The First to Discover the Sex Chromosomes

When women rarely went to high school, Nettie Maria Stevens (1861-1912) wanted to be a research scientist. We don’t know a lot about her personal life, but she became a biologist. And though she received little credit for it during her lifetime, she was the first to discover the sex chromosomes.

Photograph of Nettie Stevens the first to discover the sex chromosomes

The Incubator (courtesy of Carnegie Institution of Washington) / Public domain

Before the 1900s, the link between Mendel’s genetic rules and gender were unclear. Scientists didn’t know what factors determined the sex of an offspring. Some believed external factors such as temperature and nutrition influenced gender. Very few thought chromosomal factors were responsible for the gender of offspring.

Early Life

Born on July 7th, 1861 in Cavendish, Vermont to Julia and Ephraim Stevens. Records of her early life are sketchy. We know her mother died relatively early in Stevens’s life but don’t know what caused her death. 

Her father, a carpenter, remarried and the family moved to Westford, Massachusetts. He earned enough to send both of his daughters to high school, though it was uncommon to educate women. Stevens graduated from Westford Academy in 1880. She and her sister, Emma, were two of three women to graduate from her high school.

Teacher, Librarian, and Student

Stevens wanted to become a scientist but needed to earn money for her higher education. She became a teacher and a librarian.

She taught courses in physiology and zoology, mathematics, Latin, and English.

After teaching for three terms, she continued her education at Westfield Normal School (now Westfield State University) completing the four-year course in only two years and being graduated with the highest scores in her class.

She enrolled in the one-year-old Stanford University in 1896. By 1899 she’d earned her B.A. and graduated with an M.A. in biology in 1900.

For a year, she did graduate work under Oliver Peebles Jenkins and his former student and assistant professor, Frank Mace MacFarland. During this time, her work in physiology focused more and more on histology.

She earned her PhD from Bryn Mawr College in 1903.

And in 1904 she received a fellowship from the Carnegie Institution of Washington.

Discovering the Sex Chromosomes

She wrote and published a research paper in 1905. “Studies in Spermatogenesis with Especial Reference to the ‘Accessory Chromosome’” was one of the 20th century’s major scientific breakthroughs. 

Stevens studied insects and discovered the sperm cells would differ by one chromosome. Some sperm cells carried a large chromosome while others carried a smaller one. She noticed that unfertilized eggs did not have this difference and concluded that the smaller chromosome was responsible for sex determination.

Today we know these two chromosomes as X and Y.


Most scientists of the time did not embrace Stevens’s findings.

Edmund Wilson, another researcher, independently made a similar discovery. Because of his higher reputation (and in my opinion, his gender), he received credit for her discovery when his own discoveries and papers were not as strong or as accurate.

She remained uncredited for her discovery until scientific research and society grew to acknowledge and search for accomplishments by women.


At 50 years old, Stevens had published more than 38 papers in cytology and experimental physiology. Finally, she was offered her dream job, a research professor at Bryn Mawr College, but was too ill to accept the position. 

She died of breast cancer on May 4, 1912. They buried Stevens in the Westford, Massachusetts cemetery beside her father and her sister.

They buried Stevens in the Westford, Massachusetts cemetery beside her father and her sister.


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

Google displayed a doodle showing Stevens peering through a microscope at XY chromosomes on July 7, 2016, her 155th birthday.

Westfield State University opened the Dr. Nettie Maria Stevens Science and Innovation Center on May 5, 2017.

The state-of-the-art building houses the university’s “STEM-related degree programs.” (Nursing and Allied Health, Chemical and Physical Sciences, Biology, Environmental Science and the master’s degree program in Physician Assistant Studies.)

It takes a special kind of strength to lead a life outside of society’s norms. This post is part of an ongoing series that celebrates women who are role models, leaders, and strong women.

Nettie Stevens was the first to discover the sex chromosomes and realize that one of them determined gender. Her discovery opened the doors of science and led to things like the identification of hereditary diseases, understanding human and animal development, and even the onset of forensic science. Tip of the hat to Dr. Nettie Stevens.

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