Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American embryologist who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries relating the role the chromosome plays in heredity.

Morgan received his PhD from Johns Hopkins University in zoology in 1890 and researched embryology during his tenure at Bryn Mawr. Following the rediscovery of Mendelian inheritance in 1900, Morgan's research moved to the study of mutation in the fruit fly Drosophila melanogaster. In his famous Fly Room at Columbia University Morgan was able to demonstrate that genes are carried on chromosomes and are the mechanical basis of heredity. These discoveries formed the basis of the modern science of genetics. He was the first person to be awarded the Nobel Prize in Physiology or Medicine for his work in genetics. During his distinguished career, Morgan wrote 22 books and 370 scientific papers, and, as a result of his work, Drosophila became a major model organism in contemporary genetics. The Division of Biology he established at the California Institute of Technology produced seven Nobel Prize winners.

Morgan was born in Lexington, Kentucky, to Charlton Hunt Morgan and Ellen Key Howard Morgan. Part of a long line of Southern aristocracy on his father's side, Morgan was a nephew of Confederate General John Hunt Morgan and his great grandfather John Wesley Hunt had been the first millionaire west of the Allegheny Mountains. Through his mother, he was the great grandson of Francis Scott Key, the author of the "Star Spangled Banner", and John Eager Howard, governor and senator from Maryland. However, following the Civil War the family had fallen on harder times with the loss of civil and property rights for those who aided the Confederacy. His father also had difficulty finding work in politics and spent much of his time coordinating veterans reunions.

Beginning at age 16 in the Preparatory Department, Morgan attended the State College of Kentucky (now the University of Kentucky). There, he focused on science; he particularly enjoyed natural history, and worked with the U.S. Geological Survey in his summers. He graduated as valedictorian in 1886 with a bachelor in science. Following a summer at the Marine Biology School in Annisquam, Massachusetts, Morgan began graduate studies in zoology at the recently founded Johns Hopkins University, the first research oriented American university. After two years of experimental work with morphologist William Keith Brooks and several publications, Morgan was eligible to receive a master of science from the State College of Kentucky in 1888, the College required two years study at another institution and an examination by the College Faculty. The College offered Morgan a full professorship; however, he chose to stay at Johns Hopkins and was awarded a relatively large fellowship to help him fund his studies.

Under Brooks, Morgan completed his thesis work on the embryology of sea spiders — collected during the summers of 1889 and 1890 at the Marine Biological Laboratory in Woods Hole, Massachusetts — to determine their phylogenetic relationship with other arthropods. He concluded that with respect to embryology they were more closely related to spiders than crustaceans. Based on the publication of this work Morgan was awarded his Ph.D. from Johns Hopkins in 1890, and was also awarded the Bruce Fellowship in Research. With the fellowship he was able to travel to Jamaica, the Bahamas and to Europe to conduct further research.

In 1891, Morgan was appointed associate professor (and head of the biology department) at Johns Hopkins' sister school Bryn Mawr College, replacing his colleague E.B. Wilson. Morgan taught all morphology related courses, while the other member of the department, Jacques Loeb, taught the physiological courses; though Loeb only stayed for one year, it was the beginning of a lifelong friendship. Morgan lectured in biology 5 days a week, giving two lectures a day. He frequently included his own recent research in his lectures, and although he was an enthusiastic teacher, his true interests were in the laboratory. During the first few years at Bryn Mawr he produced descriptive studies of sea acorns, ascidian worms and frogs.

In 1894 Morgan was granted a year's absence to conduct research in the laboratories of Stazione Zoologica in Naples, where Wilson had worked two years earlier. At the laboratory in Naples he worked with German biologist Hans Driesch, whose research in the experimental study of development piqued Morgan's interest. Among other projects that year, Morgan completed an experimental study of ctenophore embryology. From his exposure in Naples and through Loeb to the Entwicklungsmechanik (roughly, "developmental mechanics") school of experimental biology — a reaction to the vitalistic Naturphilosophie that was exteremly influential in 19th century morphology — Morgan's work shifted from traditional, largely descriptive morphology to an experimental embryology that sought physical and chemical explanations for organismal development.

