to accept the chromosomal theory of sex determination, as well as chromosomes serving as the physical basis for Mendelian inheritance ( M organ 1911a ; A llen 1978 ; M aienschein 1984 ). In 1912, after his Drosophila discoveries and with new cytological evidence from the phylloxerans, Morgan unequivocally ascribed differences in the male and female parthenogenetic phylloxeran eggs to differences in their sex chromosomes. (Prior to this, he admitted in this paper, “the value of the chromosome hypothesis in sex determination” might have seemed to “hang in the balance.”) ( M organ 1912 , p. 479).

Sturtevant considered Morgan's phylloxeran and aphid work, which he continued to pursue in Woods Hole until 1915, as very important in confirming the chromosome hypothesis. “[Morgan] showed that the facts, which at first seemed quite inconsistent with the chromosome interpretation of sex determination, were in fact intelligible only in terms of that interpretation,” Sturtevant wrote. “This was one of Morgan's most brilliant achievements, involving great skill and patience in the collecting and care of the animals, insight in seeing what were the critical points of study, and ability to recognize and to follow up on unexpected facts. The results were of importance in serving to demonstrate the role of the chromosomes in sex determination, at a time when that importance was seriously questioned by many biologists” ( S turtevant 1959 , pp. 289–290).

MENDEL AND MUTATIONS: MICE AND FRUIT FLIES

The rediscovery of Gregor Mendel's work in 1900 brought questions of evolution and heredity to the fore in biological circles. Morgan began investigations of Mendelian inheritance in 1905, when he started breeding rats and mice ( K ohler 1994 ). In the summer of 1907, he caught a house mouse in Woods Hole with the “sport,” or mutation, of a pure white belly. “Later, I caught two more such mice and, in the same closet, another typical house mouse. In the neighborhood, I have caught a few other white-bellied mice,” Morgan reported (which summons a vision of an agile Morgan chasing mice around Woods Hole). Morgan then obtained white-bellied mice from Iowa and New York and began a series of breeding experiments crossing the wild sport with various domesticated races. “My intention was to familiarize myself at first hand with the process of Mendelian inheritance,” he wrote, by observing the varieties of coat color that his crosses produced ( M organ 1911b , p. 88).

At some point in the midst of this work, probably in 1908, Morgan started breeding Drosophila, not for Mendelian studies, but as a foray into experimental evolution. Morgan's studies of regeneration had led him to reject Darwin's concept of new species arising by natural selection of minute, random, continuous variations. Instead, Morgan entertained Hugo de Vries' concept that species evolved through discrete jumps, which de Vries called mutations ( A llen 1978 ). De Vries had predicted that, under certain conditions, animals can enter “mutating periods.” With Drosophila, Morgan wanted to see if he could induce such a mutating period through intensive inbreeding ( A llen 1975 ; K ohler 1994 ).

The first researchers to use Drosophila experimentally were William E. Castle and his students at Harvard University ( C astle et al . 1906 ) and Morgan was clearly influenced by this work. It is not certain who gave Morgan his first Drosophila stocks; but “certainly some of the early material was collected in grocery stores which existed then in Woods Hole,” Sturtevant wrote ( S turtevant 2001 , p. 3). Morgan himself in a letter to A. F. Blakeslee in 1935, responding to whether he had obtained his first stocks of flies from F. E. Lutz, wrote “…if so, I have forgotten it” ( C arlson 2004 , p. 168).

For many months, Morgan's inbreeding experiments with Drosophila turned up nothing, and when Ross Harrison visited Morgan's Columbia University lab in January 1910, Morgan referred to it as “two years' work wasted” ( K ohler 1994 , p. 41). Yet, soon after, a stream of mutants began appearing in his stocks, starting with an atypical thorax pattern. Overjoyed, Morgan at first thought he had succeeded in inducing a de Vriesian mutating period in the fly ( K ohler 1994 ). But soon the sex-limited Mendelian ratios Morgan observed for white-eye and other mutations drew his attention from his original focus on experimental evolution to an analysis of the chromosomes in sex determination and inheritance, despite his prior skepticism toward the chromosomal theory. As Castle later wrote, “Morgan was too good a scientist to hold a conclusion after he believed it had been clearly disproved” ( C arlson 2004 , p. 163).

Beginning in 1913 and continuing through the 1920s, Morgan brought members of his Columbia University “fly room,” particularly A. H. Sturtevant and C. B. Bridges, to the MBL each summer, where they carried out their Drosophila research in the Crane Building ( S turtevant 1959 ) ( Figure 3 ). “This did not mean any interruption in the Drosophila experiments,” Sturtevant later wrote. “All the cultures were loaded into barrels—big sugar barrels—shipped by express, and what you started in New York, you'd finish (in Woods Hole) and vice versa.” They traveled from New York by boat, lugging cages of chickens, pigeons, mice, and other animals Morgan was working on, and “when Morgan got to Woods Hole, he plunged deeply into work on marine forms, even while his work with Drosophila was actively going on,” Sturtevant wrote ( S turtevant 2001 , p. 4). From 1917 to 1924, for example, Morgan conducted an extended study of regeneration and “intersex” mutations in the fiddler crab at the MBL ( M organ 1920 , 1924 ). Also accompanying Morgan to Woods Hole since their marriage in 1904 was biologist Lilian Vaughan (Sampson) Morgan, who investigated amphibian breeding and development at the MBL in the 1890s and later transitioned into genetics. In 1913, Lilian Morgan cofounded what is now the Children's School of Science in Woods Hole ( K eenan 1983 ).

Morgan and friends: T. H. Morgan called this photo, taken at the MBL in the summer of 1919, “Solving the Problems of the Universe.” Clockwise from left: T. H. Morgan, Calvin Bridges (kneeling), Franz Schrader, E. E. Just, A. H. Sturtevant, and an unidentified person. Courtesy of MBL Archives.

Morgan's success with Drosophila in establishing a material basis for the Mendelian theory of inheritance was a triumph of the experimentalist approach, which would come to dominate biological research in the 20th century. Yet the diversity of Morgan's studies at the MBL over more than 50 years indicates he also appreciated the naturalist Louis Agassiz's dictum, which is still displayed in the MBL Library: “Study Nature, not Books.” In Morgan's time, the naturalist and the experimentalist traditions seemed to pose a dichotomy: descriptive vs . quantitative, holistic vs . reductionistic. Morgan did not choose either/or: he adopted, with great success, something of both.

Acknowledgments

The authors are grateful to Jane Maienschein and Garland E. Allen for helpful discussions about Morgan and for generously providing feedback and suggestions on the manuscript. The authors also thank Catherine N. Norton, library director, and Diane M. Rielinger, archivist, of the Marine Biological Laboratory Woods Hole Oceanographic Institution Library, Woods Hole, Massachusetts, for archival assistance.

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Thomas hunt morgan and his legacy, thomas h. morgan.

by Edward B. Lewis 1995 Nobel Laureate in Physiology or Medicine

Thomas Hunt Morgan was awarded the Nobel Prize in Physiology or Medicine in 1933. The work for which the prize was awarded was completed over a 17-year period at Columbia University, commencing in 1910 with his discovery of the white-eyed mutation in the fruit fly, Drosophila.

Morgan received his Ph. D. degree in 1890 at Johns Hopkins University. He then went to Europe and is said to have been much influenced by a stay at the Naples Marine Laboratory and contact there with A. Dohrn and H. Driesch. He learned the importance of pursuing an experimental, as opposed to descriptive, approach to studying biology and in particular embryology, which was his main interest early in his career. A useful account of Morgan’s life and works has been given by G. Allen (ref. 1).

Thomas Hunt Morgan with fly drawings.

