This chapter 7 is about Nettie Stevens, a pioneering figure whose contributions to genetics laid the groundwork for modern understandings of sex determination.
Stevens' work not only advanced scientific knowledge but also exemplified the profound impact of diverse perspectives in research. As we delve into her life and achievements, we explore how her unique experiences as a woman in science during the early 20th century shaped her groundbreaking discoveries.
Early life and education
Nettie Maria Stevens was born on July 7, 1861, in Cavendish, Vermont.
Initially, Stevens pursued a career in education, teaching high school subjects while nurturing a deep interest in biology and zoology. This passion led her to seek further education, culminating in her enrolment at Stanford University in 1899, where she earned both her B.A. and M.A. in biology in 1900.
After completing her degrees, Stevens was eager to delve into the pressing questions of heredity and sex determination, particularly in light of the recent rediscovery of Mendel's laws of inheritance. In 1903, she began her Ph.D. studies at Bryn Mawr College under the mentorship of Thomas Hunt Morgan, where she focused on cytology and embryology. Her academic journey included a year studying in Europe, where she worked with prominent scientists like Theodor Boveri. [1]
Securing her research
Nettie Stevens faced several significant obstacles in securing funding for her research, particularly during a time when women were often marginalized in the scientific community.
Financial constraints
After completing her Ph.D., Stevens faced financial difficulties that threatened her ability to continue her research. She had limited savings and was concerned about having to return to teaching, which would hinder her scientific ambitions. This financial pressure motivated her to seek external funding. With encouragement from Morgan, she applied for a Carnegie Institution research fellowship to support her research.
Gender bias in science
As a woman in a male-dominated field, Stevens contended with pervasive gender biases that often-devalued women's contributions to science. Many contemporaries doubted the capabilities of female scientists, which made it more difficult for her to gain recognition and support for her work.[2]
Limited research opportunities
During Stevens' time, there were few institutional supports for women pursuing scientific research. Many prestigious institutions were closed to women or offered limited opportunities for advancement. This environment necessitated that Stevens seek funding from organizations that were just beginning to recognize the importance of supporting female researchers.
Need for strong recommendations:
To secure funding from the Carnegie Institution, Stevens relied heavily on recommendations from influential figures like Thomas Hunt Morgan and Edmund Beecher Wilson. While she ultimately received their support, she had to navigate the complexities of establishing herself within a network that was often reluctant to acknowledge women’s contributions.[3]
In 1905, Stevens received the fellowship, which provided her with the necessary resources to conduct her research.
What determines the sex of the child?
Following Gregor Mabel’s work, cytologists had observed under their microscopes that a child receives the same number of chromosomes from both parents by 1900, and they were well aware of this fact.
However, no one had been able to demonstrate a connection between the function of chromosomes and Mendel's laws.
The big question in genetics was then simple: what determines the sex of the child?
At that time, gender determination was a real mystery. For decades, scientists believed that sex was due to what a woman ate during pregnancy or her body temperature. However, Nettie began to doubt these theses and therefore began to investigate.
Discovery of sex chromosomes
Stevens's pivotal research began with her studies on the yellow mealworm (Tenebrio molitor). In her experiments, she observed that male mealworms produced two types of sperm: one carrying a large chromosome (later identified as the X chromosome) and another carrying a smaller chromosome (the Y chromosome). She found that when sperm with the large chromosome fertilized an egg, female offspring were produced, while fertilization by sperm with the small chromosome resulted in male offspring.
Nettie Stevens had to face ambiant skepticism
Stevens faced skepticism regarding her research on sex chromosomes. At the time, many scientists believed that sex determination was influenced by environmental factors rather than genetic ones. This prevailing attitude made it challenging for her findings to gain immediate acceptance within the scientific community.
Her fiercest critic was Thomas Hunt Morgan, who was her professor at Bryn Mawr—a prominent researcher, but also skilled in self-promotion, making him very influential in the public eye. Morgan was considered arrogant and stubborn; he came from a dynasty of financiers and generals who had shaped the history of the United States, and a relative of his wrote the American national anthem.
But what did he have against Nettie's thesis?
Morgan could not believe it was possible to attribute a hereditary trait to a chromosome, let alone to sex, as at that time little was known about genes and DNA. It took decades to discover the exact mechanism of transmission.
However, Nettie did not get discouraged; she continued her genetic research, presenting more and more new evidence and theses.[4]
In 1910, Thomas Hunt Morgan continued to publicly assert that Miss Stevens was contributing anecdotes and that the illustrations accompanying her articles were products of excessive imagination. In fact, he was lying knowingly, unwilling to accept that a woman had outsmarted him in research that he, as a leading expert, had never encountered before.[5]
While biologists and the public believed Morgan, Bryn Mawr College created a special fund so that Nettie could continue her experiments. She no longer needed to scavenge for money, wandering from one foundation to another to pay for laboratory materials as well as for her struggling students.
