Thomas Hunt Morgan

Geneticist

• Born: September 25, 1866
• Birthplace: Lexington, Kentucky
• Died: December 4, 1945
• Place of death: Pasadena, California

American geneticist

Nineteenth-century geneticist and embryologist Thomas Hunt Morgan developed the gene-chromosome theory of inheritance—the idea that genes located on chromosomes encode information inherited by the next generation. Morgan and his research team also confirmed the roles of the X and Y chromosomes in determining gender at fertilization.

Born: September 25, 1866; Lexington, Kentucky

Died: December 4, 1945; Pasadena, California

Primary field: Biology

Specialties: Genetics; physiology; cellular biology

Early Life

Thomas Hunt Morgan was born in Lexington, Kentucky, on September 25, 1866. The Morgans were a distinguished Southern family; his brother, John Hunt Morgan, was a Confederate general during the Civil War, and his mother, Ellen Key Howard, was the granddaughter of Francis Scott Key, who wrote the lyrics to the national anthem of the United States.

As a child, Morgan was fascinated by nature. He was an avid collector of bird’s eggs and fossils. At fourteen, he was accepted into the preparatory department of the Agricultural and Mechanical College of Kentucky, now the University of Kentucky. He earned a bachelor’s degree in zoology in 1886, graduating with highest honors. Morgan then enrolled at Johns Hopkins University, where he earned his PhD in 1890. His doctoral research involved the evolutionary relationships of sea spiders to other organisms.

Morgan spent a year doing postdoctoral work at Johns Hopkins before he accepted a teaching position at Bryn Mawr College, a women’s college. There, he met his future wife, Lilian Vaughan Sampson, who was one of his students; they married in 1904. That same year, Morgan accepted the post of chair of experimental zoology at Columbia University, where he would remain for the next twenty-four years.

At Columbia, Morgan began researching heredity. At the time, it was not known that chromosomes are made of DNA and encode heritable traits. Morgan and his contemporaries had observed chromosomes under the microscope as ropelike structures inside cells. Many scientists of the era believed that the distribution of chromosomes during cell division was the mechanism for inheritance; however, Morgan was not yet convinced.

Around 1857, the monk Gregor Mendel had discovered what he called factors, later known as genes, during plant-breeding experiments. He knew nothing of cells or chromosomes, but he carefully kept track of the results of his crosses (matings) between plants and developed a simple method for predicting ratios of offspring produced from certain crosses. Mendel’s work was initially forgotten, probably because few scientists of his era understood it. It was rediscovered just as Morgan was entering the new field of genetics and would prove crucial to his success.

Early in his career, Morgan was opposed to Mendelian genetics and Darwin’s theory of natural selection. The results of his own experiments would eventually change his mind on both counts, and his research would play a large role in convincing the scientific community that Mendel’s factors were actually physical structures, or genes, found on chromosomes.

Life’s Work

At Columbia, Morgan assembled a team of young researchers in his lab. In sharp contrast to the hierarchical nature of most labs, Morgan encouraged his research associates to share ideas and debate with him. This open atmosphere contributed to the team’s accomplishments and later served as a role model for other laboratories.

Morgan chose to study heredity in the fruit fly Drosophila melanogaster, as its small size, rapid reproduction rate, and the fact that it has only four chromosomes made it ideal for genetic research. Morgan’s goal was to find a mutant fly and investigate the inheritance of the mutation. For two years, his team subjected fruit flies to X-rays, acids, and toxins in an attempt to induce a mutation. The team eventually found what they were looking for: a single male fly that had white eyes instead of the normal red. It was a naturally occurring mutant.

When Morgan crossed the mutant fly with its red-eyed sister, all the offspring had red eyes; the white-eye trait seemed to have disappeared. Morgan deduced that the red-eye trait was dominant, and white was recessive. However, an organism with genes for both a dominant and a recessive trait is still able to pass on the recessive trait to its offspring. Morgan interbred the red-eyed litter, and the white-eye trait reappeared in the “grandchildren”—but only in the grandsons. Every white-eyed fly was male.

