Professor Thomas Hunt Morgan, Geneticist

Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American evolutionary biologist, geneticist, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role that the chromosome plays in heredity. Photo credit: Unknown – http://wwwihm.nlm.nih.gov/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=549067; This image is one of several created for the 1891 Johns Hopkins yearbook of 1891.

 

Thomas Hunt Morgan received his Ph.D. from Johns Hopkins University in zoology in 1890. Following the rediscovery of Mendelian inheritance in 1900, Morgan began to study the genetic characteristics of the fruit fly Drosophila melanogaster. In his famous Fly Room at Columbia University, Morgan demonstrated that genes are carried on chromosomes and are the mechanical basis of heredity. These discoveries formed the basis of the modern science of genetics. As a result of his work, Drosophila became a major model organism in contemporary genetics. The Division of Biology which he established at the California Institute of Technology has produced seven Nobel Prize winners.

 

Morgan was born in Lexington, Kentucky, to Charlton Hunt Morgan and Ellen Key Howard Morgan. Part of a line of Southern planter elite 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. Beginning at age 16, Morgan attended the State College of Kentucky (now the University of Kentucky). He focused on science and particularly enjoyed natural history. He worked with the U.S. Geological Survey in his summers and graduated as valedictorian in 1886 with a BS degree. Following a summer at the Marine Biology School in Annisquam, Massachusetts, Morgan began graduate studies in zoology at Johns Hopkins University. After two years of experimental work with morphologist William Keith Brooks, Morgan received a master of science degree from the State College of Kentucky in 1888. 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, 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. He used the fellowship to travel to Jamaica, the Bahamas and to Europe to conduct further research. Nearly every summer from 1890 to 1942, Morgan returned to the Marine Biological Laboratory to conduct research. He became very involved in governance of the institution, including serving as an MBL trustee from 1897 to 1945.

 

In 1890, Morgan was appointed associate professor (and head of the biology department) at Johns Hopkins’ sister school Bryn Mawr College. 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. There 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 (commonly known as comb jellies, that live in marine waters worldwide. 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 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 and 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.

 

When Morgan returned to Bryn Mawr in 1895, he 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 (1897). He began a series of studies on different organisms’ ability to regenerate. He looked at grafting and regeneration in tadpoles, fish and earthworms; in 1901 he published his research as Regeneration. Beginning in 1900, Morgan started working on the problem of sex determination, which he had previously dismissed when Nettie Stevens discovered the impact of the Y chromosome on gender. He also continued to study the evolutionary problems that had been the focus of his earliest work. In 1904, E. B. Wilson invited Morgan to join him at Columbia University. This move freed him to focus fully on experimental work. When Morgan took the professorship in experimental zoology, he became increasingly focused on the mechanisms of heredity and evolution. He had published Evolution and Adaptation (1903); like many biologists at the time, he saw 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. 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. Around 1908 Morgan started working on the fruit fly Drosophila melanogaster, and encouraging students to do so as well. 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. With Fernandus Payne, he mutated Drosophila through physical, chemical, and radiational means. Morgan began cross-breeding experiments to find heritable mutations, but they had no significant success for two years. Castle had also had difficulty identifying mutations in Drosophila, which were tiny. 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. A second generation cross produced white-eyed males – a gender-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 gender-linked, the trait was probably carried on one of the Y or X chromosomes, and (3) other genes were probably carried on specific chromosomes as well. 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. The 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.

 

Morgan’s 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 1933 Morgan was awarded the Nobel Prize in Physiology or Medicine. 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.

 

Morgan 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. In 1945, at age 79, he experienced a severe heart attack and died from a ruptured artery.

 

Below is Thomas Hunt Morgan’s Drosophila melanogaster genetic linkage map. This was the first successful gene mapping work and provides important evidence for the chromosome theory of inheritance. The map shows the relative positions of allelic characteristics on the second Drosophila chromosome. The distance between the genes (map units) are equal to the percentage of crossing-over events that occurs between different alleles.

 

Thomas Hunt Morgan’s Drosophila melanogaster genetic linkage map. This was the first successful gene mapping work and provides important evidence for the Boveri-Sutton chromosome theory of inheritance. The map shows the relative positions of allelic characteristics on the second Drosophila chromosome. The distance between the genes (map units) are equal to the percentage of crossing-over events that occurs between different alleles. This gene linkage map shows the relative positions of allelic characteristics on the second Drosophila chromosome. The alleles on the chromosome form a linkage group due to their tendency to form together into gametes. The distance between the genes (map units) are equal to the percentage of crossing-over events that occurs between different alleles. This diagram is also based on the findings of Thomas Hunt Morgan in his Drosophila cross. Graphic credit: Twaanders17 – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=40694655

 

Source: https://www.ncbi.nlm.nih.gov; Wikipedia

 

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