Courtesy of the Caltech Archives. © California Institute of Technology. All rights reserved. Commercial use or modification of this material is prohibited.

In 1928 he moved with several of his group to Pasadena, where he joined the faculty of the California Institute of Technology (or Caltech) and became the first chairman of its Biology Division. What factors were responsible for the successes that Morgan and his students achieved at Columbia University and how did these factors carry over to the Caltech era first under Morgan’s, and later G.W. Beadle’s leadership? It is convenient to consider three time periods:

Morgan and the Columbia Period (1910 to 1928)

Morgan attracted extremely gifted students, in particular, A.H. Sturtevant, C.B. Bridges, and H.J. Muller (Nobel Laureate, 1946). They were to discover a host of new laws of genetics, while working in the “Fly Room,” in the Zoology Department at Columbia.

Throughout their careers Morgan and these students worked at the bench. The investigator must be on top of the research if he or she is to recognize unexpected findings when they occur. Sturtevant has stated that Morgan would often comment about experiments that led to quite unexpected results: “they [the flies] will fool you every time.”

Morgan attracted funding for his research from the Carnegie Institution of Washington. That organization recognized the basic research character of Morgan’s work and supported research staff members in Morgan’s group, such as C.B. Bridges and Morgan’s artist, Edith Wallace, who was also curator of stocks. The Carnegie grants required nothing more than an annual report from the investigators. Federal support had not yet started and although universities were able to finance costs associated with teaching they were usually unable to support basic research.

C. Bridges, P. Reed, T.H. Morgan, A.H. Sturtevant, E.M. Wallace.

During the Columbia period Morgan was clearly in his prime. His style of doing science must have been of paramount importance. He was not afraid to challenge existing dogma. He had become dissatisfied, even skeptical, of the formalistic treatment that genetics had taken in the period between the rediscovery of Mendelism in 1901 and 1909. He ridiculed explanations of breeding results that postulated more and more hereditary factors without any way of determining what those factors were. He wanted to know what the physical basis of such factors might be. At that time it was generally assumed that chromosomes could not be the carriers of the genetic information. He wanted a suitable animal and chose Drosophila, because of its short life cycle, ease of culturing and high fecundity. Also, large numbers of flies could be reared inexpensively — an important factor during this period when there were very few funds available to support basic research. Morgan was very thrifty when it came to purchasing laboratory equipment and supplies — but, according to Sturtevant, generous in providing financial help to his students. At the start of the work hand lenses were used. Only later did Bridges introduce stereoscopic microscopes. Bridges also devised a standard agar-based culture medium. Prior to that, flies were simply reared on bananas. In addition, Bridges built the basic collection of mutant stocks, mapped virtually all of the genes and later, at Caltech, drew the definitive maps of the salivary gland chromosomes. His enormous research output may in part be attributed to his being a staff member of the Carnegie Foundation with consequent freedom from teaching and other academic obligations.

Morgan’s first attempts to find tractable mutations to study were quite disappointing. Fortunately, he persevered and found the white-eyed fly 1 . This led to his discovery of sex-linked inheritance and soon with the discovery of a second sex-linked mutant, rudimentary, he discovered crossing over.

H. Sturtevant in the Drosophila stock room of the Kerckhoff Laboratories.

Sturtevant (ref. 2) has described how chromosomes finally came to be identified as the carriers of the hereditary material. In a conversation with Morgan in 1911 about the spatial relations of genes in the nucleus, Sturtevant, who was still an undergraduate, realized that the sex-linked factors might be arranged in a linear order. He writes that he went home and spent the night constructing a genetic map based on five sex-linked mutations that by then had been discovered. In 1912 Bridges and Sturtevant identified and mapped two groups of autosomal (not sex-linked) factors and a third such group was identified by Muller in 1914. The four linkage groups correlated nicely with the four pairs of chromosomes that Drosophila was known to possess. Proof that this correlation was not accidental came when Bridges used the results of irregular segregation of the sex chromosomes (or non-disjunction) to provide an elegant proof that the chromosomes are indeed the bearers of the hereditary factors or genes as they are now known. Bridges published this proof in 1916 in the first paper of volume I of the journal Genetics.

Sturtevant often commented on Morgan’s remarkable intuitive powers. Thus, Sturtevant describes how after explaining some puzzling results to Morgan, Morgan replied that it sounded like an inversion 2 . Sturtevant went on to provide critical evidence, purely from breeding results, that inversions do occur; it was only later that inversions were observed cytologically.

It seems clear that Morgan was not only a stimulating person but one who recognized good students, gave them freedom and space to work, and inspired them to make the leaps of imagination that are so important in advancing science.

Morgan and the Caltech Period (1928 to 1942)

Robert A. Millikan with cosmic ray equipment.

Morgan was invited by the astronomer, G.E. Hale, to chair a Biology Division at the California Institute of Technology (Caltech). Hale had conceived the idea of creating Caltech some years earlier and had already recruited R.A. Millikan (Nobel Laureate in Physics, 1923) and A.A. Noyes to head the Physics and Chemistry Divisions, respectively. According to Sturtevant, Morgan told his group at Columbia of Hale’s invitation and of how it was not possible to say no to Hale. Morgan accepted and came to Caltech in 1928. He brought with him Sturtevant, who came as a full professor, Bridges, and T. Dobzhansky, who later became a full professor. In addition to Sturtevant and Dobzhansky, the genetics faculty consisted of E.G. Anderson and S. Emerson. J. Schultz, who like Bridges was a staff member of the Carnegie Institution of Washington, participated in the teaching of an advanced laboratory course in genetics.

During this second period, many geneticists visited the Biology Division for varying periods of time. Those from foreign countries included D. Catcheside, B. Ephrussi, K. Mather, and J. Monod (1965 Nobel Laureate). Visiting professors included Muller and L.J. Stadler. B. McClintock (1983 Nobel Laureate) came as a National Research Fellow in the early 1930s.

Morgan was well known outside of the scientific community and attracted interesting people. Professor Norman Horowitz, who was a graduate student in the Biology Division during this period, tells me that he remembers Morgan giving a tour of the Biology Division to the well-known author, H.G. Wells.

Back (left to right): Wildman, Beadle, Lewis, Wiersma; standing: Keighley, Sturtevant, Went, Haagen-Smit, Mitchell, Van Harreveld, Alles, Anderson; seated (back row): Borsook, Emerson; (front row): Dubnoff, Bonner, Tyler, Horowitz.

J.R. Goodstein (ref. 3) has described how the Rockefeller Foundation and private donors provided financial support to the Biology and other Divisions during this period. Such assistance was essential at that time, since Caltech is a private institution and received no support from the state or the federal government.

Edward B. Lewis with Drosophila .

In the latter half of this period, Morgan returned to his interest in marine organisms and did not follow the newer developments in genetics. Instead it was largely Sturtevant who carried on the Morgan legacy as far as genetics was concerned. Sturtevant also allowed his graduate students considerable freedom to choose their thesis projects and to consult with him on those projects or indeed on any matter. I was fortunate to have been one such student, commencing in 1939. Sturtevant’s door was always open to students and faculty. I well remember Morgan coming to Sturtevant’s office to discuss matters affecting the Division.

Sturtevant told us that the award of the Nobel Prize to Morgan in 1933 was an important factor in elevating the prestige and status of the Biology Division at the Institute. At the time, the only other Nobel Laureate at Caltech was Millikan. From 1942 to 1946, the Division was managed by a committee chaired by Sturtevant.

Beadle and the Caltech Period (1946 to 1961)

Beadle and Pauling with molecular model.

