In the spring of 1910, Thomas Hunt Morgan peered into one of the hundreds of glass milk bottles that filled his cramped and famously messy laboratory at Columbia University and spotted something he had never before seen. Among the thousands of red-eyed fruit flies he had been breeding for nearly a year was a single male with white eyes.
That one small fly was the first in a series of discoveries by Morgan and his team that would transform the study of heredity. Over the following decades, Morgan’s “fly group” showed that genes are physical units lined up in a fixed order along chromosomes and that their positions could be mapped, laying the foundation of modern genetics.
Carnegie Science made much of this research possible, providing sustained funding to the group from 1915 until 1945 and publishing its findings in a series of volumes, which were accompanied by exquisitely detailed ink drawings and watercolors of fruit flies by Edith Wallace, Morgan’s technician and the team’s artist. Her illustrations, representing the fly group’s work, are Object 24 in our #Carnegie125 series.
Before the Flies
Morgan was born into a prominent Kentucky family in 1866. After earning his Ph.D. in zoology at Johns Hopkins University, he joined the faculty of Bryn Mawr College in 1891 as an associate professor of biology, eventually rising to chair the department over his 13 years there. At Hopkins and Bryn Mawr, Morgan was an embryologist, studying the factors that influence how embryos develop.
As Morgan’s career progressed, heredity began to emerge as a major focus of biological research. In 1900, scientists had rediscovered the work of Gregor Mendel, whose experiments with pea plants suggested that traits are passed down in discrete units according to predictable ratios. But how heredity actually worked inside living cells remained one of the greatest scientific challenges of the day, and Morgan, like many biologists, was not yet convinced that Mendel’s ideas held the answer.
Even so, the role of chromosomes was beginning to come into focus, and some of the most important early evidence came from Morgan’s own circle. In 1903, Morgan championed the application of one of his most promising Bryn Mawr graduate students, Nettie Stevens, to the newly founded Carnegie Institution of Washington (now known as Carnegie Science). With Morgan’s support, Stevens received Carnegie grants in 1904 and 1905 that funded her groundbreaking work showing that the X and Y chromosomes are associated with sex determination. It was Morgan’s first connection to Carnegie, and an early sign of the chromosomal link to heredity that his own research would later confirm.
By then, Morgan had moved on from Bryn Mawr. In 1904, he joined Columbia University’s zoology department as professor of experimental zoology, where he would begin his work with the fruit fly that would transform both his career and the field of genetics.
The Fly Room
Morgan first learned about the fruit fly, Drosophila melanogaster, through Carnegie’s own Station for Experimental Evolution at Cold Spring Harbor in New York. There, the Carnegie-funded William E. Castle had been using the fly in studies of inbreeding, and his colleague Frank E. Lutz—who also received Carnegie support—recommended flies as a research organism to Morgan, who was searching for inexpensive options that could be bred in the limited space available to him at Columbia. By 1909, Morgan was breeding Drosophila in earnest.
The fly turned out to be an ideal subject. At about a quarter of an inch long, it was large enough to study without a high-powered microscope, but small enough that thousands could be housed in bottles in a small laboratory, feeding on the yeasts that grow on rotting fruit. A female could lay hundreds of eggs, and a new generation could be raised in under two weeks, so researchers could follow traits through dozens of generations in a single year. Males and females were easy to tell apart, variations like eye color and wing shape were plainly visible, and the fly had only four pairs of chromosomes, making the relationship between traits and chromosomes far easier to untangle than in other species.
Morgan’s laboratory, which came to be known as the “fly room,” occupied a space barely 16 by 23 feet on the top floor of Columbia’s zoology building. Shelves climbed the walls, lined with empty milk bottles, borrowed from the university lunchroom, that served as fly habitats, each stocked with mashed bananas for food. Eight desks were squeezed into the room, where Morgan’s students and assistants carried out their methodical work. Flies were bottled and fed, bred and crossbred, then put to sleep with ether so they could be counted and sorted under hand lenses, with any mutants separated out.
