Meet the American who unlocked the science of gender, Nettie Stevens, female research pioneer


Dr. Nettie Stevens broke down barriers for women and opened up doors of knowledge for all humankind.

She decoded the science between the two genders. 

A pioneering cytogeneticist and researcher at Bryn Mawr College, outside Philadelphia, Stevens discovered in 1905 that sex is determined by hereditary traits passed through chromosomes. 

Females are born with a pair of XX chromosomes, inheriting an X from both the mother and father. Males are born with XY chromosomes; the X is from the mother, the Y from the father. 


Stevens further discovered that the determinant X or Y chromosome was passed through the father. 

Stevens studied insects. But the same chromosomal factors have proven to determine sex in most species, including humans, on the planet. 

Bryn Mawr college researcher Nettie Stevens made a groundbreaking discovery in 1905. She found that sex was determined by the existence of XX or XY chromosomes in mealworms. The same factor determines gender across most other species, including humans. (Public Domain)

Humans have 23 pairs of chromosomes. One pair, known as the sex chromosomes, determines male or female gender. 

It’s settled science today. But her work revolutionized the burgeoning field of genetics at the turn of the 20th century. 

“She was a real prodigy,” Gregory Davis, an associate professor of biology at Bryn Mawr today, told Fox News Digital.

More importantly, perhaps, he said, “She had guts!”

“She was a real prodigy … She had guts!”

Research fields such as biology are still dominated today by people with Y chromosomes. An XX carrier in the field was positively extraordinary more than 100 years ago. 

Stevens was more than just a symbolic female figure, however. She brazenly challenged the assumptions of many powerful people in biology, including her own colleagues and powerful mentor, Thomas H. Morgan. 


Aristotle and his pupil, Alexander. Aristotle speculated that the sex of a child was determined by environmental conditions at the time of conception. Scientists attributed sex to environmental factors for more than 2,000 years, until American scientist Nettie Stevens discovered the existence of X and Y chromosomes in 1905.  (Getty Images)

Even more dramatically, Stevens solved a mystery that had eluded the greatest minds in the world for thousands of years. 

The differences between male and female in form, function, physiology and many other factors were obvious in humans — and in most other species — for millennia. 


Scientists long suggested environmental or circumstantial factors determined the sex of a child. 

One of the most famous minds of the ancient world claimed that heat was the primary force in sex determination.

“Aristotle’s advice for older men looking to produce a male heir: Have sex in the summer,” author Rachel Swaby wrote in her 2015 book, “Headstrong: 52 Women Who Changed Science — and the World.”

Aristotle’s theory sounds like folk medicine hokum today. 

But the leading science in sex determination had changed little from Ancient Greece to the day Stevens entered the lab. 

human chromosomes

Human karotype, 23 pairs of chromosomes; bottom right, the pair of sex chromosomes XY or XX determines the sex.  (BSIP/Universal Images Group via Getty Images)

Environmental causes were still the leading theories behind gender determination.

Stevens’ sex-chromosome discovery “was the culmination of more than 2,000 years of speculation and experiment on how an animal, plant or human becomes male or female,” Stephen Brush wrote for “The History of Science Society” in 1978. 

The indomitable spirit of Stevens would face one final challenge after she solved the eternal mystery of the sexes.

She never quite got credit for her work at the time because, among other reasons, she did not have a Y chromosome. 

Born female and ‘obviously brilliant’

Nettie Maria Stevens was born on July 7, 1861 in Cavendish, Vermont, to Ephraim and Julia Maria (Adams) Stevens. 

Both were apparently from longtime Massachusetts families.

Pickett's Charge

The tragedy of the Civil War began just three months before Nettie Stevens was born in July 1861.  (Universal History Archive/Universal Images Group via Getty Images)

The Civil War began to tear apart the nation just three months before Stevens was born. Her family soon suffered its own tragedy. 

The future genetics pioneers had just turned two years old when her mother died of unknown causes.

The family moved to Westford, Massachusetts, where Stevens eventually proved a standout student at Westford Academy — a public school, despite its name, still in existence today.


She was reportedly one of only three girls to graduate from the high school in more than a decade. 

“She was obviously brilliant and worked very hard,” said Davis. But even then, options for brilliant women were limited.

old school desk

Nettie Stevens was teaching high school into her 30s when she decided in the 1890s to pursue her passion for scientific research. Vintage school desks and memorabilia donated to the Henry Schoolhouse Museum in Wayan, Idaho, are shown here. (Jon G. Fuller/VWPics/Universal Images Group via Getty Images)

Stevens became a schoolteacher in Lebanon, New Hampshire, but still burned with greater intellectual passion. 

Incredibly, well into her 30s, she moved all the way across the country to study at the upstart new Leland Stanford Junior University in California, now Stanford University.

“Stevens was finally able to pursue biology,” according to author Swaby.


“She spent much of her lab time corralling protozoa. As an undergraduate, she discovered two new species, slotting it into the new genus of ciliates.”

“Stanford provided a solid foundation in biology, but Stevens was a rising talent, drawn to the cutting-edge research happening back east.”

Stevens, in other words, was already stretching the bounds of known science.

She received a bachelor’s degree in 1899 and a master’s degree in biology in 1900. 

Her future, and her contributions to science, would be forged at Bryn Mawr College. The all-XX school offered rare opportunities for women in science. 