In 1946, Sturtevant and Linus Pauling (who was awarded Nobel Prizes in Chemistry, 1954 , and Peace, 1962 ) persuaded Beadle, who was then Professor of Biology at Stanford University, to become chairman of the Biology Division. Beadle carried on the Morgan tradition of strongly supporting basic research and maintaining a stimulating intellectual atmosphere. During the early 1930s Beadle had been a National Research Fellow in the Division. He had collaborated with Sturtevant on a monumental study of inversions and together they wrote a textbook of genetics. He had collaborated also during that time with Sterling Emerson, and with E.G. Anderson. Beadle was clearly a part of the Morgan legacy.

Beadle in lab coat.

George Beadle and B. Ephrussi using microscopes.

Both photos courtesy of the Caltech Archives. © California Institute of Technology. All rights reserved. Commercial use or modification of this material is prohibited.

Beadle received the Nobel Prize in Physiology or Medicine in 1958 for work carried out at Stanford University on the biochemical genetics of the bread mold, Neurospora. In his biographical memoir on Beadle, Horowitz (ref. 4) describes how, while postdoctoral fellows in the Biology Division, Beadle and Ephrussi decided to pursue an early discovery by Sturtevant; namely, that a diffusible substance must be involved in the synthesis of the brown eye pigment of Drosophila. Sturtevant had shown that the vermilion eye color mutation is non-autonomously expressed in flies that are mosaic for the vermilion mutation and its wild-type allele. Beadle and Ephrussi designed at Caltech a set of experiments, involving transplantation of larval imaginal eye discs, to study the vermilion -plus hormone, as they called the diffusible substance. They carried out these experiments in Paris in Ephrussi’s laboratory. They were able to show that another eye color gene, cinnabar, lacks a cinnabar -plus substance and that the wild-type vermilion and cinnabar genes control sequential steps in a biochemical pathway leading to the brown eye pigment. Beadle correctly realized that the fungus Neurospora would provide better genetic material for exploring such pathways. Beadle and E. Tatum (co-winner with Beadle of the Nobel Prize) and colleagues at Stanford were then successful in dissecting the biochemical pathways that are involved in the synthesis of vitamins and many amino acids in that organism. The Neurospora findings opened a new era, now known as molecular genetics.

During Beadle’s tenure as chairman, N.H. Horowitz, H.K. Mitchell, R.D. Owen, and R.S. Edgar were added to the faculty in genetics. [I had come as an instructor in 1946 before Beadle had arrived]. Horowitz and Mitchell had been associated with Beadle at Stanford and played major roles in developing the one-gene one-enzyme hypothesis that led to the award of the Nobel Prize to Beadle and Tatum.

Beadle was responsible for persuading Delbrück to return to Caltech as a full professor. Delbrück had not been offered an appointment at Caltech after his tenure in the Division in the 1930s as a post-doctoral fellow and had taken a faculty position at Vanderbilt University. Other appointments during Beadle’s chairmanship that added strength in animal virology were R. Dulbecco (1975 Nobel Laureate), and M. Vogt. Howard Temin was one of Dulbecco’s graduate students and later a cowinner with Dulbecco of the Nobel Prize in 1975. R. Sperry (1981 Nobel Laureate) joined the faculty as a full professor in 1954 and continued his work on split brains that he had begun at the University of Chicago.

R. Dulbecco, G. Beadle, M. Delbrück and H. Temin.

Basic research gradually became well supported financially by Federal Agencies commencing with the Office of Naval Research, the Atomic Energy Commission and finally by the National Institutes of Health and the National Science Foundation. Such support was essential to obtain the personnel, equipment and supplies needed by the new fields of molecular and microbial genetics which flourished and indeed flowered during Beadle’s chairmanship.

During this third period there were many postdoctoral research fellows in the Biology Division, including S. Benzer (Crafoord Prize in 1993), who was a post-doctoral fellow in Delbrück’s group from 1949 to 1951, and was later recruited in 1967 as full professor. J. Weigle was a visiting professor and a valuable member of the Delbrück group. There were visits by F. Jacob (Nobel Laureate, 1965) and J. Watson (Nobel Laureate, 1962). B. McClintock returned in 1946 for a short visit, working with one of the graduate students, Jessie Singleton, perfecting a method of analyzing the chromosomes of Neurospora. Interestingly, R. Feynman , Caltech professor of physics (Nobel Laureate in Physics, 1965), spent part of an academic year working with R. Edgar and other members of the Delbrück group.

George Beadle at blackboard.

Beadle had remarkably versatile skills. He early abandoned his research on Neurospora in order to devote full time to being chairman. He was very successful in finding donors to endow postdoctoral fellowships and new buildings. The fellowships were often used to support visits by foreign scientists who otherwise would not have had been able to come to the USA. As in the previous period, teaching loads were kept light and much teaching was conducted in the form of seminars and journal clubs. The biology faculty was by and large a harmonious group and students were allowed considerable freedom to choose their professors. As one of a number of measures of the success of this atmosphere, the Nobel Prize in Physiology or Medicine was awarded to Professors Delbrück, Dulbecco and Sperry, as already noted, and in my case as well, for work carried out in the Division under the leadership of Beadle.

George Beadle and students.

1. Mutant and Wild Fly

2. Inversion

1. Allen, G., Thomas Hunt Morgan, pp. 1-447, Princeton University Press, Princeton, N.J. (1978). 2. Sturtevant, A.H., A History of Genetics, pp. 1-165, Harper and Rowe, New York (1965). 3. Goodstein, J.R., Millikan’s School, W. W. Norton and Co., New York. pp. 1-318 (1991). 4. Horowitz, N.H. Biographical Memoirs, vol. 59, pp. 26-52, National Academy Press, Washington, D. C. (1990).

First published 20 April 1998

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Thomas Hunt Morgan and the Chromosome Theory of Heredity

Thomas Hunt Morgan (1866-1945)

On September 25 , 1866 ,  American evolutionary biologist , geneticist , embryologist , and science author Thomas Hunt Morgan was born. He is famous for his experimental research with the  fruit fly by which he established the chromosome theory of heredity . Thomas Hunt Morgan was awarded the  Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role that the chromosome plays in heredity .

“Except for the rare cases of plastid inheritance, the inheritance of all known cooacters can be sufficiently accounted for by the presence of genes in the chromosomes. In a word the cytoplasm may be ignored genetically.” — Thomas Hunt Morgan, ‘Genetics and the Physiology of Development’, The American Naturalist (1926),  60 , 491

Thomas Hunt Morgan – Early Years

Thomas Hunt Morgan was born in Lexington, Kentucky. He joined the State College of Kentucky, today known as the University of Kentucky. Morgan mostly studied natural science and worked with the U.S. Geological Survey during the summer. Morgen started his graduate studies at the recently founded Johns Hopkins University. Under William Keith Brooks, Morgan was able to complete his thesis on the embryology of sea spiders he collected at the Marine Biological Laboratory in Woods Hole, Massachusetts. In 1890, Morgan was awarded his Ph.D. from Johns Hopkins and was also awarded the Bruce Fellowship in Research. He used his scholarship to travel to Jamaica, the Bahamas and Europe where he conducted further research.

Morphological Research

Also in 1890, Thomas Morgan was appointed associate professor at Johns Hopkins’ sister school Bryn Mawr College. There he taught all morphology-related courses and studied sea acorns, ascidian worms and frogs. Through the years, Morgan got enthusiastic for experimental biology, influenced by the German biologist Hans Driesch in Naples. Back then, there was a considerable scientific debate over the question of how an embryo developed. Basically, the two sides evolved around Wilhelm Roux who believed that hereditary material was divided among embryonic cells, which were predestined to form particular parts of a mature organism, and Hans Driesch who (among his followers) thought that development was due to epigenetic factors, where interactions between the protoplasm and the nucleus of the egg and the environment could affect development. Morgan collaborated with Driesch and they demonstrated that blastomeres isolated from sea urchin and ctenophore eggs could develop into complete larvae, contrary to the predictions of Roux’s supporters. Further, Thomas Morgan was able to show that sea urchin eggs could be induced to divide without fertilization by adding magnesium chloride.