Morgan now had enough evidence to prove that chromosomes are the carriers of genetic traits. Mendel had explained dominant and recessive inheritance, but Morgan’s work proved something new. Since the trait was found only in males, Morgan determined that the white-eye trait must be carried on one of the sex chromosomes. He dubbed it a sex-limited trait, a term that would later be replaced by sex-linked.

Under the microscope, scientists can tell the difference between the chromosomes of male and female fruit flies. Just as in humans, females have two X chromosomes, and males have an X and a Y. To be born male, a fly (or person) must inherit an X from the mother and a Y from the father. A female also inherits an X from the mother, but she inherits a second X chromosome from the father.

Because only the male flies had white eyes, Morgan determined that the trait was carried on the X chromosome. In a way, having two X chromosomes is like having a computer file and a backup, both carrying virtually the same information. If a mutation occurs on one X chromosome, a female has a backup file (the other X) and still appears normal. A male has only one X chromosome, however, so his file has no backup; therefore, a male with a mutation on the X chromosome will display the mutation, like the white-eyed trait.

In the case of the fruit flies, subsequently breeding a carrier female—one who looks normal but has the hidden mutation on one of her two X chromosomes—with a white-eyed male could potentially produce white-eyed female flies. It is for this reason that inbreeding among closely related people is taboo in most cultures. Just as in the culture of inbred flies, relatives have a higher chance of carrying the same mutation and producing an affected child. Scientists now know that sex linkage is the same mechanism by which humans pass on color blindness, hemophilia, and muscular dystrophy. These conditions are rare in women because they are carried on the X chromosome, so a woman would have to inherit the same trait from both parents to be affected.

In 1911, geneticist Alfred Henry Sturtevant, Morgan’s lab assistant, discovered a way to map the genes in fruit flies. He showed that genes are arranged in a line on a chromosome. Morgan, Sturtevant, and the rest of the research team published their results in The Mechanism of Mendelian Heredity in 1915.

Morgan became chairman of the new Division of Biological Sciences at the California Institute of Technology (Caltech) in 1928. Though he described himself as a “laboratory animal,” he proved to be an able administrator, and with the help of generous endowments, he developed a biology program at Caltech that emphasized research and experimentation. He was awarded the Nobel Prize in Physiology or Medicine for his work on chromosomes and heredity in 1933.

While at Caltech, Morgan revisited embryology and regeneration, concepts he had been interested in earlier in his career. One of his better-known embryology experiments showed that gravity does not affect the early development of frog zygotes. He published several works on the topic of regeneration, based on his studies of the regrowth of appendages in the hermit crab.

Impact

The work of Morgan and his team transformed the study of biology from a purely descriptive science into an experimental one. His legacy, however, is perhaps best measured by the achievements of his students. Geneticist Hermann J. Muller, who worked with Morgan at Columbia, was awarded the Nobel Prize in Physiology or Medicine in 1946 for demonstrating that X-rays can damage DNA, thereby inducing mutations. George Wells Beadle, also a geneticist and a student of Morgan’s at Caltech, won the 1958 Nobel Prize in Physiology or Medicine. Distinguished Rockefeller Foundation fellow Max Delbrück also worked with Morgan at Caltech.

Morgan was passionate about his research and patient with his students. In a time when even university students wore suits and ties, Morgan used a string for a belt. Although he worked incessantly, rarely taking time off, he made time for his family. His wife, Lilian, herself a scientist, worked with her husband in the lab while raising the couple’s four children. Morgan died on December 4, 1945, in Pasadena, California.

Bibliography

Morgan, Thomas Hunt. A Critique of the Theory of Evolution. Charleston: Forgotten, 2012. Print. Reprint of Morgan’s 1916 work that presents counter-arguments to Darwin’s theory of evolution.

---. Sex-Linked Inheritance in Drosophila. Ann Arbor: U of Michigan P, 2012. Print. Reprint of Morgan’s 1916 work on the genetics of fruit flies. Reviews his theories of inheritance ratios.

Shine, Ian. Thomas Hunt Morgan: Pioneer of Genetics. Lexington: UP of Kentucky, 2009. Print. Reviews Morgan’s life, work, and career in genetics. Also discusses his research procedures.