Illustration depicts an aerial view of ‘Senator Leland Stanford’s Farm at Palo Alto, California,’ 1888. Stanford founded Stanford University, and the illustration depicts the university’s proposed buildings. (Photo by Interim Archives/Getty Images)

Writes Swaby, “Stanford provided a solid foundation in biology, but Stevens was a rising talent, drawn to the cutting-edge research happening back east at Bryn Mawr — the small Pennsylvania college was a hub for genetics research.”

The science behind gender

Stevens blossomed at Bryn Mawr, under the tutelage of Thomas H. Morgan, and Edmund B. Wilson, who went on to Columbia University but remained collegial with the upstart scientist.

Scientist Gregor Mendel

Painting showing Gregor Mendel, Austrian scientist who experimented with garden peas to establish the principle of heredity. (Getty Images)

She received her PhD at Bryn Mawr in 1903 at the age of 41. 

She had risen at a time when women in her hometown rarely graduated from high school to hold a doctorate in science and stand on the cusp of great breakthroughs in science. 


Stevens was working at an incredible time in genetics. 

Gregor Mendel, an Augustinian friar in Europe, had pioneered genetics research in the previous century, but his work was only discovered around the world in 1900.


Stevens spent much of her time at Stanford studying protozoa and discovered two new species of the ciliates. (Encyclopaedia Britannica/Universal Images Group via Getty Images)

“Mendel established the laws of inheritance,” said Davis, the Bryn Mawr biology professor. “At some point there was a suggestion that the factors that Mendel was talking about could be associated with things people could see in cells called chromosomes.”

Stevens began looking into the theory — to see if hereditary traits such as sex were in fact passed on through chromosomes.

“The study involved plucking the tiny gonads from mealworms, beetles and butterflies and fixing them in a solution.”

The work was far from glamorous.

“The study involved plucking the tiny gonads from mealworms, beetles and butterflies and fixing them in a solution,” Swaby reports.

“Stevens then secured the preserved sex organs in paraffin blocks in order to slice them into thin pieces without crushing the structures … When it was done right, Stevens could see a whole line of chromosomes laid out before her.”

Stevens published her extraordinary findings in a 1905 report called “Studies in Spermatogenesis.”

Mouse tissue

Nettie Stevens embedded the sex organs of insects into parrafin so that she could slice and study them under a microscope. Image here shows paraffin-embedded mouse spleen for the purpose of scientific study.  (Universal History Archive/Universal Images Group via Getty Images)

The science in it was clear-cut and revolutionary. A mystery that eluded the greatest minds in history had been solved.

The world of academia, however, is not so cleanly constructed as the building blocks of nature.

Wilson drew the same conclusions about sex-determination via chromosomes independently the same year — and even peer-reviewed Stevens’ study, said Davis. 

There remains debate over who actually made the discovery first.

Glory in the arena, and credit for sex-determination in the eyes of history, however, would later go to Stevens’ former mentor. 

“Before Nobel Prize-winner Morgan had a religious conversion to chromosomes [determining sex identity], he was an opponent of the idea.”

Thomas H. Morgan won the Nobel Prize in Medicine in 1933 “for his discoveries concerning the role played by the chromosome in heredity,” in the words of the Nobel Foundation. 

Thomas Hunt Morgan

Nobel Prize-winning geneticist Thomas Hunt Morgan with a microscope at a laboratory. Morgan originally rejected the findings by Nettie Stevens that would later help him win the Nobel Prize in 1933.  (Getty Images)

Morgan made additional advances in the field. 

But his global honor was built on the breakthrough that Stevens made years earlier.

“The irony,” Davis said, “is that before he had a religious conversion to chromosomes [determining sex identity], Morgan was an opponent of the idea. He eventually comes around, but not until he finds [the] same phenomemon studying fruit flies.”

Career cut short

Nettie Maria Stevens died on May 4, 1912, in Baltimore after battling breast cancer. 

She was just 50 years old. 

Nettie Stevens

American geneticist Nettie Stevens answered one of the most enduring questions in science in 1905 when she discovered that a child’s sex was determined by the existence of X and Y chromosomes among the 23 pairs of chromosomes in the human genome. Females have a pair of XX chromosomes, while males have XY chromosomes, with the X or Y sex determinant passed on through the father. The same X or Y dynamic determines the sex in most plant or animal species.  (Courtesy Bryn Mawr Special Collections)

Perhaps Stevens would have won a Nobel Prize on her own she had only lived longer.

Instead, her brilliance was devoted to scientific research for less than a decade. She started old and died young. 

Yet she left an outsized impact on the advance of gender research.

Stevens has in recent years begun to achieve the recognition that evaded her in life. She’s been the subject of, or featured in, several books. 

“Women in Science: 50 Fearless Pioneers Who Changed the World,” was published in 2016. 

“Stolen Science: Thirteen Untold Stories of Scientists and Inventors Almost Written out of History,” was published in 2021. 


Both were written for young readers.

Stevens was inducted into the National Women’s Hall of Fame in 1994. 

“She continues to serve as an inspiration of a woman who persevered in face of the odds.”

Westfield State University in Westfield, Massachusetts, where she first began her college studies before going to Stanford, dedicated the Dr. Nettie Maria Stevens Science and Innovation Center in 2017.

“For a lot of our students, she continues to serve as an inspiration of a woman who persevered in face of the odds,” said Davis, the professor at Bryn Mawr, which remains an all-women’s school.

Few people praised Stevens more than Morgan, the mentor who went on to win the Nobel Prize. 


He wrote a powerful obituary, famous in the field, following the death of Stevens.

“Her single-mindedness and devotion, combined with keen powers of observation; her thoughtfulness and patience, united to a well-balanced judgment, accounts, in part, for her remarkable accomplishment,” he wrote.

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