At the time there was considerable scientific debate over the question of how an embryo developed. Following Wilhelm Roux's mosaic theory of development, some believed that hereditary material was divided among embryonic cells, which were then predestined to form particular parts of a mature organism. Driesch and others 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 was in the latter camp; his work with Driesch demonstrated that blastomeres isolated from sea urchin and ctenophore eggs could develop into complete larvae, contrary to the predictions (and experimental evidence) of Roux's supporters. A related debate involved the role of epigenetic and environmental factors in development; on this front Morgan showed that sea urchin eggs could be induced to divide without fertilization by adding magnesium chloride, work which was continued by Jacques Loeb (who became well known for creating fatherless frogs using the method).

Morgan returned to Bryn Mawr in 1895 and was promoted to full professor. Morgan's main lines of experimental work involved regeneration and larval development; in each case, his goal was to distinguish internal and external causes to shed light on the Roux - Driesch debate. He wrote his first book, The Development of the Frog's Egg, published in 1897. He began a series of studies on different organisms ability to regenerate. He looked at grafting and regeneration in tadpoles, fish and earthworms and in 1901 this work was published as Regeneration. Beginning in 1900, he started working on the problem of sex determination; he also continued to study the evolutionary problems that had been the focus of his earliest work.

On June 4, 1904, Morgan married Lilian Vaughan Sampson (1870 - 1952), who had entered graduate school in biology at Bryn Mawr the same year Morgan joined the faculty; she put aside her scientific work in the early years of their marriage, but would later contribute significantly to Morgan's Drosophila work. Later in 1904, E.B. Wilson — still blazing the path for his younger friend — invited Morgan to join him at Columbia University, which at last freed him to focus fully on experimental work.
One of their four children (one boy and three girls) was Isabel Morgan (1911 – 1996) (marr. Mountain), who became a virologist at Johns Hopkins, specializing in polio research.

By 1904, when Morgan took a professorship in experimental zoology at Columbia University, he was becoming increasingly focused on the mechanisms of heredity and evolution. The previous year, he had published Evolution and Adaptation; like many biologists at that time, he saw clear evidence for biological evolution (as in the common descent of similar species) but rejected Darwin's proposed mechanism of natural selection acting on small, constantly produced variations. Extensive work in biometry seemed to indicate that continuous natural variation had distinct limits and did not represent heritable changes. Embryological development posed an additional problem in Morgan's view, as selection could not act on the early, incomplete stages of highly complex organs such as the eye. The common solution of the Lamarckian mechanism of inheritance of acquired characters, which featured prominently in Darwin's theory, was increasingly rejected by biologists. According to Morgan's biographer Garland Allen, he was also hindered by his views on taxonomy: he thought that species were entirely artificial creations that distorted the continuously variable range of real forms, while he held a "typological" view of larger taxa and could see no way that one such group could transform into another. But while he would remain skeptical of natural selection for many years, his theories of heredity and variation were radically transformed through his conversion to Mendelism.

In 1900 three scientists, Carl Correns, Erich von Tschermak and Hugo De Vries had rediscovered the work of Gregor Mendel, and with it the foundation of genetics. De Vries had gone on to propose that new species are created by mutation, bypassing the need for either Lamarckism or Darwinism. Morgan dismissed both of these evolutionary theories, and was actually seeking to prove Hugo De Vries' mutation theory with his experimental heredity work. He was initially quite skeptical of Mendel's laws of heredity (as well as the related chromosomal theory of sex determination), which were being considered as a possible basis for natural selection.