Regeneration

Thomas Morgan returned to Bryn Mawr in 1895 and was appointed full professor upon his arrival. His first book ‘ The Development of the Frog’s Egg’ was published two years later. He further started a series of studies on different organisms’ ability to regenerate, which he published in 1901 with the title ‘ Regeneration ‘.

In a typical Drosophila genetics experiment, male and female flies with known phenotypes are put in a jar to mate; females must be virgins. Eggs are laid in porridge which the larva feed on; when the life cycle is complete, the progeny are scored for inheritance of the trait of interest., image: cudmore, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons

Evolutionary Theories

Morgan joined Columbia University in 1904 in order to focus fully on his experimental work. His research shifted more and more towards the mechanisms of heredity and evolution. Like many biologists back then, he did see evidence for biological evolution but rejected Darwin’s proposed mechanism of natural selection acting on small, constantly produced variations. However, while Morgan was skeptical of natural selection for many years, his theories of heredity and variation were radically transformed through his conversion to Mendelism. Around 1900, Carl Correns ,[ 6 ]  Erich von Tschermak and Hugo De Vries rediscovered Gregor Mendel ‘s work and with it the foundation of genetics.[ 4 ] As Morgan had dismissed both evolutionary theories, he was seeking to prove De Vries’ mutation theory with his experimental heredity work.

Working with the Fruit Fly

Just like C. W. Woodworth and William E. Castle , Thomas Morgan started to work on the fruit fly Drosophila melanogaster around 1908. Together with Fernandus Payne , he mutated Drosophila through physical, chemical, and radiational means and started cross-breeding experiments to find heritable mutations. After no significant finding during their two years of work, a series of heritable mutants appeared, some of which displayed Mendelian inheritance patterns. For instance, Morgan noticed a white-eyed mutant male among the red-eyed wild types. When white-eyed flies were bred with a red-eyed female, their progeny were all red-eyed. A second generation cross produced white-eyed males — a sex-linked recessive trait, the gene for which Morgan named white. He also found a pink-eyed mutant that showed a different pattern of inheritance. In 1911, Morgan published a paper concluding that some traits were sex-linked, the trait was probably carried on one of the sex chromosomes, and other genes were probably carried on specific chromosomes as well.

Genetic Linkage

Morgan and his students whom he had motivated to study flies as well counted the mutant characteristics of thousands of fruit flies and studied their inheritance. The observation of a miniature-wing mutant, which was also on the sex chromosome but sometimes sorted independently to the white-eye mutation, led Morgan to the idea of genetic linkage and to hypothesize the phenomenon of crossing over. Morgan proposed that the amount of crossing over between linked genes differs and that crossover frequency might indicate the distance separating genes on the chromosome.

Mendelian Chromosome Theory

During the following years more and more biologists accepted the Mendelian chromosome theory, which was independently proposed by Walter Sutton and Theodor Boveri , and elaborated and expanded by Morgan and his students. However, the details of the increasingly complex theory, as well as the concept of the gene and its physical nature, were still controversial. Still, due to Thomas Morgan’s success on fruit flies, numerous labs across the globe took up fruit fly genetics and Columbia became the center of an informal exchange network, through which promising mutant Drosophila strains were transferred from lab to lab. Drosophila became one of the first, and for some time the most widely used, model organisms.

Inheritance of eye color in fruit flies according to Morgan

Later Years

During his later career, Morgan returned to embryology and worked to encourage the spread of genetics research to other organisms and the spread of the mechanistic experimental approach to all biological fields. He also became a critic of the growing eugenics movement, which adopted the ideas of genetics in support of racism. Thomas Morgan’s fly-room at Columbia became famous, and he found it easy to attract funding and visiting academics. In 1927 after 25 years at Columbia, and nearing the age of retirement, he received an offer from George Ellery Hale to establish a school of biology in California.[ 5 ] In 1919 he was elected a Foreign Member of the Royal Society, which awarded him the Darwin Medal in 1924 and the Copley Medal in 1939. From 1927 to 1931 he was president of the National Academy of Sciences, of which he had been a member since 1909. In 1928 Morgan was elected to the American Academy of Arts and Sciences. In the same year he was elected a foreign member of the Göttingen Academy of Sciences. In 1933 he received the Nobel Prize for Medicine . In 1935 he was accepted as a corresponding member of the Prussian Academy of Sciences. From 1923 he was Corresponding Member and from 1932 Honorary Member of the Soviet Academy of Sciences.

Thomas Hunt Morgan had throughout his life suffered with a chronic duodenal ulcer. In 1945, at age 79, he experienced a severe heart attack and died from a ruptured artery.

References and Further Reading:

• [1]  Thomas Hunt Morgan at nature
• [2]  Thomas Hunt Morgan at the Nobel Prize Foundation Webpage
• [3]  Thomas Hunt Morgan at Britannica Online
• [4]  Gregor Mendel and the Rules of Inheritance , SciHi blog
• [5]  George Ellery Hale – Large Telescopes and the Spectroheliograph , SciHi Blog
• [6]  Carl Correns and the Principles of Heredity , SciHi Blog
• [7] Thomas Hunt Morgan via Wikidata
• [8]  Thomas Hunt Morgan and fruit flies , 2016,  Khan Academy  @ youtube
• [9]  Kenney, D. E.; Borisy, G. G. (2009).  “Thomas Hunt Morgan at the Marine Biological Laboratory: Naturalist and Experimentalist” .  Genetics .  181  (3): 841–846.
• [10]  Morgan, Thomas Hunt; Alfred H. Sturtevant, H. J. Muller and C. B. Bridges (1915).  The Mechanism of Mendelian Heredity . New York: Henry Holt.
• [11]  Timeline for Thomas Hunt Morgan, via Wikidata

Tabea Tietz

Related posts, norman borlaug and the green revolution, sidney fox and his research for the origins of life, frederick william twort and the bacteriophages, wilhelm pfeffer – a pioneer of plant physiology, one comment.

Morgan was also in 1928, a Life Member of the Marine Biological Laboratory, a fraternal organization of scientists across the country. (It was located in Woods Hole, MA) along with George Papanicolaou, MD, PhD, who was a regular member of the MBL. Morgan quoted Papanicolaou’s PhD thesis in his 1913 “Heredity and Sex” book. pages 183-85. Morgan was responsible for opening the doors for Dr. Pap to get his first job at Cornell Medical College/NY Hospital in 1914. So in a sense, Morgan was a catalyst to the Pap Test for cervical cancer that Papanicolaou is know for. He (Pap) was born on May 13, 1883, a Friday. So much for being a unlucky day.

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Thomas Hunt Morgan, Genetic Recombination, and Gene Mapping

Sturtevant Uses Crossing-Over Data to Construct the First Genetic Map

Soon after Morgan presented his hypothesis, Alfred Henry Sturtevant, a 19-year-old Columbia University undergraduate who was working with Morgan, realized that if the frequency of crossing over was related to distance, one could use this information to map out the genes on a chromosome. After all, the farther apart two genes were on a chromosome, the more likely it was that these genes would separate during recombination. Therefore, as Sturtevant explained it, the "proportion of crossovers could be used as an index of the distance between any two factors" (Sturtevant, 1913). Collecting a stack of laboratory data, Sturtevant went home and spent most of the night drawing the first chromosomal linkage map for the genes located on the X chromosome of fruit flies (Weiner, 1999).

