Nettie Stevens and the Discovery of the Sex Chromosomes

For a woman growing up in the aftermath of the American Civil War, a research career in science was next to unheard of. But Nettie Stevens didn’t just beat the odds to become one of America’s earliest female geneticists – she also helped set the foundation for modern understandings of chromosomes by determining the role of X and Y chromosomes in biological sex.

Largely forgotten in her own life, Stevens has more recently received newfound recognition as an example of the tremendous influence women, sich as Rosalind Franklin, have had on the advancement of science – even if they were denied the recognition they deserved in their own lifetimes.


Early Life and Career

Born in Vermont in 1861 – the same year the American Civil War began – Stevens held an early drive to go into higher education, but lacked the financial means make the jump from high school to college. Instead, Stevens worked as a teacher to make ends meet while studying at Westford Academy in Massachusetts, becoming one of only three women to graduate with a degree in her class. From Westford, Stevens completed degrees at the newly opened Stanford University and Bryn Mawr in Pennsylvania, where she achieved a PhD in cellular biology at the age of 39.

Even as Stevens was finishing her degree in 1903 at Bryn Mawr, Austrian friar Gregor Mendel’s research on genetics was seeing a renewed burst of interest, with scientists busy diving into research on the factors behind hereditary genetics. At the same time, long-held assumptions, particularly surrounding the conditions that dictated biological sex, remained stubbornly rigid in the scientific community. For centuries, the determining factor of whether a child was biologically male or female was believed based on environmental factors, such as the temperature of the mother’s womb. Even as scientists were uncovering genetic basis for certain traits, the belief environmentally-determined sex remained stubbornly pervasive in the community.

Steven’s research was one of the first to challenge this assumption. In her research, she looked at the chromosomes of insects – mostly mealworms, beetles, and butterflies – by inspecting thousands of paper-thin cross sections of their gonads under the microscope. After studying and cataloging the individual chromosomes, Stevens found evidence that biological sex was based on the particular assortment of chromosomes – what we now know as X/Y sex determination. 

An Unrecognized Discovery

In 1905, Steven published her findings, which definitively made the case for biological sex as the makeup of X and Y chromosomes. The discovery effectively made her one of the first scientists in the world to understand how chromosomes may be involved in sex determination.

However, several factors prohibited Stevens from receiving the recognition she deserved. Most prominently was the research of another scientist, Edmund Beecher Wilson, who earlier that year had published his own findings on X/Y sex determination through his own independent research. While normally two scientists independently coming to the same conclusions in the in such as short amount of time from each other would jointly receive recognition, in part because Wilson was an already well-known scientist – and in part likely because he was a man – he received the recognition for the discovery, while Stevens discovery was discounted.

Even as Wilson went on to become one of the most famous geneticists of his time, Stevens faded into obscurity, dying not six years later from breast cancer in 1912. It would take decades for her name to resurface, as an example of those like Rosalind Franklin whose contributions to science were discounted in favor of their male peers, largely on the basis of sex.

Stevens’ Legacy Today

In hindsight, Stevens’ findings in the early 1900s were spot-on, going further than even Wilson’s discovery to make the (ultimately incorrect) claim of chromosomes as being the determinate of sex, without the influence of any external or environmental factors. Between her and Wilson, the discovery of chromosomal sex determination set the foundations for generations of future geneticists to unlock the mysteries of our chromosomes, especially in how certain traits are passed along the X or Y chromosome.

Today, Stevens’ name is being re-discovered, over a hundred years after her death. In 1994, Stevens was added to the National Women’s Hall of Fame, and on the 155th anniversary of her birthday in 2016 was recognized with a Google Doodle, which directed internet searches to learn more about her own scientific discoveries. In time, perhaps her name will finally see its rightful place alongside Wilson’s as the co-discoverer of X/Y sex determination, and receive the recognition she deserved years ago.


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Photo credits:

  • Bryn Mawr College Special Collections, Public domain, via Wikimedia Commons
  • The Incubator (courtesy of Carnegie Institution of Washington), Public domain, via Wikimedia Commons

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