Following C.W. Woodworth and William E. Castle, around 1908 Morgan started working on the fruit fly Drosophila melanogaster, and encouraging students to do so as well. With Fernandus Payne, he mutated Drosophila through physical, chemical, and radiational means, and began cross-breeding experiments to find heritable mutations. However, they had no significant success for two years. Castle had also had difficulty identifying mutations in Drosophila, hardly unusual given the flies' tiny size. Finally in 1909, a series of heritable mutants appeared, some of which displayed Mendelian inheritance patterns; in 1910 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, while a second generation cross produced white eyed males — a sex-linked recessive trait, the gene for which Morgan named white. Morgan also discovered a pink eyed mutant that showed a different pattern of inheritance. In a paper published in Science in 1911, he concluded that (1) some traits were sex-linked, (2) the trait was probably carried on one of the sex chromosomes, and (3) other genes were probably carried on specific chromosomes as well.

Morgan and his students became more successful at finding mutant flies; they counted the mutant characteristics of thousands of fruit flies and studied their inheritance. As they accumulated multiple mutants, they combined them to study more complex inheritance patterns. 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. He relied on the discovery of the Belgian Professor Frans Alfons Janssens of the University of Leuven who described the phenomenon in 1909 and had called it 'chiasmatypie'. 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; later English geneticist J.B.S. Haldane suggested that the unit of measurement for linkage be called the morgan. Morgan's student Alfred Sturtevant developed the first genetic map in 1913.

In 1915 Morgan, Sturtevant, Calvin Bridges and H.J. Muller wrote the seminal book The Mechanism of Mendelian Heredity. Geneticist Curt Stern called the book "the fundamental textbook of the new genetics" and C.H. Waddington noted that "Morgan's theory of the chromosome represents a great leap of imagination comparable with Galileo or Newton". In the following years, most biologists came to accept the Mendelian chromosome theory, which was independently proposed by Walter Sutton and Theodor Boveri in 1902/1903 and elaborated and expanded by Morgan and his students. Garland Allen characterized the post 1915 period as one of normal science, in which "The activities of 'geneticists' were aimed at further elucidation of the details and implications of the Mendelian chromosome theory developed between 1910 and 1915." However, the details of the increasingly complex theory, as well as the very concept of the gene and its physical nature, were still controversial. Critics such as W.E. Castle pointed to contrary results in other organisms suggesting that genes interact with each other, while to Richard Goldschmidt and others, there was no compelling reason to view genes as discrete units residing on chromosomes.

Because of Morgan's dramatic success with Drosophila, many other labs throughout the world took up fruit fly genetics. 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. Morgan's group remained highly productive, but Morgan largely withdrew from doing fly work himself and gave his lab members considerable freedom in designing and carrying out their own experiments. Instead, Morgan returned to embryology and worked to encourage the spread of genetics research to other organisms and the spread of the mechanistic experimental approach (Enwicklungsmechanik) to all biological fields. After 1915, he also became a strong critic of the growing eugenics movement, which frequently co-opted the ideas of genetics in support of racism and worse.

John Hopkins awarded Morgan an honorary LL.D. and the University of Kentucky awarded him an honorary Ph.D. He was elected a member of the National Academy of Sciences and made a foreign member of the Royal Society. In 1924 Morgan received the Darwin Medal. His fly-room at Columbia became world 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.

Morgan moved to California to head the Division of Biology at the California Institute of Technology in 1928. In establishing the biology division, Morgan wanted to distinguish his program from those offered by Johns Hopkins and Columbia, with research focused on genetics and evolution; experimental embryology; physiology; biophysics and biochemistry. He was also instrumental in the establishment of the Marine Laboratory at Corona del Mar. He wanted to attract the best people to the Division at Caltech, so he took Bridges, Sturtevant, Jack Shultz and Albert Tyler from Columbia and took on Theodosius Dobzhansky as an international research fellow. More scientists came to work in the Division including George Beadle, Boris Ephrussi, Edward L. Tatum, Linus Pauling, Frits Went, and Sidney W. Fox.