When creating his map, Sturtevant started by placing six X-linked genes in order. B was a gene for black body color. C was a gene that allowed color to appear in the eyes. Flies with the P gene had vermilion eyes instead of the ordinary red, and flies with two copies of the recessive O gene had eyes that appeared a shade known as eosin. The R and M factors both affected the wings. Sturtevant placed C and O at the same point because they were completely linked and were always inherited together — in other words, he never saw any evidence for recombination between C and O. Sturtevant then placed the remainder of the genes in the order shown in Figure 3 (Sturtevant, 1913). Crossover events were tracked by examining the F 2 progeny in crosses for "new" phenotypes.

For example, to find the distance between P (vermilion eyes) and M (long wings), Sturtevant performed crosses between flies that had long wings and vermilion eyes and flies that had small wings and red eyes. These crosses resulted in F 1 flies that either had long wings and red eyes or long wings and vermilion eyes. Sturtevant then crossed these two types of F 1 flies and analyzed the offspring for evidence of recombination. Unexpected phenotypes observed in the male F 2 progeny from this cross were then examined. (Because very little recombination occurs in the male germ line of Drosophila , only the female F 1 chromosomes are considered for predicting phenotypes [Figure 4].) Sturtevant noted four classes of male flies in this F 2 generation, as shown in Table 1.

The two additional classes of flies that appeared in this generation (long wings with red eyes and rudimentary wings with vermilion eyes) could only be explained by recombination occurring in the female germ line.

Sturtevant then worked out the order and the linear distances between these linked genes , thus forming a linkage map. In doing so, he computed the distance in an arbitrary unit he called the "map unit," which represented a recombination frequency of 0.01, or 1%. Later, the map unit was renamed the centimorgan (cM), in honor of Thomas Hunt Morgan, and it is still used today as the unit of measurement of distances along chromosomes.

In addition to describing the order of the genes on the X chromosome of fruit flies, Sturtevant's 1913 paper elucidated a number of other interesting points, including the following:

• The relationship between crossing over and genetic map distance
• The effects of multiple crossover events
• The fact that a first crossover can inhibit a second crossover (a phenomenon called interference , which is described later in this article)

Mapping Genes Using Recombination Frequency

To better understand how Sturtevant arrived as his results, let's take a closer look at the process he followed. In Figure 5, the gray-eosin and yellow-red flies are the parental lines, and all the alleles for these traits are linked on the X chromosome. Therefore, any gray-red or yellow-eosin male offspring are recombinants. As you can see, two recombinants result from the cross. We count only the male progeny because the males have one X chromosome and dominance will not obscure any phenotypes (Robbins, 2000). Of course, crossing over can occur only in the female fruit flies, which have two X chromosomes. Thus, in this cross, the female F 1 gametes provide the parental and recombinant gametes that we observe in the F 2 progeny.

In order to calculate the recombination frequency we use the following formula:

Substituting the values from our data set, we arrive at the following:

Deviations from Expected Results Revealed Genetic Interference

In short, Sturtevant realized that double recombination events could occur if genes were far apart. Moreover, not only did Sturtevant's data suggest that double-crossing over occurred, but it also suggested that an initial crossover event could inhibit subsequent events by way of a phenomenon Sturtevant referred to as interference.

To understand how Sturtevant arrived at this conclusion, take a look at the data shown in Figure 6 (Sturtevant, 1913). As you can see, Sturtevant examined recombination events between B (body color), CO (two eye color genes that were closely linked), and R (rudimentary wings), and compared the frequencies of crossover events. When B and CO did not separate, Sturtevant noticed that the "gametic ratio," or presence, of CO/R recombinants was approximately 1:2 (3,454:6,972). However, when a crossover between B/CO (N = 60) occurred, there was a much lower likelihood (approximately 1:6.5) of a crossover between CO/R (N = 9). This finding is indicative of interference.

Interference phenomena are still being studied today, and research has shown that interference can act over extremely large distances of the genome. For example, Kenneth J. Hillers and Anne M. Villeneuve recently demonstrated that in Caenorhabditis elegans , interference can actually occur over half the genome of the organism . They demonstrated this by fusing multiple chromosomes together and observing that crossovers still occurred a single time (Hillers & Villeneuve, 2003).

Complete and Incomplete Linkage

When Sturtevant drew his chromosomal map, he placed the C and O genes at the same location because they were always inherited together (Figure 3; Sturtevant, 1913). Genes that are so close together on a chromosome that they are always inherited as a single unit show a relationship referred to as complete linkage . In fact, two genes that are completely linked can only be differentiated as separate genes when a mutation occurs in one of them. There is no other way to identify genes with complete linkage from single genes that show multiple phenotypes .

On the other hand, the phenomenon known as incomplete linkage occurs when two genes show linkage with a recombination level greater than 0% and less than 50%. In incomplete linkage, all expected types of gametes are formed, but the recombinant gametes occur less often than the parental gametes.

In addition, if two genes are on the same chromosome and are far enough apart that they undergo recombination at least 50% of the time, the genes are independently assorting and do not show linkage. Genes independently assort at a distance of 50 cM or more apart. This means that no statistical test would allow researchers to measure linkage.

Finally, linked genes that do not independently assort show statistical linkage . Statistical linkage is detected as deviation from independent assortment that favors the parental gametes. Syntenic genes are genes that are physically located on the same chromosome, whether or not the genes themselves exhibit linkage (Passarge et al. , 1999). Therefore, all linked genes are syntenic, but not all syntenic genes show genetic linkage.

References and Recommended Reading

Blixt, S. Why didn't Gregor Mendel find linkage? Nature 256 , 206 (1975) doi:10.1038/256206a0 ( link to article )

Bridges, C. B. Salivary chromosome maps with a key to the banding of the chromosomes of Drosophila melanogaster . Journal of Heredity 26 , 60–64 (1935)

———. A revised map of the salivary gland X chromosome. Journal of Heredity 29 , 1113 (1938)

Hillers, K., & Villeneuve, A. Chromosome-wide control of meiotic crossing over in C. elegans . Current Biology 13 , 1641–1647 (2003) doi:10.1016/j.cub.2003.08.026

Morgan, T. H. Random segregation versus coupling in Mendelian inheritance. Science 34 , 384 (1911)

Passarge, E., et al . Incorrect use of the term "synteny." Nature Genetics 23 , 387 (1999) ( link to article )

Pierce, B. Genetics: A Conceptual Approach. (New York, W. H. Freeman & Co., 2005)

Punnett, R. C. Linkage in the sweet pea ( Lathyrus odoratus ). Journal of Genetics 13 , 101–123 (1923)

———. Linkage groups and chromosome number in Lathyrus . Proceedings of the Royal Society of London: Series B, Containing Papers of a Biological Character 102 236–238. (1927)

Robbins, R. J. Introduction to sex-limited inheritance in Drosophila . Electronic Scholarly Publishing Foundations of Classical Genetics Project. http://www.esp.org/foundations/genetics/classical/thm-10a.pdf (2000) (accessed May 19, 2008)

Sturtevant, A. H. The linear arrangement of six sex-linked factors in Drosophila , as shown by their mode of association. Journal of Experimental Zoology 14 , 43–59 (1913)

Weiner, J. Time, Love, Memory: A Great Biologist and His Quest for the Origins of Behavior (New York, Random House, 1999)

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Morgan’s Work on Drosophila | Genetics

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In the early decades of the twentieth century T.H. Morgan and his associates A.H. Sturtevant, H.J. Muller, C.B. Bridges and a few others at Columbia University, New York were actively engaged in studies on Chromosome Theory of Heredity with a very suitable material Drosophila.

From their studies on mutants in Drosophila they could assign several genes to chromosomes. For their extensive researches on Drosophila, T.H. Morgan became the first geneticist to be awarded Nobel Prize in Medicine in 1934.