Morgan, known affectionately as “the Boss,” was down-to-earth and quick with a joke, and he ran the fly room as a collaborative place filled with talented young researchers. Among them were Alfred Sturtevant, Calvin Bridges, and Hermann J. Muller, all of whom earned their Ph.D.s with Morgan. Sturtevant and Bridges, who had first joined the lab as Columbia undergraduates, would continue working with him for decades. The team also included Edith Wallace, a trained biologist who had joined the lab as Morgan’s personal technician in 1908 and would become the group’s gifted artist, remaining with Morgan until her retirement in 1944. The fly room soon became a destination for any scientist who wanted to take part in the newest work in genetics.
The White-Eyed Fly
It was in the fly room, in the spring of 1910, that Morgan first noticed the surprising white-eyed male fly that would launch his group’s major discoveries. To Morgan, it was an enormously exciting find: a distinct change that had arisen spontaneously, in other words, a mutation. When he bred the male with red-eyed females, their offspring all had red eyes. But when those offspring were bred together, white eyes reappeared in the next generation in roughly the proportion Mendel’s laws predicted for a recessive trait, with one striking twist. Every one of the white-eyed flies was male.
Morgan reasoned that the gene for eye color must be carried on the X chromosome. Males have just one X chromosome, inherited from their mothers, so a recessive mutation on it always shows. Females have two, so a non-mutant copy on the second X masks the mutation. Morgan announced his findings in 1910. The discovery, and others that quickly followed, finally convinced him that Mendel had been right.
More mutations soon appeared in the fly room’s bottles, and by the end of 1911 the fly group had identified some 40 of them. Many traits tended to be inherited together. Black body color traveled with curved wings, for instance, but neither was tied to sex. The group sorted the traits into four “linkage groups,” matching the fly’s four pairs of chromosomes. One group was much smaller than the other three, just as one of the fly’s chromosomes is much smaller than the rest. It was compelling evidence that chromosomes carry genes.
Yet the group found that linked traits occasionally separated. Morgan proposed that paired chromosomes could exchange segments, a process he called “crossing over,” and that genes located farther apart from each other on a chromosome were more likely to be split when this happened. (This work would later be expanded by Carnegie geneticist Barbara McClintock, who further defined heredity by demonstrating the existence and prominence of “jumping genes,” which have been described as “the wind in the sails of evolution.”)
Through the course of his work, Sturtevant realized that the frequency of these separations could reveal how far apart the genes sit. He went home, worked through the data, and returned to the fly room the next day with the first genetic map of a chromosome. Genes, it seemed, were lined up along chromosomes like beads on a string.
The map opened an enormous new task. If every gene had a place on a chromosome, then every new mutant had to be bred, crossed, and tracked through generation after generation to find it. Until then, the work had been funded by Columbia and by Morgan himself, but the project was quickly outgrowing those resources, and the group doing it was about to scatter. Sturtevant would earn his Ph.D. in 1914, Muller in 1915, and Bridges in 1916, and once they graduated, the university could no longer support them, though much of the mapping remained to be done.
An Extramural Department of Carnegie
Though Morgan was initially uneasy about accepting outside money, the prospect of losing his gene mappers in the middle of the job led him to Carnegie Science, whose support for basic research in heredity and evolution was already well known. Under its second president, Robert S. Woodward, the institution had shifted away from scattered small grants, like those given to Nettie Stevens, toward sustained support for proven investigators, and Morgan’s fly project already looked much like the organized, long-term research Carnegie ran in its own departments.
Morgan met with Woodward in April 1914 and formally applied for funding that May. In December, the Carnegie trustees approved a three-year grant of $3,600 a year to cover the salaries of Sturtevant and Bridges as they completed the chromosome mapping project.
The investment paid off quickly and proved to be the start of a fruitful relationship that would last three decades. In 1915, the group published The Mechanism of Mendelian Heredity, which recast Mendel’s laws in terms of chromosomes and laid out the principles of linkage, crossing over, and the linear arrangement of genes.
Morgan had expected the mapping to take about three years. He underestimated how the work would grow, since every answer produced more flies, more mutants, and more questions. In 1917, he asked Carnegie for a renewal, which the institution readily granted. In 1919, the arrangement became permanent, with Morgan, Sturtevant, and Bridges made research associates of the institution. As historian Robert Kohler put it, the fly group “became in effect an extramural department of the Carnegie Institution, encysted in Columbia’s department of zoology but largely independent of it.”
