Nettie Stevens, an American Scientist Who Revolutionized the Field of Genetics

This American scientist revolutionized the field of genetics by making one of the most important contributions to history when she asked herself how the sex of organisms is determined.

Nettie Stevens, an American Scientist Who Revolutionized the Field of Genetics
Nettie Stevens (1861-1912), discovered that biological sex is determined by a configuration of chromosomes. Photo: Wikipedia

On July 7, 1861, Nettie Maria Stevens was born, an American scientist who revolutionized the field of genetics by making one of the most important contributions to history by asking how the sex of organisms is determined.

From an early age, she showed a great passion for research, and despite not having sufficient economic resources and her condition as a woman at that time, she managed to enter Stanford University at the age of 35, received her doctorate in 1903 at Bryn Mawr College in Philadelphia and focused on research.

Nettie devoted herself to insect research in the field of cytogenetics, particularly the mealworm. In his work Studies in Spermatogenesis with Special Reference to the "Accessory Chromosome" (1905), Nattie concluded that what establishes sex are two types of spermatozoa: those that possess the X chromosome and those that possess the Y, and depending on which one fertilizes an egg, the following results are obtained: XX produces a female and XY a male, with these statements he changed the previous structural scheme of chromosomes and revolutionized the study of genetics.

Nattie also stood out for the methodical way in which he made his microscopic observations, as well as for the rigorous and detailed description of the methods he used, in many of which he made relevant innovations.

Why was Nettie Stevens's discovery important?

By the time of her death in 1912, Nettie Stevens had achieved enough prestige as a biologist and geneticist to receive praise from her mentor and future Nobel laureate, Thomas Hunt Morgan, in the journal Science. But to get there was not easy.

Years earlier, when in 1905 Nettie Stevens published Studies in Spermatogenesis with Special Reference to the Accessory Chromosome and managed to demonstrate that chromosomes were responsible for determining the sex of organisms, she had to fight to make herself heard and become part of the history of embryology and cytogenetics against an entire scientific community that relegated women and their discoveries to the background.

Nettie Stevens had to work to survive. She worked as a teacher and librarian, postponing her dream of going to university to become a researcher until she was able to save enough money. And she did.  At a time when women were predestined to marry and be mothers or at most aspire to be teachers, nurses, or secretaries, Nettie Stevens carved a niche for herself in the scientific community despite having started her career at the age of 35.

She shone academically from childhood, but her career was cut short for eleven years after graduating in 1880. Lack of resources to afford higher education led Nettie Stevens to work as a teacher and librarian, but after saving enough for just over a decade, in 1896, at the age of 35, she enrolled at the prestigious Stanford University. Just six years later, Nettie Stevens had already graduated, completed her master's degree, and published her doctoral dissertation.

Despite living in a difficult time for women, and even more complicated for those who chose to dedicate themselves to science, the talent of Nettie Stevens did not go unnoticed by the geneticist and future Nobel Prize winner Thomas Hunt Morgan, who incorporated her into his research team and took it upon himself to personally direct her work.

Nettie Stevens decided to direct her work towards the investigation of chromosomes and Mendelian inheritance, a hot topic in the scientific society of the time. Between the late 19th and early 20th centuries, most researchers agreed that the sex determination of organisms depends on factors internal to the egg. However, there was disagreement as to which factors are the determinants, and there was even another current that suggested that external factors could also play a role.

Contrary to the belief of her mentor, who defended the role of the cytoplasm in embryonic development, Nettie Stevens focused on studying the relationship between chromosomes and sex determination by examining different insects in detail. While observing the mealworm, Nettie Stevens discovered that males were capable of producing reproductive cells with both types of X and Y chromosomes, while females only did so with the X chromosomes, which led her to deduce that sex is inherited as a chromosomal factor.

Continuing in this line, she observed and analyzed fifty species of beetles and nine species of flies, and in 1905 she published the work in which his conclusions were collected. Under the title Studies in Spermatogenesis with Special Reference to the "Accessory Chromosome," she found that chromosomes exist as paired structures of cells and that if the egg was fertilized by a sperm carrying the X chromosome it would give rise to a female, while if it contained the Y chromosome it would produce a male.

Her work had revolutionized the world of genetics, and her contribution was substantial, but Nettie Stevens would still have to overcome some more obstacles to achieve the recognition she deserved. Firstly, because the scientific community, still obsessed with environmental contributions, was slow to embrace her theory, and secondly because Nettie Stevens' research coincided in time and conclusions with those of Edmund B. Wilson, one of the most prestigious geneticists of the time, who would end up receiving the priority of the discoveries.

Her status as a woman automatically relegated her to the background even though the article published by Wilson in the journal Science mentioned that "her findings were in agreement with the observations of Nettie Stevens", which showed that the geneticist was aware of Stevens' work and in a certain way recognized her priority.

The truth is that the work of Nettie Stevens was more decisive both for its quality and for the amount of experimental information provided, all in a meticulous and detailed style. Aware of the secondary role played by women scientists at that time, Nettie Stevens filled her publications with quotations and references to the work of other female colleagues to praise their work and try to give them the visibility that they did not have.

The scientific work of Nettie Stevens produced profound and important changes in biology by establishing the basis of what this discipline would become in the following decades. She worked and signed numerous papers on her own, but she also worked together with other scientists, geneticists, and biologists of the time, such as T.H. Morgan, E.B. Wilson, A.M. Boring, H.D. King, and H. Randolph.

She died early, at the age of 53, on May 4, 1912, due to breast cancer, but in the few years of her scientific career and despite the difficulties she faced simply because she was a woman, Nettie Stevens achieved transcendental advances and contributions that are still relevant today. Nettie Stevens was a woman ahead of her time who fought to gain recognition in the world of science at a time when women did not have it easy.