Some of the experiments performed in Morgan’s laboratory indicated linkage because the genes did not assort independently. In Drosophila the normal fly has grey body and long wings. There are some recessive mutations due to which the body colour of the fly is black and the wings are underdeveloped or vestigeal.

It was proposed by Bridges that in linkage experiments the wild normal type allele be represented by the sign +, and mutants recessive to wild type by abbreviated initials in small letters such as b for black, and v for vestigial. Since linked genes are present on the same chromosome, they are represented by their symbols above and below a horizontal line, genes on one homologue being above the line, those on the other below the line.

Bridges crossed a wild type female fly with grey body and long wings with a male having black body and vestigial wings. The F 1 was wild type with grey body and long wings. The females from F 1 were testcrossed with double recessive males i.e. black vestigial.

Now if the genes are assorting independently, the test cross would yield all four combinations in the ratio of 1: 1: 1: 1. But the actual results showed that the parental types (grey long and black vestigial) were more and the recombination’s (grey vestigial and black long) were less than expected (Fig. 8.2).

Obviously the genes were linked. One more striking feature was revealed by these experiments, that is, the absence of crossover in the Drosophila male. Due to this when a male fly was used as a double recessive parent in a testcross, there were recombinants present in the F 2 progeny.

Reciprocal testcrosses where female flies were employed as double recessives, recombinants were absent in the F 2 progeny. The crosses provide excellent illustrations of linkage and absence of crossing over in male Drosophila. The female silk moth (Bombyx) is another example where crossing over does not take place.

The Drosophila school performed numerous experiments in their efforts to understand the chromosome theory of heredity. They had accumulated a large number of mutants for various characters. It was possible to isolate stocks of flies carrying two mutant genes for different characters on the same chromosome.

Interestingly, they also found some hereditary characters which were linked to the sex chromosome X in females. Characters such as white eyes, yellow body, miniature wings and a few others were therefore said to be sex-linked.

In all D. melanogaster was found to have four linked groups of genes. Drosophila has four pairs of chromosomes (Fig. 8.3). Each linkage group in Drosophila was associated with one pair of chromosomes and the linkage groups were numbered 1, 2, 3 and 4.

Further, the number of genes in each linkage group was found to be proportional to the size of chromosomes. Thus, the tiniest dot-like chromosomes are found to carry the smallest linkage group of only about 12 genes whereas the larger chromosomes each had more than 150 linked genes.

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Article Contents

Early years: marine organisms, morphology, and experimental embryology, regeneration and artificial parthenogenesis: from earthworms to sea urchins, sex determination: insect studies, mendel and mutations: mice and fruit flies, acknowledgements.

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Thomas Hunt Morgan at the Marine Biological Laboratory: Naturalist and Experimentalist

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Diana E Kenney, Gary G Borisy, Thomas Hunt Morgan at the Marine Biological Laboratory: Naturalist and Experimentalist, Genetics , Volume 181, Issue 3, 1 March 2009, Pages 841–846, https://doi.org/10.1534/genetics.109.101659

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Anecdotal, Historical and Critical Commentaries on Genetics

IN the early 1910s, researchers at the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts, might have wondered why a colleague, Thomas Hunt Morgan ( Figure 1 ), began shipping fruit flies from his Columbia University lab to the MBL each summer. After all, the Woods Hole currents supplied the MBL with a rich variety of marine organisms and Morgan, an avid practitioner of experimental embryology, made good use of them.

T. H. Morgan in 1920. This portrait of Morgan was taken by A. F. Huettner. Courtesy of MBL Archives.

Yet those who knew Morgan well would not have been surprised by his insect stocks. A keen naturalist, Morgan studied a veritable menagerie of experimental animals—many of them collected in Woods Hole—as a student and later researcher at the MBL from 1890 to 1942. Moreover, Morgan always had a diversity of investigations going on simultaneously. “This was the way Morgan worked: he wasn't happy unless he had a lot of different irons in the fire at the same time,” wrote A. H. Sturtevant, Morgan's long-term collaborator ( Sturtevant 2001 , pp. 4–5). In Morgan's first 3 decades at the MBL, for instance, he studied at least 15 different species, including the now-famous fruit fly, while investigating a variety of problems related to his central interests in development and heredity ( Morgan 1888–1937 ; Marine   Biological   Laboratory 1909 ).

Morgan was also a vocal proponent of experimentalism, and at the MBL he (quite successfully) joined with Jacques Loeb in arguing for a quantitative, predictive foundation for biological studies ( Allen 1969 ). Morgan was interested only in scientific problems that could be experimentally tested. Deeply wary of ungrounded hypotheses, he sought not overarching theories, but experimental methods that would allow him to identify proximate causes. This stance would triumph in Morgan's work with the fruit fly, Drosophila melanogaster . Morgan initially began breeding this animal in his search for an experimental approach to evolution: he was testing an alternative to the theory of natural selection, which he felt was insufficient to explain the origin of new species. But when a sex-linked mutation appeared in his Columbia University stocks in 1910, Morgan's attention was diverted to analyzing the material basis of sex determination and inheritance. By 1912, he and his colleagues were mapping the location of genes on chromosomes. These epoch-making studies launched the field of experimental genetics.

Morgan's penchant for maintaining multiple, diverse lines of investigation paid off in important ways, as this review of his work at the MBL up through the mid-1920s shows. First, Morgan was able to synthesize his research on many different organisms in his book Regeneration ( Morgan 1901 ), which today provides a useful and insightful perspective on regenerative medicine. Second, evidence from originally distinct studies conceptually converged for Morgan. An example is his post-1910 work at the MBL on the insects phylloxeran and aphid, which confirmed his early Drosophila results on the relationship of the chromosomes to sex determination and inheritance.

Morgan's dual characteristics as a “naturalist and experimentalist” ( Figure 2 ) place him historically in an era when biology was transitioning from a descriptive and often speculative field to an experimental one ( Allen 1969 ). Yet they may indicate also why Morgan was a successful scientist, one who received the first Nobel Prize ever awarded in genetics in 1933 and became the first in a now-long list of Nobel Laureates affiliated with the MBL. Morgan's appreciation of natural diversity and his wide-ranging investigations, coupled with his skepticism toward a priori theories, could have left him flailing in a biological wilderness. What anchored him was his strict experimentalism, his insistence on choosing problems that could be analytically tested.

Naturalist, experimentalist, and trustee. This plaque in the lobby of Lillie Laboratory at the MBL commemorates T. H. Morgan's long-term and wide-ranging activities at the laboratory. Courtesy of Matthew Person.

In 1886, when T. H. Morgan was 20 years old and about to start graduate studies in zoology at Johns Hopkins, he attended the summer marine laboratory in Annisquam, Massachusetts, where he first learned how to collect and handle marine organisms for basic biological research. “Altogether, I am delighted with myself for being here and without doubt the work will be of the greatest assistance to me next winter,” he wrote to a friend ( Allen 1978 , p. 25). As it turned out, the Annisquam laboratory closed down after that summer, and its benefactors moved its glassware, apparatus, boats, furniture, and fixtures to Woods Hole, where they established the MBL in 1888 ( Lillie 1944 ). When Morgan died in 1945, he was “the last surviving personal link” between the MBL and its predecessor at Annisquam, wrote Edwin G. Conklin of Princeton University, Morgan's close friend and 45-year colleague at the MBL ( Conklin 1947 , p. 14).