The following year, Carnegie’s support rose to about $11,000 a year. Morgan wrote excitedly to a colleague that “the salaries of the boys will be considerably increased. I am to have an artist and technical assistant, with a fund for running expenses and another for apparatus … So if [you] come back again, you will find the flies humming.”
The flies were indeed humming. As new mutants piled up, Morgan and Bridges had begun describing the fly room’s mutant strains in a series of volumes, which Carnegie published chromosome by chromosome as part of the Carnegie Monograph Series. The first volume, on the X chromosome, appeared in 1916, followed by volumes on the second chromosome in 1919 and the third in 1923, by which time the group had identified more than 400 mutant strains representing more than 160 genes. Illustrating these volumes were drawings and watercolor plates by Edith Wallace, now supported with Carnegie funding, showing readers exactly what a selection of the mutations looked like in the fly body.
With stable support, the group’s work continued to expand and in 1926, Morgan distilled their findings in The Theory of the Gene. Its central idea, that genes are laid out on chromosomes in a linear and knowable order, had become a cornerstone of the field of genetics.
Pasadena and the Prize
By the late 1920s, Morgan was widely regarded as the foremost geneticist in the world. In 1927, George Ellery Hale, founding director of Carnegie’s Mount Wilson Observatory and a driving force behind the transformation of Throop Polytechnic Institute into the world-class Caltech, began recruiting him to Pasadena. The following year, at the age of 63, Morgan left Columbia to organize Caltech’s new Division of Biology, bringing the core of the fly group with him.
Carnegie’s support followed them west, continuing through 1945, three years after Morgan himself retired. At Caltech, Bridges continued as a Carnegie employee, while Sturtevant became a Caltech professor, and Jack Schultz took his place on the Carnegie team. While Morgan turned increasingly back toward experimental embryology, the Carnegie-funded group pressed ahead with mapping the fly’s chromosomes in ever greater detail.
Working in his Caltech laboratory on the morning of October 20, 1933, Morgan was interrupted by a long-distance telephone call from New York. Assuming it was one of his children, he was surprised to hear a telegram from Stockholm read aloud: he had been awarded the Nobel Prize in Physiology or Medicine “for his discoveries concerning the role played by the chromosome in heredity.”
The prize recognized what Morgan and his colleagues had established through their studies with the fruit fly: that genes are carried on chromosomes in a linear order that can be mapped. The presentation speech hailed Morgan as Mendel’s heir, marveling that hundreds of hereditary factors had been located on chromosomes so small they had to be measured in thousandths of a millimeter. The fly group’s methods helped turn biology from a purely observational discipline into an experimental, quantitative science, and the fruit fly they championed remains one of the most widely used model organisms in biology today. The prize bore Morgan’s name, but he considered it a shared achievement, dividing the prize money among his own children and those of Sturtevant and Bridges. It honored the work of the entire fly group and the Carnegie support that sustained it.
This would establish Carnegie’s Nobel heritage as the first of four prizes in Physiology or Medicine recognizing the contributions of Carnegie research in genetics. In 1969, Alfred Hershey shared the prize for his work on the replication mechanism and structure of viruses. In 1983, Barbara McClintock was recognized with a solo prize for her jumping gene work. Both were Carnegie employees working out of Carnegie’s Department of Genetics at Cold Spring Harbor Laboratory in New York. In 2006, then-recently departed Carnegie Staff Scientist Andrew Fire shared the prize for the discovery of RNA interference, or RNAi.
Edith Wallace’s Flies
Through it all, Edith Wallace was drawing flies. Over more than three decades with the group, she produced black-and-white drawings in India ink on ivory artist’s board and watercolor plates painted on fine Canson paper. Working at an extraordinary microscopic level of detail, she captured the mutant forms with scientific precision, from subtle shifts in eye color and body shade to the exact placement of bristles and the veining of wings.
Many of Wallace’s original illustrations are now preserved in the Caltech Archives. Together with the volumes they illustrated, they form a vivid record of the fly room’s discoveries and of Carnegie Science’s commitment to the researchers who made them.
Source: carnegiescience.edu