In accordance with his reputation, Morgan held numerous prestigious positions in American science organizations. From 1927 to 1931 Morgan served as the President of the National Academy of Sciences; in 1930 he was the President of the American Association for the Advancement of Science; and in 1932 he chaired the Sixth International Congress of Genetics in Ithaca, New York. In 1933 Morgan was awarded the Nobel Prize in Physiology or Medicine; he had been nominated in 1919 and 1930 for the same work. As an acknowledgement of the group nature of his discovery he gave his prize money to Bridges', Sturtevant's and his own children. Morgan declined to attend the awards ceremony in 1933, instead attending in 1934. The 1933 rediscovery of the giant polytene chromosomes in the salivary gland of Drosophila may have influenced his choice. Until that point, the lab's results had been inferred from phenotypic results, the visible polytene chromosome enabled them to confirm their results on a physical basis. Morgan's Nobel acceptance speech entitled "The Contribution of Genetics to Physiology and Medicine" downplayed the contribution genetics could make to medicine beyond genetic counselling. In 1939 he was awarded the Copley Medal by the Royal Society.

He received two extensions of his contract at Caltech, but eventually retired in 1942, becoming professor and chairman emeritus. George Beadle returned to Caltech to replace Morgan as chairman of the department in 1946. Although he had retired, Morgan kept offices across the road from the Division and continued laboratory work. In his retirement he returned to the questions of sexual differentiation, regeneration, and embryology. Morgan had throughout his life suffered with a chronic duodenal ulcer, and in 1945 he experienced a severe heart attack and died from a ruptured artery.

Morgan was interested in evolution throughout his life. He wrote his thesis on the phylogeny of sea spiders (pycnogonids) and wrote 4 books about evolution. In Evolution and Adaptation (1903) he argued the anti-Darwinist position that selection never could produce wholly new species by acting on slight individual differences. He also rejected Darwin's theory of sexual selection and the Neo-Lamarckian theory of the inheritance of acquired characters. Morgan was not the only scientist attacking natural selection. The period 1875 - 1925 has been called 'The eclipse of Darwinism'. After discovering many small stable heritable mutations in Drosophila he gradually changed his mind. The relevance of mutations for evolution is that only characters that are inherited can have an effect in evolution. Since Morgan (1915) 'solved the problem of heredity', he was in a unique position to examine critically Darwin's theory of natural selection. In A Critique of the Theory of Evolution (1916) he discussed questions such as: Does selection play any role in evolution? How can selection produce anything new? Is selection no more than the elimination of the unfit? Is selection a creative force? After eliminating some misunderstandings and explaining in detail the new science of Mendelian heredity and its chromosomal basis, Morgan concludes that "the evidence shows clearly that the characters of wild animals and plants, as well as those of domesticated races, are inherited both in the wild and in domesticated forms according to the Mendel's Law". "Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the organism". Injurious mutations have practically no chance of becoming established. Far from rejecting evolution as the title of his 1916 book may suggest, Morgan not only laid the foundation of the science of genetics, but by doing that, he also laid the theoretical foundation for the mechanism of evolution: natural selection. Heredity was a central plank of Darwin's theory of natural selection, but Darwin could not provide a working theory of heredity. Therefore, Darwinism could not progress without a correct theory of genetics. Morgan created that foundation and that is why Morgan's work is so fundamental for the neo-Darwinian synthesis despite his criticism at the beginning of his career. Even so, much work on the Evolutionary Synthesis remained to be done.

Morgan left an important legacy in genetics. Some of Morgan's students from Columbia and Caltech went on to win their own Nobel Prizes, including George Wells Beadle and Hermann Joseph Muller. Nobel prize winner Eric Kandel has written of Morgan, "Much as Darwin's insights into the evolution of animal species first gave coherence to nineteenth century biology as a descriptive science, Morgan's findings about genes and their location on chromosomes helped transform biology into an experimental science."

The Thomas Hunt Morgan School of Biological Sciences at the University of Kentucky is named for Morgan. In Morgan's honor, the Genetics Society of America annually awards the Thomas Hunt Morgan Medal to one of its members who has made a significant contribution to the science of genetics. Thomas Hunt Morgan's discovery was illustrated on a 1989 stamp issued in Sweden, showing the discoveries of eight Nobel Prize winning geneticists. A junior high school in Shoreline, Washington, was named in Morgan's honor for the latter half of the 20th century.