At Johns Hopkins, Morgan trained with embryologist W. K. Brooks, who promoted the use of marine organisms for studies of early development, as was then practiced at the Naples Zoological Station and other European marine laboratories. Through Brooks' arrangement, Morgan spent the summer of 1889 at the U.S. Fish Commission Laboratory in Woods Hole, and the following summer Morgan was one of 20 investigators at the nascent MBL, which had opened in 1888. During these two summers, Morgan collected and studied sea spiders for his doctoral research. Morgan, like Brooks, was then working within the paradigm of descriptive morphology; in his thesis, he sought to trace the phylogenetic relations of sea spiders with other arthropods by studying their embryological development. In 1891, after defending his thesis and accepting an assistant professorship at Bryn Mawr College, Morgan returned to the MBL and did so again for the next two summers.

Morgan's activities in those years are not much noted in descriptions of the MBL written by his contemporaries. Yet Morgan's profile in Woods Hole rose significantly after he spent 10 months at the Naples Zoological Station in 1894–95, carrying out research with the German embryologist Hans Driesch. Through Driesch, Morgan came into direct contact with the European school of experimental embryology that had begun in the 1870s with Wilhelm His, who developed methods for sectioning embryos and argued for a cleaving of the field from phylogenetic studies. Morgan had already been attracted to these new methods and this approach and had devised experiments on teleost and echinoderm eggs at the MBL in 1893. Other MBL investigators were interested, too. A few months before Morgan left for Naples, W. M. Wheeler translated Wilhelm Roux's manifesto for an experimental and mechanistic approach to embryology, or Entwicklungsmechanik , and presented it as a Friday Night Lecture at the MBL ( Roux 1895 ; Maienschein 1991 ).

Morgan's Naples experiments, which were designed to identify causal factors controlling development of the egg cell, made a singular impression on his Woods Hole contemporaries. Edmund B. Wilson, Morgan's longtime friend and colleague at Columbia University and at the MBL, described a “beautiful experiment” Morgan conducted in Naples in which he manipulated the relative position of frog blastomeres and gave “most conclusive evidence that each of the (first) two blastomeres contains all the materials, nuclear and cytoplasmic, necessary for the formation of a whole body, and that these materials may be used to build a whole body or half-body, according to the grouping that they assume” ( Morgan 1895 ; Wilson 1897 , p. 319). In another experiment, Driesch and Morgan showed with ctenophore eggs that if part of the cytoplasm is removed, the remainder gives rise to incomplete larvae showing defects corresponding to the part removed ( Driesch and Morgan 1895 ). “Thus the way was prepared for theories of organ-forming germ regions in the egg and later of ‘organ-forming substances,’” wrote Frank R. Lillie in his history of the MBL. “The chapter in experimental embryology that immediately follows from this is a long one, with important contributions from Woods Hole investigators,” particularly Wilson, Conklin, Lillie, and Morgan himself ( Lillie 1944 , p. 128).

After his Naples stay, Morgan next returned to the MBL as an investigator in 1897. At that point, he also became deeply involved in organizational matters at the MBL and was named a trustee, a position he would hold for the rest of his life ( Conklin 1947 ). Over the next five years, Morgan's research interests at the MBL would dovetail closely with those of Jacques Loeb, whom MBL director C. O. Whitman had recruited to establish a department of physiology at the MBL in 1894. Loeb had also been influenced by Driesch and was even more adamant than Morgan in his experimentalist, mechanistic approach to biology. Together, Morgan and Loeb waged battle in Woods Hole against the descriptive, phylogenetic tradition. “Loeb has been here [in Woods Hole] … all summer and I have learned to know him so much better,” Morgan wrote to Driesch in 1899. “We agree on so many fundamental views (and differ on these points from most of the people here) that we have become very good friends and strong allies. We have done battle with nearly all the other good morphologists and still survive their united assaults” ( Allen 1978 , p. 326).

The first line of research Loeb developed at the MBL was regeneration ( Lillie 1944 ). Morgan, beginning in 1897, also pursued experiments on regeneration in a variety of organisms, including earthworms, hermit crabs, and teleost fish. By 1901 he had published more than 30 articles and a book on the topic, and much of this work was carried out at the MBL ( Marine   Biological   Laboratory 1909 ; Maienschein 1991 ). Morgan saw regeneration as essentially similar to normal development; he understood the value of using regenerative organisms to study both. One outcome of Morgan's extensive studies on regeneration was they led him to initially reject the Darwinian theory of natural selection, particularly the idea that adaptations have arisen due to their usefulness. How could the regenerative power have been slowly acquired through selection, Morgan argued, since it is useful to the animal only if the injured part entirely regenerates in a single generation? “The building up of the complete regeneration by slowly acquired steps, that cannot be decisive in the battle for existence, is not a process that can be explained by the theory (of natural selection),” he wrote ( Morgan 1901 , p. 129). These considerations would lead Morgan to explore alternative mechanisms for the origin of species, which later led to his experimental use of fruit flies.

In 1899, Morgan was at the front lines of Jacques Loeb's spectacular discovery of artificial parthenogenesis at the MBL, which brought the lab much publicity ( Loeb 1899 ). Morgan reported in 1896 and in 1898 on the induction of artificial asters in sea urchin eggs by the use of hypertonic seawater and in 1899 on the effects of various salt solutions on unfertilized sea urchin eggs, finding that they caused irregular cell division ( Morgan 1896 , 1898 , 1899 ). These latter experiments, carried out at the MBL, nearly “come to a complete anticipation of Jacques Loeb's famous discovery,” Lillie wrote ( Lillie 1944 , p. 133). According to A. H. Sturtevant, later Morgan's long-term collaborator, there was “at the time a rather general feeling that Loeb had taken more credit than was due him for his discovery of artificial parthenogenesis,” and Morgan “clearly felt that Loeb had been secretive about his own work and had used every opportunity to find out just what Morgan was doing”; however, Morgan “was not as resentful as were some other members of the Woods Hole group on his behalf” ( Sturtevant 1959 , p. 288). The artificial parthenogenesis episode seems not to have unduly troubled Morgan; he and Loeb remained friendly until Loeb's death in 1924.

Beginning about 1906, Morgan also pursued studies of sex determination in insects at the MBL, focusing on phylloxerans and on aphids that he later reported collecting from bearberry plants at nearby Quissett, Massachusetts ( Morgan 1915 ). The relationship of the chromosomes to sex determination was at that time a topic of vigorous investigation at the MBL, particularly by Thomas H. Montgomery, Edmund B. Wilson, and Nettie Stevens, a former student of Morgan's at Bryn Mawr ( Marine   Biological   Laboratory 1909 ). Parallel cytological studies by Wilson and Stevens in 1905 (not at the MBL) provided strong evidence that the X chromosome determined sex, but Morgan remained unconvinced, believing the cytoplasm and physiological development played a more important role.

The evolution of Morgan's thought on sex determination is apparent in his papers on the phylloxerans and aphids, both of which have a life-cycle phase in which parthenogenetic eggs produce both males and females. Morgan wanted to find out what in the egg determines what sex it will become. In 1906, Morgan reported finding no discernible difference between the chromosomes of the male-producing and the female-producing phylloxeran eggs, nor in their cytoplasm. In watching them develop, though, he noted one importance difference: “the precocious development of the relatively enormous reproductive organs of the male,” suggesting that “a pre-existing mass of cytoplasm from which the testis develops may be present in the egg.” This led him to suggest that “the immediate determination of the sex is a cytoplasmic phenomenon” ( Morgan 1906 , p. 206). In 1908, he reported having discovered that somatic cells in the female phylloxeran have six chromosomes, while those in the male have only five; thus at some point in the parthenogenetic egg the ones that will become male lose a chromosome. He was still considering cytoplasmic influences: “It follows that the egg as well as the sperm has the power of determining sex by regulating the number of its chromosomes,” he wrote ( Morgan 1908 , p. 57). In a 1909 study of both phylloxerans and aphids, Morgan concluded that the sex of the egg is determined to be male or female before any change in chromosome number. “Clearly, the egg as well as the sperm contains factors that determine sex,” he wrote ( Morgan 1909 , p. 235).

