Nettie Stevens and the X chromosome

By Jaime Seltzer

When Nettie Stevens was growing up in the late 1800s, scientists were not certain how characteristics were passed from parents to children. In 1903, Walter S. Sutton proposed chromosomes were responsible for how traits passed from generation to generation, but this had not yet been experimentally proved. There were still many scientists who thought that environmental factors such as what a pregnant woman ate or how hot or cold it was could influence the gender of her baby.

Nettie Stevens displayed her cleverness from an early age, at the top of her class in whichever school she attended. Westford Academy’s charter stated that the school was “free to any nationality, age, or sex”, and so Nettie attended with several other girls her own age, including her sister Emma. She graduated in 1880 and then went on to Westfield Normal School from about 1881 to 1883, completing the equivalent of a four-year degree in two years. While there she earned perfect scores in her geometry, chemistry, and algebra classes.

She was the first to study female eggs rather than male testis, in part due to how difficult the eggs were to work with, but also because science at the time viewed males as the ‘default’ organism, while women were an exotic variation on the theme.

After she graduated, Nettie taught high school for a few years. Records do not clarify her exact position, but it appears that she taught as well as was principal for a brief while. However, in 1896, she decided to continue her education at Stanford University. She took about a full semester’s courseload before she was admitted as a freshman, but then was awarded advanced status less than four months later!

It was while at Stanford that Nettie first began to focus on histology, the study of cells. She worked under Frank Mace McFarland, and inherited his position at the Investigator’s Room at the Hopkins Seaside Laboratory in Pacific Grove, California, from which she worked while tweaking her master’s thesis.

Then it was on to Bryn Mawr, where she studied under Thomas Hunt Morgan. She had only been studying there six months before the quality of her work was rewarded with a fellowship to study abroad.

It was in the very early 1900s that Nettie met Theodor Boveri in Germany. He was studying chromosomes, and Nettie learned a great deal from the work he was doing; she would study with him again between 1908 and 1909.

Over the course of Nettie’s work at Stanford and Bryn Mawr she had discovered two new kinds of ciliates and written papers to describe their life cycle and characteristics, which included detailing the number of chromosomes present and speculated about how the position of the nucleus played a role in when cell division would occur. She submitted one of her papers in order to earn a doctoral degree, which she received in 1903.

Nettie had gone as far as she could in her schooling, so it was time to find a job. Of course, this was the challenging part. Women were not as hirable as men. However, Nettie’s work had been so brilliant, and she had hobnobbed with so many different important scientists of the day that her references would knock anyone’s socks off. Even the president of Bryn Mawr had included a glowing recommendation. After waiting on tenterhooks for months, she finally won a grant that allowed her to commit fully to her research.

Nettie Stevens

The real Nettie Stevens and her card in the Women in Science card game.

In 1905, she won the Ellen Richards Prize (yes, that Ellen Richards) of $1000 to study chromosomes and sex determination. In 1905, $1000 was equivalent to a year’s salary and this helped her be a bit more financially secure. Later that year, Stevens published her first paper stating that sex was determined by chromosomes. Wilson published a paper with a similar premise near-simultaneously, though Wilson’s paper was more equivocal and hemmed and hawed about environmental influence perhaps also playing a part. Each researcher cited the other, as they had shared their work with one another.

Nettie had chosen five species of insects to study and described their chromosomes and those chromosomes’ relationship to sex in her characteristic clipped, matter-of-fact manner of writing, though differences in size in the insects’ chromosomes muddied the water. Nettie stuck to her guns, however, eventually performing studies in flies, aphids and fifty species of beetle; though since the genetic mechanism of gender is not the same in every species, some did not have the typical XX or XY configuration of sex chromosomes. Nettie also did experiments to disprove the environmental influence question once and for all, determining that no matter the temperature of her enclosure, the same number of gentleman and lady aphids were born, further stymying the traditionalist thinkers.

By 1911, Nettie was an assistant professor at Bryn Mawr, but she had little time to enjoy her newfound title: she died shortly thereafter of breast cancer. Morgan wrote her obituary for the journal Science, in which he praised her efforts for pages but concluded with a snippy little paragraph that seemed to imply that Nettie was an awesome experimentalist but lacked ‘vision’. This appears to be particularly sour grapes, since Morgan was one of the old guard who let go of his environmental-determinism beliefs late in the game. It was Nettie whose vision allowed her to try on new ideas for size, and flexibly adopt or dismiss them based on her findings. She was the first to study female eggs rather than male testis, in part due to how difficult the eggs were to work with, but also because science at the time viewed males as the ‘default’ organism, while women were an exotic variation on the theme.

In her short professional life, Nettie published at least thirty-eight scientific papers. Bryn Mawr’s cytology department kept her favorite shiny microscope, nicknamed lovingly “the Nettie Maria”, at first in use and then on display for many years.

Bailey, M.B. and Choquette, C.J. (1981, August). Nettie Maria Stevens (1861—1912): Her Life and Contributions to Cytogenetics. Proceedings of the American Philosophical Society, 125(4): 292-311. Retrieved from
Hagen, J., Allchin, D., & Singer, F. (2009). Doing Biology (pp. 37-47). Glenview, IL: Harper Collins. Retrieved from
Morgan, T. H. (1912, October 11). The Scientific Work of Miss Nettie M. Stevens. Science, 36(928), 468-470. Retrieved October 6, 2015, from

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