She contributed even more to the word of genetics
Her work provided critical evidence supporting Gregor Mendel’s laws of inheritance. By demonstrating that specific traits (such as sex) could be directly traced to particular chromosomes, Stevens reinforced the idea that traits are inherited according to predictable patterns. This helped to solidify the foundation of modern genetics, confirming that inheritance follows specific rules rather than being random or solely environmental.
Her research laid foundational principles for modern genetics and cytology, marking a significant shift in scientific understanding of heredity.
Following her discovery, Stevens’ research illuminated how variations in chromosome number and type can lead to differences in phenotype. By exploring how different combinations of sex chromosomes result in male or female offspring, she contributed to a broader understanding of genetic diversity and its implications for evolution and species adaptation.
The recognition of sex chromosomes opened new avenues for studying evolutionary biology. Understanding how sex determination works at the chromosomal level allowed scientists to explore how these mechanisms influence reproductive strategies, population dynamics, and even speciation processes.
Stevens' discoveries paved the way for subsequent research in genetics, including work on genetic disorders linked to sex chromosomes, such as Turner syndrome (X0) and Klinefelter syndrome (XXY). Her findings have had lasting impacts on fields ranging from medicine to evolutionary biology, influencing how scientists approach questions of heredity and genetic variation
Impact and legacy
Despite her achievements, Stevens faced challenges in gaining recognition during her lifetime. She was not invited to speak at major conferences where her contemporaries presented their findings, reflecting the gender biases prevalent in science at the time.
However, her louder opponent, Thomas Hunt Morgan came to accept and build upon these concepts, particularly through his studies on fruit flies (Drosophila melanogaster), which ultimately led to his Nobel Prize in 1933 for discoveries concerning the role played by the chromosome in heredity!
It has frequently been assumed that Nettie Stevens and Edmund Wilson collaborated closely on the topic of chromosomal sex determination, with Stevens acting as Wilson's assistant.
Many textbooks have attributed the findings solely to Wilson, given his status as a more established researcher. Even Thomas Hunt Morgan has received credit, particularly after being awarded the Nobel Prize.
However, it is evident from subsequent publications after 1905 that both Wilson and Morgan were reluctant to fully embrace the chromosomal theory of sex determination, clinging to their alternative hypotheses for several years.
In 1905, Stevens secured a position at Bryn Mawr College as an associate in experimental morphology while also maintaining her affiliation with the Carnegie Institution as a research assistant. That same year, she was awarded the Ellen Richards Prize of $1,000 for her paper titled “Studies on the Germ Cells of Aphis.”
Despite some acknowledgment from Morgan regarding her contributions, Stevens received limited recognition for her pivotal role in elucidating chromosomal sex determination. This lack of accolades can be partly attributed to her untimely death from breast cancer in 1912, just seven years after her groundbreaking work was published.
She left behind more than 40 scientific papers.
Even years later, many scientists continued to adhere to externalist theories of sex determination, and some still advocate for environmental influences today. This resistance to change slowed the scientific community's acceptance of Stevens' conclusions, which are now celebrated as pioneering. Interestingly, both Morgan and Wilson were invited to present their theories at a conference in 1906, while Stevens was notably excluded from the speaking roster. Her contributions to our understanding of heredity and its significance in development clearly warrant greater recognition.
Parallels to sustainability
Stevens's work offers surprising insights for modern sustainability practices:
Inherent characteristics matter:
Just as an organism's sex is determined by inherent chromosomal factors, a company's core values and practices fundamentally shape its sustainability potential.
Systematic observation leads to breakthroughs:
Stevens's methodical approach to research mirrors the need for businesses to systematically assess their environmental impact and social practices.
Challenging prevailing assumptions:
Stevenschallenged existing theories about sex determination. Similarly, companies must challenge assumptions about "business as usual" to achieve true sustainability.
Conclusion
This discovery was revolutionary as it provided definitive evidence linking chromosomal configurations to sex determination, challenging the prevailing belief that environmental factors influenced gender. Stevens' works ultimately revolutionized our understanding of genetics and sex determination, marking a significant shift from earlier beliefs that attributed gender to environmental influences. Her contributions laid foundational principles for modern genetics.
Her work reminds us that fundamental characteristics matter, careful observation leads to breakthrough insights, and challenging existing paradigms can revolutionize our understanding. By applying these principles to sustainability, companies can work towards more meaningful, impactful, and lasting changes in their environmental and social practices.
This article is part of a series exploring the importance of intersectionalty in science for innovation and sustainability