At the dawn of the 20th century, biologist Nettie Stevens resolved a question that had puzzled humanity for millennia: why do men and women exist as distinct sexes? Stevens, conducting groundbreaking research at Bryn Mawr, uncovered the secret behind this divide—sex chromosomes.
Her work laid the foundation for our understanding of genetics. Thanks to her discoveries and subsequent research, we now know that gender is a hereditary trait, determined by the father's sperm. Yet, throughout history, this mystery sparked countless speculative theories.
Aristotle, for instance, believed a man's body temperature during intercourse dictated the child's gender. As noted in the textbook *Developmental Biology*, he advised older men to adjust their temperatures accordingly if they wished for sons.
In 19th-century Europe, it was commonly thought that parental nutrition influenced the baby's sex—poorer diets supposedly resulted in boys, while wealthier diets produced girls. Over the centuries, many bizarre ideas surfaced.
In the 18th century, French physiologist Michel Procope-Couteau suggested that removing one testicle or ovary could determine the child's gender. He even proposed lying on one's side during conception to influence the outcome. These ideas may sound far-fetched today, but they reflect the era's confusion about sex determination.
Today, we know these theories were incorrect, thanks largely to Stevens' research. Born in Vermont in 1861, Stevens didn't pursue higher education until she was 35. She earned her bachelor's and master's degrees from Stanford University and later pursued a Ph.D. at Bryn Mawr College, a rare achievement for women at the time.
By the early 1900s, the idea of chromosomes carrying genetic information was still novel. Mendel's work had only recently been rediscovered, and scientists were eager to understand how traits, including gender, were passed through generations.
Stevens wanted to uncover whether—and how—gender was genetically inherited. Using a microscope to examine Tenebrio molitor chromosomes, she revealed a truth hidden for millennia. She noticed that female Tenebrio cells contained 20 large chromosomes, while males had 20 chromosomes, but one was significantly smaller.
"This seems to be the phenomenon of chromosomes determining gender," she wrote in her report. She concluded that the difference stemmed from the sperm of the mealworms. Some sperm carried a smaller chromosome, resulting in male offspring, while others produced females.
"Nettie Stevens' findings represent over two millennia of human inquiry into the formation of sex in animals and humans," wrote historian Stephen Brush. Her work also provided crucial support for Mendel's gene theory, which became foundational to modern biology.
Stevens' colleague and former mentor, Edmund Beecher Wilson, is often credited as the discoverer of sex chromosomes. Wilson, like Stevens, was studying the issue and published similar findings around the same time. However, in the species he examined, males lacked a chromosome that females possessed. This is rarer in nature compared to Stevens' XY chromosome model, which serves as the basis for human gender determination.
Wilson's paper predated Stevens', and his academic reputation was higher, so he was often recognized as the pioneer in this field. However, Stevens' conclusion was more accurate, with stronger evidence. Wilson still believed environmental factors influenced gender, while Stevens insisted it depended solely on chromosomes. Although both theories couldn't be fully proven at the time, history proved Stevens correct for humans and mammals.
This is a classic example of the "Matilda Effect," named after abolitionist Matilda Gage, highlighting how women's contributions are often credited to their male peers. Stevens isn't the only female scientist to face this fate—Rosalind Franklin, whose work was pivotal in discovering DNA, suffered a similar oversight.
Stevens succumbed to breast cancer in 1912. The New York Times acknowledged her contributions, calling her "one of the few outstanding female scientists and a leader in contemporary biology." While this was a modest tribute, her impact on science far exceeded this description.
Written by Brian Resnick
Translated by Wang Ke
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