Then, an event in Morgan's Drosophila research, which he had initiated in about 1908, catalyzed a distinct change of course in his thought. In May 1910, Morgan discovered a male fly with a white-eyed mutation in his Drosophila stocks at Columbia University. By June, he had done enough crosses to realize he had in his white-eyed fly “a splendid case of sex-limited inheritance,” as he wrote to a friend from Woods Hole ( Schwartz 2008 , p. 179). Morgan submitted his classic Science paper describing his new Drosophila results from Woods Hole, and it was published in July ( Morgan 1910 ). Although Morgan described the expression of the white-eyed mutation in males only, it is noteworthy that he did not mention chromosomes in this paper. Yet Morgan soon found additional sex-linked traits in Drosophila, which he first publicly reported in a lecture at the MBL in July 1910. These findings would lead him to accept the chromosomal theory of sex determination, as well as chromosomes serving as the physical basis for Mendelian inheritance ( Morgan 1911a ; Allen 1978 ; Maienschein 1984 ). In 1912, after his Drosophila discoveries and with new cytological evidence from the phylloxerans, Morgan unequivocally ascribed differences in the male and female parthenogenetic phylloxeran eggs to differences in their sex chromosomes. (Prior to this, he admitted in this paper, “the value of the chromosome hypothesis in sex determination” might have seemed to “hang in the balance.”) ( Morgan 1912 , p. 479).

Sturtevant considered Morgan's phylloxeran and aphid work, which he continued to pursue in Woods Hole until 1915, as very important in confirming the chromosome hypothesis. “[Morgan] showed that the facts, which at first seemed quite inconsistent with the chromosome interpretation of sex determination, were in fact intelligible only in terms of that interpretation,” Sturtevant wrote. “This was one of Morgan's most brilliant achievements, involving great skill and patience in the collecting and care of the animals, insight in seeing what were the critical points of study, and ability to recognize and to follow up on unexpected facts. The results were of importance in serving to demonstrate the role of the chromosomes in sex determination, at a time when that importance was seriously questioned by many biologists” ( Sturtevant 1959 , pp. 289–290).

The rediscovery of Gregor Mendel's work in 1900 brought questions of evolution and heredity to the fore in biological circles. Morgan began investigations of Mendelian inheritance in 1905, when he started breeding rats and mice ( Kohler 1994 ). In the summer of 1907, he caught a house mouse in Woods Hole with the “sport,” or mutation, of a pure white belly. “Later, I caught two more such mice and, in the same closet, another typical house mouse. In the neighborhood, I have caught a few other white-bellied mice,” Morgan reported (which summons a vision of an agile Morgan chasing mice around Woods Hole). Morgan then obtained white-bellied mice from Iowa and New York and began a series of breeding experiments crossing the wild sport with various domesticated races. “My intention was to familiarize myself at first hand with the process of Mendelian inheritance,” he wrote, by observing the varieties of coat color that his crosses produced ( Morgan 1911b , p. 88).

At some point in the midst of this work, probably in 1908, Morgan started breeding Drosophila, not for Mendelian studies, but as a foray into experimental evolution. Morgan's studies of regeneration had led him to reject Darwin's concept of new species arising by natural selection of minute, random, continuous variations. Instead, Morgan entertained Hugo de Vries' concept that species evolved through discrete jumps, which de Vries called mutations ( Allen 1978 ). De Vries had predicted that, under certain conditions, animals can enter “mutating periods.” With Drosophila, Morgan wanted to see if he could induce such a mutating period through intensive inbreeding ( Allen 1975 ; Kohler 1994 ).

The first researchers to use Drosophila experimentally were William E. Castle and his students at Harvard University ( Castle   et al . 1906 ) and Morgan was clearly influenced by this work. It is not certain who gave Morgan his first Drosophila stocks; but “certainly some of the early material was collected in grocery stores which existed then in Woods Hole,” Sturtevant wrote ( Sturtevant 2001 , p. 3). Morgan himself in a letter to A. F. Blakeslee in 1935, responding to whether he had obtained his first stocks of flies from F. E. Lutz, wrote “…if so, I have forgotten it” ( Carlson 2004 , p. 168).

For many months, Morgan's inbreeding experiments with Drosophila turned up nothing, and when Ross Harrison visited Morgan's Columbia University lab in January 1910, Morgan referred to it as “two years' work wasted” ( Kohler 1994 , p. 41). Yet, soon after, a stream of mutants began appearing in his stocks, starting with an atypical thorax pattern. Overjoyed, Morgan at first thought he had succeeded in inducing a de Vriesian mutating period in the fly ( Kohler 1994 ). But soon the sex-limited Mendelian ratios Morgan observed for white-eye and other mutations drew his attention from his original focus on experimental evolution to an analysis of the chromosomes in sex determination and inheritance, despite his prior skepticism toward the chromosomal theory. As Castle later wrote, “Morgan was too good a scientist to hold a conclusion after he believed it had been clearly disproved” ( Carlson 2004 , p. 163).

Beginning in 1913 and continuing through the 1920s, Morgan brought members of his Columbia University “fly room,” particularly A. H. Sturtevant and C. B. Bridges, to the MBL each summer, where they carried out their Drosophila research in the Crane Building ( Sturtevant 1959 ) ( Figure 3 ). “This did not mean any interruption in the Drosophila experiments,” Sturtevant later wrote. “All the cultures were loaded into barrels—big sugar barrels—shipped by express, and what you started in New York, you'd finish (in Woods Hole) and vice versa.” They traveled from New York by boat, lugging cages of chickens, pigeons, mice, and other animals Morgan was working on, and “when Morgan got to Woods Hole, he plunged deeply into work on marine forms, even while his work with Drosophila was actively going on,” Sturtevant wrote ( Sturtevant 2001 , p. 4). From 1917 to 1924, for example, Morgan conducted an extended study of regeneration and “intersex” mutations in the fiddler crab at the MBL ( Morgan 1920 , 1924 ). Also accompanying Morgan to Woods Hole since their marriage in 1904 was biologist Lilian Vaughan (Sampson) Morgan, who investigated amphibian breeding and development at the MBL in the 1890s and later transitioned into genetics. In 1913, Lilian Morgan cofounded what is now the Children's School of Science in Woods Hole ( Keenan 1983 ).

Morgan and friends: T. H. Morgan called this photo, taken at the MBL in the summer of 1919, “Solving the Problems of the Universe.” Clockwise from left: T. H. Morgan, Calvin Bridges (kneeling), Franz Schrader, E. E. Just, A. H. Sturtevant, and an unidentified person. Courtesy of MBL Archives.

Morgan's success with Drosophila in establishing a material basis for the Mendelian theory of inheritance was a triumph of the experimentalist approach, which would come to dominate biological research in the 20th century. Yet the diversity of Morgan's studies at the MBL over more than 50 years indicates he also appreciated the naturalist Louis Agassiz's dictum, which is still displayed in the MBL Library: “Study Nature, not Books.” In Morgan's time, the naturalist and the experimentalist traditions seemed to pose a dichotomy: descriptive vs . quantitative, holistic vs . reductionistic. Morgan did not choose either/or: he adopted, with great success, something of both.

The authors are grateful to Jane Maienschein and Garland E. Allen for helpful discussions about Morgan and for generously providing feedback and suggestions on the manuscript. The authors also thank Catherine N. Norton, library director, and Diane M. Rielinger, archivist, of the Marine Biological Laboratory Woods Hole Oceanographic Institution Library, Woods Hole, Massachusetts, for archival assistance.

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