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  • Writer's pictureDale DeBakcsy

Where Sex Begins: The Chromosomal Investigations of Nettie Stevens.

If you were to ask an ancient Greek how it is determined that a baby is born a boy or a girl, they would have had some interesting and very compelling theories to offer you. One camp held that it was determined by which of the male's testes the child originated from, while another believed that what really matters is what side of the womb the fetus develops in, until Aristotle put everybody on notice by declaring once and for all, and definitively, that it is temperature that makes the difference, because men are governed by the element fire, and women by the cooler element water.

For two millennia, our ideas about sex determination were little more advanced than those of the ancient Greeks, with prenatal environment considered the most important factor for determining whether a child, sexless at conception, emerged a boy or a girl. Temperature continued to be cited as an important factor, but the nutrition consumed by the mother was also deemed to be crucial, and such was the common wisdom until 1905, when a Bryn Mawr cytologist by the name of Nettie Stevens (1861-1912) published "Studies in Spermatogenesis with Especial Reference to the 'Accessory Chromosome,'" a daring paper that smashed all previous theories and established on a definite basis the chromosomal, hereditary nature of gender.

As important as her work was, we know astonishingly little about her life. She was born three months after the start of the US Civil War in Vermont to Julia and Ephraim Stevens, one of four children of whom only two survived to adulthood. Julia Stevens died in 1863 and Nettie was raised primarily by her stepmother, whom her father married in 1865. The year of Stevens's birth put her firmly between two generations of expectations and opportunities. Born two decades earlier, and her career would have firmly been that of a gifted and vastly overqualified governess or school teacher. Born two decades later, and her talents would have opened a series of doors that would have resulted in their steady and early development.

As it stood, Stevens underwent several starts and stops to her career as she attempted to navigate her way through the grey area of the late nineteenth century educational system. She attended public school in Westford, Massachusetts and her academic performance there allowed her to continue on as one of the rare women students attending Westford Academy, where she graduated in 1880. Stevens had clear gifts, but she was also driven by the need to be financially independent, and in 1880 that meant finding a job as a teacher, as paying research positions for women interested in biology were not, as of yet, an option. She took up a position as a Latin, English, mathematics, and biology teacher in Lebanon, New Hampshire and furthered her scientific education as best she could at Westfield Normal School, where she graduated at the top of her class in 1883.

The next thirteen years, then, were devoted to the sort of necessary, money-earning work that would allow her to maintain what she prized most of all, her independence. She worked as a librarian and a teacher in three different cities while waiting for an opportunity to take the next step in her education. That opportunity arose in 1896 when she heard about a new university on the West Coast, founded in 1891 and open to women applicants - Leland Stanford Junior University.

In a move that seems doubly bold in view of her history of cautious and pragmatic career decisions, she decided to move out to California in 1896 and join Stanford as a special-case student preparatory to earning her full freshman status in 1897, and an advanced status a few months after that. She worked at Stanford with Frank Mace MacFarland, a nudibranch authority who steered Stevens towards histology, the study of organic tissues by microscope. She earned her bachelor's degree in 1899 and her Master's in 1900 with her thesis Studies on Ciliate Infusoria.

1900 was a crucial year not only for Nettie Stevens, when she earned her Master's and settled in to the work at Bryn Mawr College that was to define the rest of her life, but for biology generally. This was the year that the genetic work of Gregor Mendel was rediscovered and verified in a paper by German botanist Carl Correns, kickstarting a new wave of investigations that would come to define modern biology. By 1903 Walter Sutton and Theodor Boveri had established that chromosomes were the carriers of genetic material, thereby providing the locus of study for any researchers interested in questions of heredity.

Stevens had a chance to study with Boveri himself at the University of Würzberg at precisely the time that he was carrying out his important investigations of chromosomal regularity in sea urchins, further concentrating her interest from cytology generally to chromosomal research. She received her PhD in 1903 for her dissertation Further Studies on the Ciliate Infusoria, Licnophor and Boveria, which focused on morphology and particularly the regenerative processes of those organisms. Her interest, however, was turning towards a new investigation of the possible hereditary basis of sex determination, work which she would need funding to carry out. She applied for a Carnegie stipend to supplement her meager salary as a reader in experimental morphology at Bryn Mawr, and received it in 1904.

Stevens would author thirty-eight papers over the next eight years of life left to her, but it was to be the paper published in 1905 describing the results of her investigations into sex determination that would earn her a pedestal in the scientific pantheon. The majority opinion of the scientific community in the early 1900s was that environmental factors determined the evolution of a fetus's sex in the womb. In 1901 Clarence McClung, working with the grasshoppers that were readily available around the University of Kansas, had hypothesized that gender was determined by the presence or absence of a second X chromosome in the cell, an observation which is true enough in grasshoppers, where females have two X chromosomes while males only have one, but was, unfortunately for McClung, not true of the larger animal kingdom.

Stevens worked with the meal worm Tenebrio molitor and noted that while eggs always possessed ten full sized chromosomes, that sperm gametes contained either ten full chromosomes or nine full chromosomes and one smaller chromosome (what we now call the Y chromosome). When an egg was fertilized by a sperm with the larger tenth chromosome, it developed into a female, and when it was fertilized by a sperm with the smaller tenth chromosome, it developed into a male. This was the death knell of the environmental hypothesis for sex determination, and also of McClung's theory that sex was caused generally by the presence of an extra "accessory" chromosome. In later studies, she expanded her research to ensure that her results were true for other species besides Tenebrio molitor (in particular aphids, beetles, and flies), while her results were confirmed independently by influential biologist EB Wilson, and in his paper on the topic he acknowledged Stevens's priority in a footnote.

Though she was first in the discovery of the hereditary basis of sex determination, she was not invited to a conference in 1906 where Wilson and her superior at Bryn Mawr (though her junior in age), Thomas Hunt Morgan, were slated to speak about their research in that topic. Morgan then compounded this omission in his official obituary of Stevens, which conveniently mis-stated the year of her work with Tenebrio molitor as 1906 instead of 1904 to make the case that Wilson and she essentially co-discovered the XY principle.

Morgan's willingness to muddy the chronology on Stevens's work to give more credit to a male colleague was mirrored in his unwillingness to credit Stevens as an early researcher in the organism that he later won the Nobel Prize for genetically describing, Drosophila melanogaster. This was a species of fly first bred by C.W. Woodworth and used in genetics research at Harvard by William E. Castle as a model organism for genetics studies, and it was one of the species that Stevens included in her researches, several years before Morgan began making it the centerpiece of his genetics experiments in 1909. In his obituary of Stevens, written in 1912, mention is made of the fact that she worked with flies generally, but the fact that she worked with that particular species is not present. This is not to say that Morgan stole the idea of working with Drosophila from Stevens - the account given in his Nobel biography states that the idea was suggested to him by entomologist Frank Eugene Lutz - but it is all the same a curious missed opportunity to recognize a predecessor in the research of his preferred organism upon the occasion of her death.

Nettie Stevens was granted less than a decade of professional work in the field she had finally found her place in at the age of forty. It was only at the end of her life that Bryn Mawr offered to create a research professorship position for her so that she would not have to expend energy scrapping up funding to supplement her official position as "Associate in Experimental Morphology," and she died of breast cancer before the new title and salary could be conferred upon her. There are so many What Ifs that accumulate about her person and career - What if she had started sooner? What if she had been given a position equal to her abilities earlier? What if those best placed to assure her legacy had worked more assiduously towards that end? It is easy to get lost in those What Ifs, and to fail to see what is right before us - 38 papers in under a decade, one of which ranks among the most important genetic works of the Twentieth Century, produced by an individual only three years into her second, mid-life, career. We know far too little of her character and voice, but the work remains, and in it we find the building blocks of our destinies, writ small but distinct, in the cells of our cousin and sometimes friend, the humble meal worm.


Marilyn Ogilvie devotes a good amount of space to Stevens in her classic Women in Science (1986) but there is no stand alone biography of her life and work. The best source is, ironically, a French article from 2008 by Simone Gilgenkrantz which places Stevens within the context of her fellow chromosomal researches. Thomas Hunt Morgan's obituary of Stevens can still be found online, which is a good source for details about her work that most authors gloss over, though of course it needs to be approached skeptically.

This piece was originally published as the 179th column in the Women in Science series.


Jul 12, 2023

Good article. 'IF' only women were accepted in days gone by, what would the world be like now. Even today there is still discrimination.

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